Re: [Xen-devel] [PATCH v2 1/1] cameraif: add ABI for para-virtual camera

[Hans Verkuil <hverkuil@xxxxxxxxx>]

On 09/12/18 12:09, Oleksandr Andrushchenko wrote:
> Hi, Hans!
> 
> Thank you for valuable comments and valid concerns!
> 
> On 09/12/2018 10:52 AM, Hans Verkuil wrote:
>> On 09/11/2018 10:29 AM, Oleksandr Andrushchenko wrote:
>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>>>
>>> This is the ABI for the two halves of a para-virtualized
>>> camera driver which extends Xen's reach multimedia capabilities even
>>> farther enabling it for video conferencing, In-Vehicle Infotainment,
>>> high definition maps etc.
>>>
>>> The initial goal is to support most needed functionality with the
>>> final idea to make it possible to extend the protocol if need be:
>>>
>>> 1. Provide means for base virtual device configuration:
>>>   - pixel formats
>>>   - resolutions
>>>   - frame rates
>>> 2. Support basic camera controls:
>>>   - contrast
>>>   - brightness
>>>   - hue
>>>   - saturation
>>> 3. Support streaming control
>>> 4. Support zero-copying use-cases
>>>
>>> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>>> ---
>>>   xen/include/public/io/cameraif.h | 1263 ++++++++++++++++++++++++++++++
>>>   1 file changed, 1263 insertions(+)
>>>   create mode 100644 xen/include/public/io/cameraif.h
>>>
>>> diff --git a/xen/include/public/io/cameraif.h 
>>> b/xen/include/public/io/cameraif.h
>>> new file mode 100644
>>> index 000000000000..38b9b3741e75
>>> --- /dev/null
>>> +++ b/xen/include/public/io/cameraif.h
>>> @@ -0,0 +1,1263 @@
>>> +/******************************************************************************
>>> + * cameraif.h
>>> + *
>>> + * Unified camera device I/O interface for Xen guest OSes.
>>> + *
>>> + * Permission is hereby granted, free of charge, to any person obtaining a 
>>> copy
>>> + * of this software and associated documentation files (the "Software"), to
>>> + * deal in the Software without restriction, including without limitation 
>>> the
>>> + * rights to use, copy, modify, merge, publish, distribute, sublicense, 
>>> and/or
>>> + * sell copies of the Software, and to permit persons to whom the Software 
>>> is
>>> + * furnished to do so, subject to the following conditions:
>>> + *
>>> + * The above copyright notice and this permission notice shall be included 
>>> in
>>> + * all copies or substantial portions of the Software.
>>> + *
>>> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS 
>>> OR
>>> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
>>> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL 
>>> THE
>>> + * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
>>> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
>>> + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
>>> + * DEALINGS IN THE SOFTWARE.
>>> + *
>>> + * Copyright (C) 2018 EPAM Systems Inc.
>>> + *
>>> + * Author: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>>> + */
>>> +
>>> +#ifndef __XEN_PUBLIC_IO_CAMERAIF_H__
>>> +#define __XEN_PUBLIC_IO_CAMERAIF_H__
>>> +
>>> +#include "ring.h"
>>> +#include "../grant_table.h"
>>> +
>>> +/*
>>> + 
>>> ******************************************************************************
>>> + *                           Protocol version
>>> + 
>>> ******************************************************************************
>>> + */
>>> +#define XENCAMERA_PROTOCOL_VERSION     "1"
>>> +
>>> +/*
>>> + 
>>> ******************************************************************************
>>> + *                  Feature and Parameter Negotiation
>>> + 
>>> ******************************************************************************
>>> + *
>>> + * Front->back notifications: when enqueuing a new request, sending a
>>> + * notification can be made conditional on xencamera_req (i.e., the generic
>>> + * hold-off mechanism provided by the ring macros). Backends must set
>>> + * xencamera_req appropriately (e.g., using 
>>> RING_FINAL_CHECK_FOR_REQUESTS()).
>>> + *
>>> + * Back->front notifications: when enqueuing a new response, sending a
>>> + * notification can be made conditional on xencamera_resp (i.e., the 
>>> generic
>>> + * hold-off mechanism provided by the ring macros). Frontends must set
>>> + * xencamera_resp appropriately (e.g., using 
>>> RING_FINAL_CHECK_FOR_RESPONSES()).
>>> + *
>>> + * The two halves of a para-virtual camera driver utilize nodes within
>>> + * XenStore to communicate capabilities and to negotiate operating 
>>> parameters.
>>> + * This section enumerates these nodes which reside in the respective 
>>> front and
>>> + * backend portions of XenStore, following the XenBus convention.
>>> + *
>>> + * All data in XenStore is stored as strings. Nodes specifying numeric
>>> + * values are encoded in decimal. Integer value ranges listed below are
>>> + * expressed as fixed sized integer types capable of storing the conversion
>>> + * of a properly formatted node string, without loss of information.
>>> + *
>>> + 
>>> ******************************************************************************
>>> + *                        Example configuration
>>> + 
>>> ******************************************************************************
>>> + *
>>> + * This is an example of backend and frontend configuration:
>>> + *
>>> + *--------------------------------- Backend 
>>> -----------------------------------
>>> + *
>>> + * /local/domain/0/backend/vcamera/1/0/frontend-id = "1"
>>> + * /local/domain/0/backend/vcamera/1/0/frontend = 
>>> "/local/domain/1/device/vcamera/0"
>>> + * /local/domain/0/backend/vcamera/1/0/state = "4"
>>> + * /local/domain/0/backend/vcamera/1/0/versions = "1,2"
>>> + *
>>> + *--------------------------------- Frontend 
>>> ----------------------------------
>>> + *
>>> + * /local/domain/1/device/vcamera/0/backend-id = "0"
>>> + * /local/domain/1/device/vcamera/0/backend = 
>>> "/local/domain/0/backend/vcamera/1"
>>> + * /local/domain/1/device/vcamera/0/state = "4"
>>> + * /local/domain/1/device/vcamera/0/version = "1"
>>> + * /local/domain/1/device/vcamera/0/be-alloc = "1"
>>> + *
>>> + *---------------------------- Device 0 configuration 
>>> -------------------------
>>> + *
>>> + * /local/domain/1/device/vcamera/0/controls = "contrast,hue"
>>> + * /local/domain/1/device/vcamera/0/formats/YUYV/640x480/frame-rates = 
>>> "30/1,15/1"
>>> + * /local/domain/1/device/vcamera/0/formats/YUYV/1920x1080/frame-rates = 
>>> "15/2"
>>> + * /local/domain/1/device/vcamera/0/formats/BGRA/640x480/frame-rates = 
>>> "15/1,15/2"
>>> + * /local/domain/1/device/vcamera/0/formats/BGRA/1200x720/frame-rates = 
>>> "15/2"
>>> + * /local/domain/1/device/vcamera/0/unique-id = "0"
>>> + * /local/domain/1/device/vcamera/0/req-ring-ref = "2832"
>>> + * /local/domain/1/device/vcamera/0/req-event-channel = "15"
>>> + * /local/domain/1/device/vcamera/0/evt-ring-ref = "387"
>>> + * /local/domain/1/device/vcamera/0/evt-event-channel = "16"
>>> + *
>>> + *---------------------------- Device 1 configuration 
>>> -------------------------
>>> + *
>>> + * /local/domain/1/device/vcamera/1/controls = "brightness,saturation,hue"
>>> + * /local/domain/1/device/vcamera/1/formats/YUYV/640x480/frame-rates = 
>>> "30/1,15/2"
>>> + * /local/domain/1/device/vcamera/1/formats/YUYV/1920x1080/frame-rates = 
>>> "15/2"
>>> + * /local/domain/1/device/vcamera/1/unique-id = "1"
>>> + * /local/domain/1/device/vcamera/1/req-ring-ref = "2833"
>>> + * /local/domain/1/device/vcamera/1/req-event-channel = "17"
>>> + * /local/domain/1/device/vcamera/1/evt-ring-ref = "388"
>>> + * /local/domain/1/device/vcamera/1/evt-event-channel = "18"
>>> + *
>>> + 
>>> ******************************************************************************
>>> + *                            Backend XenBus Nodes
>>> + 
>>> ******************************************************************************
>>> + *
>>> + *----------------------------- Protocol version 
>>> ------------------------------
>>> + *
>>> + * versions
>>> + *      Values:         <string>
>>> + *
>>> + *      List of XENCAMERA_LIST_SEPARATOR separated protocol versions 
>>> supported
>>> + *      by the backend. For example "1,2,3".
>>> + *
>>> + 
>>> ******************************************************************************
>>> + *                            Frontend XenBus Nodes
>>> + 
>>> ******************************************************************************
>>> + *
>>> + *-------------------------------- Addressing 
>>> ---------------------------------
>>> + *
>>> + * dom-id
>>> + *      Values:         <uint16_t>
>>> + *
>>> + *      Domain identifier.
>>> + *
>>> + * dev-id
>>> + *      Values:         <uint16_t>
>>> + *
>>> + *      Device identifier.
>>> + *
>>> + *      /local/domain/<dom-id>/device/vcamera/<dev-id>/...
>>> + *
>>> + *----------------------------- Protocol version 
>>> ------------------------------
>>> + *
>>> + * version
>>> + *      Values:         <string>
>>> + *
>>> + *      Protocol version, chosen among the ones supported by the backend.
>>> + *
>>> + *------------------------- Backend buffer allocation 
>>> -------------------------
>>> + *
>>> + * be-alloc
>>> + *      Values:         "0", "1"
>>> + *
>>> + *      If value is set to "1", then backend will be the buffer
>>> + *      provider/allocator for this domain during XENCAMERA_OP_BUF_CREATE
>>> + *      operation.
>>> + *      If value is not "1" or omitted frontend must allocate buffers 
>>> itself.
>>> + *
>>> + *------------------------------- Camera settings 
>>> -----------------------------
>>> + *
>>> + * unique-id
>>> + *      Values:         <string>
>>> + *
>>> + *      After device instance initialization each camera is assigned a
>>> + *      unique ID, so it can be identified by the backend by this ID.
>>> + *      This can be UUID or such.
>>> + *
>>> + * controls
>>> + *      Values:         <list of string>
>>> + *
>>> + *      List of supported camera controls separated by 
>>> XENCAMERA_LIST_SEPARATOR.
>>> + *      Camera controls are expressed as a list of string values w/o any
>>> + *      ordering requirement.
>>> + *
>>> + * formats
>>> + *      Values:         <format, char[4]>
>>> + *
>>> + *      Formats are organized as a set of directories one per each
>>> + *      supported pixel format. The name of the directory is the
>>> + *      corresponding FOURCC string label. The next level of
>>> + *      the directory under <formats> represents supported resolutions.
>> So how will this work for a pixelformat like V4L2_PIX_FMT_ARGB555X?
>>
>> As mentioned before, we display such formats as 'XXXX-BE', i.e. char[7].
> ok, then I'll change this to char[7] and put a note on big-endian:
> 
>   *      If format represents a big-endian FOURCC code, then "-BE"
>   *      suffix must be added, case insensitive.

Since the fourcc is case-sensitive, I'd keep -BE case sensitive as well.
Your decision, though.

> 
>>
>> I assume the pixelformats you use here are based on the V4L2_PIX_FMT_ 
>> fourccs?
>>
>> Note that there is no real standard for fourcc values, so if you want to
>> support a Windows backend as well, then you'll need mappings from whatever
>> Windows uses to the V4L2 fourccs.
>>
>> The V4L2_PIX_FMT_ fourccs are entirely V4L2 specific.
>>
>> So you have to define here whose fourccs you are using.
> I thought that [1] defines all these values, but if this is not the case,
> then I think we can use what V4L2 uses. In this case,
> what would be the best reference in the protocol to those fourcc
> codes? For example, I can reference [2] in the protocol's assumptions:
> 
>   
> ******************************************************************************
>   *                              Assumptions
>   
> ******************************************************************************
> ...
>   * - all FOURCC mappings used for configuration and messaging are
>   *   Linux V4L2 ones: 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/videodev2.h

Yeah, I'd pick videodev2.h. We document the formats fairly carefully in
our spec.

[1] is useless: there are many, many more formats (often HW specific) and
variants of formats, just look in our spec at how many Bayer variants there
are.

> 
> 
>>
>>> + *
>>> + * resolution
>>> + *      Values:         <width, uint32_t>x<height, uint32_t>
>>> + *
>>> + *      Resolutions are organized as a set of directories one per each
>>> + *      supported resolution under corresponding <formats> directory.
>>> + *      The name of the directory is the supported width and height
>>> + *      of the camera resolution in pixels.
>> So how will this work for sources like HDMI where the resolution depends on
>> the signal? Will this only show the received resolution? And if so, what
>> happens if there is no signal? Or the resolution changes?
>>
>> If you want to support such devices, you'll need to define the behavior.
>>
>> It might be OK to rely on the XenbusStateReconfiguring state, but I think
>> this would have to be described somewhere. I.e. if the source is 
>> disconnected,
>> then you enter state XenbusStateReconfiguring, when a new source is 
>> connected,
>> then you setup a new configuration based on the newly detected timings. But
>> what to do when the timings change? Reconfiguring again? Whether this is an
>> option depends in part on how long it takes before the new configuration
>> becomes active.
> Well, IMO this is something which is a part of the backend's
> logic: backend may decide to tear down the whole virtual camera
> device, e.g. going into closed state, or it can try reconfiguring the device
> or whatever.
> 
> One note on such complex use-cases: we are trying to keep the protocol
> as simple as possible not trying to mimic really complex HW.
> If need be to support this one can probably just pass through that
> HW device to a guest, so it can benefit from all those features.

This is not a complex use-case. The only difference is that you don't have an
integrated sensor, but instead a connector through which you receive a video
signal.

If you want you can consider this a 'remote hotpluggable sensor' use-case.

> Most use-cases we see only need captured frames and most of the work
> of getting those is handled by the backend. And the configuration
> most of the time should remain static, e.g. all the parameters are defined
> for known HW and VMs are configured with specific use-cases in mind.
> Thus, the backend should hide all the real complexity from a virtual
> camera device driver as much as it can (BTW, frontend might not be a
> V4L2 device driver, but a unikernel application for instance)

It is my feeling that you need to test this use-case before finalizing
this spec. It does not have to include this functionality for the first
version, but at least you should know that it can be added later.

>>
>>> + *
>>> + * frame-rates
>>> + *      Values:         <numerator, uint32_t>/<denominator, uint32_t>
>>> + *
>>> + *      List of XENCAMERA_FRAME_RATE_SEPARATOR separated supported frame 
>>> rates
>>> + *      of the camera expressed as numerator and denominator of the
>>> + *      corresponding frame rate.
>> Note that there are devices that allow a range of resolutions and framerates
>> instead of a discrete set of resolutions/framerates.
>>
>> Whether or not that is something you want to care about here is another 
>> matter.
>>
>> A device that has a range of resolutions typically has a scaler on board.
>> A range of framerates typically means that it has fine-grained control over
>> the sensor pixelclock and whatever else is related to the framerate.
> Yes, that was intentionally that we only support discrete values,
> please see above - most of the time you don't need a fully blown
> virtual camera

I actually think it makes sense to restrict yourself here to a list of discrete
values.

>>
>>> + *
>>> + *------------------- Camera Request Transport Parameters 
>>> ---------------------
>>> + *
>>> + * This communication path is used to deliver requests from frontend to 
>>> backend
>>> + * and get the corresponding responses from backend to frontend,
>>> + * set up per virtual camera device.
>>> + *
>>> + * req-event-channel
>>> + *      Values:         <uint32_t>
>>> + *
>>> + *      The identifier of the Xen camera's control event channel
>>> + *      used to signal activity in the ring buffer.
>>> + *
>>> + * req-ring-ref
>>> + *      Values:         <uint32_t>
>>> + *
>>> + *      The Xen grant reference granting permission for the backend to map
>>> + *      a sole page of camera's control ring buffer.
>>> + *
>>> + *-------------------- Camera Event Transport Parameters 
>>> ----------------------
>>> + *
>>> + * This communication path is used to deliver asynchronous events from 
>>> backend
>>> + * to frontend, set up per virtual camera device.
>>> + *
>>> + * evt-event-channel
>>> + *      Values:         <uint32_t>
>>> + *
>>> + *      The identifier of the Xen camera's event channel
>>> + *      used to signal activity in the ring buffer.
>>> + *
>>> + * evt-ring-ref
>>> + *      Values:         <uint32_t>
>>> + *
>>> + *      The Xen grant reference granting permission for the backend to map
>>> + *      a sole page of camera's event ring buffer.
>>> + */
>>> +
>>> +/*
>>> + 
>>> ******************************************************************************
>>> + *                               STATE DIAGRAMS
>>> + 
>>> ******************************************************************************
>>> + *
>>> + * Tool stack creates front and back state nodes with initial state
>>> + * XenbusStateInitialising.
>>> + * Tool stack creates and sets up frontend camera configuration
>>> + * nodes per domain.
>>> + *
>>> + *-------------------------------- Normal flow 
>>> --------------------------------
>>> + *
>>> + * Front                                Back
>>> + * =================================    
>>> =====================================
>>> + * XenbusStateInitialising              XenbusStateInitialising
>>> + *                                       o Query backend device 
>>> identification
>>> + *                                         data.
>>> + *                                       o Open and validate backend 
>>> device.
>>> + *                                                |
>>> + *                                                |
>>> + *                                                V
>>> + *                                      XenbusStateInitWait
>>> + *
>>> + * o Query frontend configuration
>>> + * o Allocate and initialize
>>> + *   event channels per configured
>>> + *   camera.
>>> + * o Publish transport parameters
>>> + *   that will be in effect during
>>> + *   this connection.
>>> + *              |
>>> + *              |
>>> + *              V
>>> + * XenbusStateInitialised
>>> + *
>>> + *                                       o Query frontend transport 
>>> parameters.
>>> + *                                       o Connect to the event channels.
>>> + *                                                |
>>> + *                                                |
>>> + *                                                V
>>> + *                                      XenbusStateConnected
>>> + *
>>> + *  o Create and initialize OS
>>> + *    virtual camera as per
>>> + *    configuration.
>>> + *              |
>>> + *              |
>>> + *              V
>>> + * XenbusStateConnected
>>> + *
>>> + *                                      XenbusStateUnknown
>>> + *                                      XenbusStateClosed
>>> + *                                      XenbusStateClosing
>>> + * o Remove virtual camera device
>>> + * o Remove event channels
>>> + *              |
>>> + *              |
>>> + *              V
>>> + * XenbusStateClosed
>>> + *
>>> + *------------------------------- Recovery flow 
>>> -------------------------------
>>> + *
>>> + * In case of frontend unrecoverable errors backend handles that as
>>> + * if frontend goes into the XenbusStateClosed state.
>>> + *
>>> + * In case of backend unrecoverable errors frontend tries removing
>>> + * the virtualized device. If this is possible at the moment of error,
>>> + * then frontend goes into the XenbusStateInitialising state and is ready 
>>> for
>>> + * new connection with backend. If the virtualized device is still in use 
>>> and
>>> + * cannot be removed, then frontend goes into the XenbusStateReconfiguring 
>>> state
>>> + * until either the virtualized device is removed or backend initiates a 
>>> new
>>> + * connection. On the virtualized device removal frontend goes into the
>>> + * XenbusStateInitialising state.
>>> + *
>>> + * Note on XenbusStateReconfiguring state of the frontend: if backend has
>>> + * unrecoverable errors then frontend cannot send requests to the backend
>>> + * and thus cannot provide functionality of the virtualized device anymore.
>>> + * After backend is back to normal the virtualized device may still hold 
>>> some
>>> + * state: configuration in use, allocated buffers, client application 
>>> state etc.
>>> + * In most cases, this will require frontend to implement complex recovery
>>> + * reconnect logic. Instead, by going into XenbusStateReconfiguring state,
>>> + * frontend will make sure no new clients of the virtualized device are
>>> + * accepted, allow existing client(s) to exit gracefully by signaling error
>>> + * state etc.
>>> + * Once all the clients are gone frontend can reinitialize the virtualized
>>> + * device and get into XenbusStateInitialising state again signaling the
>>> + * backend that a new connection can be made.
>>> + *
>>> + * There are multiple conditions possible under which frontend will go from
>>> + * XenbusStateReconfiguring into XenbusStateInitialising, some of them are 
>>> OS
>>> + * specific. For example:
>>> + * 1. The underlying OS framework may provide callbacks to signal that the 
>>> last
>>> + *    client of the virtualized device has gone and the device can be 
>>> removed
>>> + * 2. Frontend can schedule a deferred work (timer/tasklet/workqueue)
>>> + *    to periodically check if this is the right time to re-try removal of
>>> + *    the virtualized device.
>>> + * 3. By any other means.
>>> + *
>>> + 
>>> ******************************************************************************
>>> + *                             REQUEST CODES
>>> + 
>>> ******************************************************************************
>>> + */
>>> +#define XENCAMERA_OP_CONFIG_SET        0x00
>>> +#define XENCAMERA_OP_CONFIG_GET        0x01
>>> +#define XENCAMERA_OP_BUF_REQUEST       0x02
>>> +#define XENCAMERA_OP_BUF_CREATE        0x03
>>> +#define XENCAMERA_OP_BUF_DESTROY       0x04
>>> +#define XENCAMERA_OP_BUF_QUEUE         0x05
>>> +#define XENCAMERA_OP_BUF_DEQUEUE       0x06
>>> +#define XENCAMERA_OP_CTRL_ENUM         0x07
>>> +#define XENCAMERA_OP_CTRL_SET          0x08
>>> +#define XENCAMERA_OP_CTRL_GET          0x09
>>> +#define XENCAMERA_OP_STREAM_START      0x0a
>>> +#define XENCAMERA_OP_STREAM_STOP       0x0b
>>> +
>>> +#define XENCAMERA_CTRL_BRIGHTNESS      0
>>> +#define XENCAMERA_CTRL_CONTRAST        1
>>> +#define XENCAMERA_CTRL_SATURATION      2
>>> +#define XENCAMERA_CTRL_HUE             3
>>> +
>>> +/* Number of supported controls. */
>>> +#define XENCAMERA_MAX_CTRL             4
>>> +
>>> +/* Control is read-only. */
>>> +#define XENCAMERA_CTRL_FLG_RO          (1 << 0)
>>> +/* Control is write-only. */
>>> +#define XENCAMERA_CTRL_FLG_WO          (1 << 1)
>>> +/* Control's value is volatile. */
>>> +#define XENCAMERA_CTRL_FLG_VOLATILE    (1 << 2)
>>> +
>>> +/* Supported color spaces. */
>>> +#define XENCAMERA_COLORSPACE_SMPTE170M 0
>>> +#define XENCAMERA_COLORSPACE_REC709    1
>>> +#define XENCAMERA_COLORSPACE_SRGB      2
>>> +#define XENCAMERA_COLORSPACE_ADOBERGB  3
>> Right, this is something I haven't done yet: replace ADOBERGB with OPRGB.
> Will do here and below, thank you for clarifying
>>
>> Adobe doesn't like others using its name for the colorspace, and in fact
>> all references in the CTA-861-G standard to Adobe have been replaced with
>> opRGB, which is the name of the official standard.
>>
>> opRGB is really identical to AdobeRGB, but if you are going to make new
>> defines you might just as well use the official name.
>>
>> I still need to make patches for the kernel to do a similar substitution.
>>
>>> +#define XENCAMERA_COLORSPACE_BT2020    4
>>> +#define XENCAMERA_COLORSPACE_DCI_P3    5
>>> +#define XENCAMERA_COLORSPACE_RAW       6
>>> +
>>> +/* Color space transfer function. */
>>> +#define XENCAMERA_XFER_FUNC_709        0
>>> +#define XENCAMERA_XFER_FUNC_SRGB       1
>>> +#define XENCAMERA_XFER_FUNC_ADOBERGB   2
>> Ditto.
>>
>>> +#define XENCAMERA_XFER_FUNC_NONE       3
>>> +#define XENCAMERA_XFER_FUNC_DCI_P3     4
>>> +#define XENCAMERA_XFER_FUNC_SMPTE2084  5
>>> +
>>> +/* Color space Y’CbCr encoding. */
>>> +#define XENCAMERA_YCBCR_ENC_601              0
>>> +#define XENCAMERA_YCBCR_ENC_709              1
>>> +#define XENCAMERA_YCBCR_ENC_XV601            2
>>> +#define XENCAMERA_YCBCR_ENC_XV709            3
>>> +#define XENCAMERA_YCBCR_ENC_BT2020           4
>>> +#define XENCAMERA_YCBCR_ENC_BT2020_CONST_LUM 5
>>> +
>>> +/* Quantization range. */
>>> +#define XENCAMERA_QUANTIZATION_FULL_RANGE    0
>>> +#define XENCAMERA_QUANTIZATION_LIM_RANGE     1
>>> +
>>> +/*
>>> + 
>>> ******************************************************************************
>>> + *                                 EVENT CODES
>>> + 
>>> ******************************************************************************
>>> + */
>>> +#define XENCAMERA_EVT_FRAME_AVAIL      0x00
>>> +#define XENCAMERA_EVT_CONFIG_CHANGE    0x01
>>> +#define XENCAMERA_EVT_CTRL_CHANGE      0x02
>>> +
>>> +/* Resolution has changed. */
>>> +#define XENCAMERA_EVT_CFG_FLG_RESOL    (1 << 0)
>>> +
>>> +/*
>>> + 
>>> ******************************************************************************
>>> + *               XENSTORE FIELD AND PATH NAME STRINGS, HELPERS
>>> + 
>>> ******************************************************************************
>>> + */
>>> +#define XENCAMERA_DRIVER_NAME          "vcamera"
>>> +
>>> +#define XENCAMERA_LIST_SEPARATOR       ","
>>> +#define XENCAMERA_RESOLUTION_SEPARATOR "x"
>>> +#define XENCAMERA_FRACTION_SEPARATOR   "/"
>>> +
>>> +#define XENCAMERA_FIELD_BE_VERSIONS    "versions"
>>> +#define XENCAMERA_FIELD_FE_VERSION     "version"
>>> +#define XENCAMERA_FIELD_REQ_RING_REF   "req-ring-ref"
>>> +#define XENCAMERA_FIELD_REQ_CHANNEL    "req-event-channel"
>>> +#define XENCAMERA_FIELD_EVT_RING_REF   "evt-ring-ref"
>>> +#define XENCAMERA_FIELD_EVT_CHANNEL    "evt-event-channel"
>>> +#define XENCAMERA_FIELD_CONTROLS       "controls"
>>> +#define XENCAMERA_FIELD_FORMATS        "formats"
>>> +#define XENCAMERA_FIELD_FRAME_RATES    "frame-rates"
>>> +#define XENCAMERA_FIELD_BE_ALLOC       "be-alloc"
>>> +#define XENCAMERA_FIELD_UNIQUE_ID      "unique-id"
>>> +
>>> +#define XENCAMERA_CTRL_BRIGHTNESS_STR  "brightness"
>>> +#define XENCAMERA_CTRL_CONTRAST_STR    "contrast"
>>> +#define XENCAMERA_CTRL_SATURATION_STR  "saturation"
>>> +#define XENCAMERA_CTRL_HUE_STR         "hue"
>>> +
>>> +/* Maximum number of buffer planes supported. */
>>> +#define XENCAMERA_MAX_PLANE            4
>>> +
>>> +/*
>>> + 
>>> ******************************************************************************
>>> + *                          STATUS RETURN CODES
>>> + 
>>> ******************************************************************************
>>> + *
>>> + * Status return code is zero on success and -XEN_EXX on failure.
>>> + *
>>> + 
>>> ******************************************************************************
>>> + *                              Assumptions
>>> + 
>>> ******************************************************************************
>>> + *
>>> + * - usage of grant reference 0 as invalid grant reference:
>>> + *   grant reference 0 is valid, but never exposed to a PV driver,
>>> + *   because of the fact it is already in use/reserved by the PV console.
>>> + * - all references in this document to page sizes must be treated
>>> + *   as pages of size XEN_PAGE_SIZE unless otherwise noted.
>>> + *
>>> + 
>>> ******************************************************************************
>>> + *       Description of the protocol between frontend and backend driver
>>> + 
>>> ******************************************************************************
>>> + *
>>> + * The two halves of a Para-virtual camera driver communicate with
>>> + * each other using shared pages and event channels.
>>> + * Shared page contains a ring with request/response packets.
>>> + *
>>> + * All reserved fields in the structures below must be 0.
>>> + *
>>> + * For all request/response/event packets:
>>> + *   - frame rate parameter is represented as a pair of 4 octet long
>>> + *     numerator and denominator:
>>> + *       - frame_rate_numer - uint32_t, numerator of the frame rate
>>> + *       - frame_rate_denom - uint32_t, denominator of the frame rate
>>> + *     The corresponding frame rate (Hz) is calculated as:
>>> + *       frame_rate = frame_rate_numer / frame_rate_denom
>>> + *   - buffer index is a zero based index of the buffer. Must be less than
>>> + *     the value of XENCAMERA_OP_CONFIG_SET.num_bufs response:
>>> + *       - index - uint8_t, index of the buffer.
>>> + *
>>> + *
>>> + *---------------------------------- Requests 
>>> ---------------------------------
>>> + *
>>> + * All request packets have the same length (64 octets).
>>> + * All request packets have common header:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |    operation   |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + *   id - uint16_t, private guest value, echoed in response.
>>> + *   operation - uint8_t, operation code, XENCAMERA_OP_XXX.
>>> + *
>>> + *
>>> + * Request to set configuration - request to set the configuration/mode
>>> + * of the camera:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_CONFIG_SET |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                            pixel format                           | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               width                               | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               height                              | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             colorspace                            | 24
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             xfer_func                             | 28
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             ycbcr_enc                             | 32
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                            quantization                           | 36
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                       displ_asp_ratio_numer                       | 40
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                       displ_asp_ratio_denom                       | 44
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          frame_rate_numer                         | 48
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          frame_rate_denom                         | 52
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 56
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * pixel_format - uint32_t, pixel format to be used, FOURCC code.
>>> + * width - uint32_t, width in pixels.
>>> + * height - uint32_t, height in pixels.
>>> + * colorspace - uint32_t, this supplements pixel_format parameter,
>>> + *   one of the XENCAMERA_COLORSPACE_XXX.
>>> + * xfer_func - uint32_t, this supplements colorspace parameter,
>>> + *   one of the XENCAMERA_XFER_FUNC_XXX.
>>> + * ycbcr_enc - uint32_t, this supplements colorspace parameter,
>>> + *   one of the XENCAMERA_YCBCR_ENC_XXX.
>> You do need to mention that ycbcr_enc is only valid for YCbCr pixelformats 
>> and
>> that it should be ignore otherwise. Or perhaps add a define just for that:
>>
>> #define XENCAMERA_YCBCR_ENC_IGNORE 0
> I will both define and put a note:
>   * ycbcr_enc - uint32_t, this supplements colorspace parameter,
>   *   one of the XENCAMERA_YCBCR_ENC_XXX. Please note, that ycbcr_enc is 
> only
>   *   valid for YCbCr pixelformats and should be ignored otherwise.
> 
>>> + * quantization - uint32_t, this supplements colorspace parameter,
>>> + *   one of the XENCAMERA_QUANTIZATION_XXX.
>>> + * displ_asp_ratio_numer - uint32_t, numerator of the display aspect ratio.
>>> + * displ_asp_ratio_denom - uint32_t, denominator of the display aspect 
>>> ratio.
>>> + * frame_rate_numer - uint32_t, numerator of the frame rate.
>>> + * frame_rate_denom - uint32_t, denominator of the frame rate.
>>> + *
>>> + * See response format for this request.
>>> + *
>>> + * Notes:
>>> + *  - frontend must check the corresponding response in order to see
>>> + *    if the values reported back by the backend do match the desired ones
>>> + *    and can be accepted.
>>> + *  - frontend may send multiple XENCAMERA_OP_CONFIG_SET requests before
>>> + *    sending XENCAMERA_OP_STREAM_START request to update or tune the
>>> + *    configuration.
>>> + */
>>> +struct xencamera_config {
>>> +    uint32_t pixel_format;
>>> +    uint32_t width;
>>> +    uint32_t height;
>>> +    uint32_t colorspace;
>>> +    uint32_t xfer_func;
>>> +    uint32_t ycbcr_enc;
>>> +    uint32_t quantization;
>>> +    uint32_t displ_asp_ratio_numer;
>>> +    uint32_t displ_asp_ratio_denom;
>>> +    uint32_t frame_rate_numer;
>>> +    uint32_t frame_rate_denom;
>>> +};
>>> +
>>> +/*
>>> + * Request current configuration of the camera:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_CONFIG_GET |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * See response format for this request.
>>> + *
>>> + *
>>> + * Request number of buffers to be used:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_BUF_REQUEST|   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |    num_bufs    |                     reserved                     | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * num_bufs - uint8_t, desired number of buffers to be used.
>>> + *
>>> + * See response format for this request.
>>> + *
>>> + * Notes:
>>> + *  - frontend must check the corresponding response in order to see
>>> + *    if the values reported back by the backend do match the desired ones
>>> + *    and can be accepted.
>>> + *  - frontend may send multiple XENCAMERA_OP_BUF_REQUEST requests before
>>> + *    sending XENCAMERA_OP_STREAM_START request to update or tune the
>>> + *    configuration.
>>> + */
>>> +struct xencamera_buf_request {
>>> +    uint8_t num_bufs;
>>> +};
>> What happens if num_bufs is 0?
> As this request's response has buffer layout, I was planning
> to use num_bufs == 0 as a way to request that layout w/o
> allocating the buffers:
>   *  - passing zero num_bufs in this request is not an error and is used
>   *    to query buffer's format
> 
>>
>> So BUF_REQUEST maps to VIDIOC_REQBUFS and once called it fixes the format
>> in place, i.e. after this you can no longer change the format.
> I'll put a note on this, thank you:
>   *  - after this request camera configuration cannot be changed, unless
>   *    streaming is stopped and buffers destroyed
> 
>>   Changing the
>> format usually means changing the buffer size, but once they are allocated
>> with REQBUFS that is no longer possible.
>>
>> If you want to change the format, then the first thing you have to do is
>> stop streaming and releasing all buffers. This last step is done with
>> VIDIOC_REQBUFS with a count of 0.
> This will be the frontend's responsibility to do so:
> 1. stop streaming
> 2. destroy buffers
> 
> In case of a V4L2 frontend this will be while serving
> "VIDIOC_REQBUFS with a count of 0".

So what if the frontend calls XENCAMERA_OP_BUF_REQUEST and never
XENCAMERA_OP_BUF_CREATE? Now it can't change the format and since it
never created buffers it can't destroy them either to trigger the
VIDIOC_REQBUFS(0) call.

BTW, once one frontend starts streaming, then other frontends can only
stream provided that they use same configuration (format). I doubt you
want to implement conversions betwee pixelformats or sizes.

So how is that handled?

>>
>> Whether or not you want to do the same thing (i.e. if num_bufs == 0, then
>> free all buffers) is up to you, but you do need a similar mechanism in your
>> API. And you need to document that once buffers are allocated, you can't
>> change the configuration anymore.
> Please see the note above
>>
>>> +
>>> +/*
>>> + * Request camera buffer creation:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_BUF_CREATE |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |      index     |                     reserved                     | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                           gref_directory                          | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * An attempt to create multiple buffers with the same index is an error.
>>> + * index can be re-used after destroying the corresponding camera buffer.
>>> + *
>>> + * index - uint8_t, index of the buffer to be created.
>>> + * gref_directory - grant_ref_t, a reference to the first shared page
>>> + *   describing shared buffer references. The size of the buffer is equal 
>>> to
>>> + *   XENCAMERA_OP_GET_BUF_DETAILS.size response. At least one page exists. 
>>> If
>>> + *   shared buffer size exceeds what can be addressed by this single page,
>>> + *   then reference to the next shared page must be supplied (see
>>> + *   gref_dir_next_page below).
>>> + *
>>> + * If XENCAMERA_FIELD_BE_ALLOC configuration entry is set, then backend 
>>> will
>>> + * allocate the buffer with the parameters provided in this request and 
>>> page
>>> + * directory is handled as follows:
>>> + *   Frontend on request:
>>> + *     - allocates pages for the directory (gref_directory,
>>> + *       gref_dir_next_page(s)
>>> + *     - grants permissions for the pages of the directory to the backend
>>> + *     - sets gref_dir_next_page fields
>>> + *   Backend on response:
>>> + *     - grants permissions for the pages of the buffer allocated to
>>> + *       the frontend
>>> + *     - fills in page directory with grant references
>>> + *       (gref[] in struct xencamera_page_directory)
>>> + */
>>> +struct xencamera_buf_create_req {
>>> +    uint8_t index;
>>> +    uint8_t reserved[3];
>>> +    grant_ref_t gref_directory;
>>> +};
>>> +
>>> +/*
>>> + * Shared page for XENCAMERA_OP_BUF_CREATE buffer descriptor 
>>> (gref_directory in
>>> + * the request) employs a list of pages, describing all pages of the shared
>>> + * data buffer:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                        gref_dir_next_page                         | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                              gref[0]                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                              gref[i]                              | 
>>> i*4+8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             gref[N - 1]                           | 
>>> N*4+8
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * gref_dir_next_page - grant_ref_t, reference to the next page describing
>>> + *   page directory. Must be 0 if there are no more pages in the list.
>>> + * gref[i] - grant_ref_t, reference to a shared page of the buffer
>>> + *   allocated at XENCAMERA_OP_BUF_CREATE.
>>> + *
>>> + * Number of grant_ref_t entries in the whole page directory is not
>>> + * passed, but instead can be calculated as:
>>> + *   num_grefs_total = (XENCAMERA_OP_BUF_REQUEST.size + XEN_PAGE_SIZE - 1) 
>>> /
>>> + *       XEN_PAGE_SIZE
>>> + */
>>> +struct xencamera_page_directory {
>>> +    grant_ref_t gref_dir_next_page;
>>> +    grant_ref_t gref[1]; /* Variable length */
>>> +};
>>> +
>>> +/*
>>> + * Request buffer destruction - destroy a previously allocated camera 
>>> buffer:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_BUF_DESTROY|   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |      index     |                     reserved                     | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * index - uint8_t, index of the buffer to be destroyed.
>>> + *
>>> + *
>>> + * Request queueing of the buffer for backend use:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_BUF_QUEUE  |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |      index     |                     reserved                     | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * Notes:
>>> + *  - frontends must not access the buffer content after this request until
>>> + *    response to XENCAMERA_OP_BUF_DEQUEUE has been received.
>>> + *  - buffers must be queued to the backend before destroying them with
>>> + *    XENCAMERA_OP_BUF_DESTROY.
>>> + *
>>> + * index - uint8_t, index of the buffer to be queued.
>>> + *
>>> + *
>>> + * Request dequeueing of the buffer for frontend use:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |_OP_BUF_DEQUEUE |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |      index     |                     reserved                     | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * Notes:
>>> + *  - frontend is allowed to access the buffer content after the 
>>> corresponding
>>> + *    response to this request.
>>> + *
>>> + * index - uint8_t, index of the buffer to be queued.
>>> + *
>>> + *
>>> + * Request camera control details:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_CTRL_ENUM  |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |      index     |                     reserved                     | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * See response format for this request.
>>> + *
>>> + * index - uint8_t, index of the control to be queried.
>>> + */
>>> +struct xencamera_index {
>>> +    uint8_t index;
>>> +};
>>> +
>>> +/*
>>> + * Request camera control change:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |  _OP_SET_CTRL  |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |       type     |                     reserved                     | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          value low 32-bit                         | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          value high 32-bit                        | 24
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 28
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * type - uint8_t, type of the control, one of the XENCAMERA_CTRL_XXX.
>>> + * value - int64_t, new value of the control.
>>> + */
>>> +struct xencamera_ctrl_value {
>>> +    uint8_t type;
>>> +    uint8_t reserved[7];
>>> +    int64_t value;
>>> +};
>>> +
>>> +/*
>>> + * Request camera control state:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |  _OP_GET_CTRL  |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |       type     |                     reserved                     | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * See response format for this request.
>>> + *
>>> + * type - uint8_t, type of the control, one of the XENCAMERA_CTRL_XXX.
>>> + */
>>> +struct xencamera_get_ctrl_req {
>>> +    uint8_t type;
>>> +};
>>> +
>>> +/*
>>> + * Request camera capture stream start:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |_OP_STREAM_START|   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + *
>>> + * Request camera capture stream stop:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |_OP_STREAM_STOP |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + *
>>> + *---------------------------------- Responses 
>>> --------------------------------
>>> + *
>>> + * All response packets have the same length (64 octets).
>>> + *
>>> + * All response packets have common header:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |    operation   |    reserved    | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                              status                               | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * id - uint16_t, copied from the request.
>>> + * operation - uint8_t, XENCAMERA_OP_* - copied from request.
>>> + * status - int32_t, response status, zero on success and -XEN_EXX on 
>>> failure.
>>> + *
>>> + *
>>> + * Set/get configuration response - response for XENCAMERA_OP_CONFIG_SET
>>> + * and XENCAMERA_OP_CONFIG_GET
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_CONFIG_?ET |    reserved    | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               status                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                            pixel format                           | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               width                               | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               height                              | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             colorspace                            | 24
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             xfer_func                             | 28
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             ycbcr_enc                             | 32
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                            quantization                           | 36
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                       displ_asp_ratio_numer                       | 40
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                       displ_asp_ratio_denom                       | 44
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          frame_rate_numer                         | 48
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          frame_rate_denom                         | 52
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 56
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * Meaning of the corresponding values in this response is the same as for
>>> + * XENCAMERA_OP_CONFIG_SET request.
>>> + *
>>> + *
>>> + * Request buffer response - response for XENCAMERA_OP_BUF_REQUEST
>>> + * request:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |_OP_BUF_REQUEST |    reserved    | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               status                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |   num_buffers  |   num_planes   |            reserved             | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                                size                               | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          plane_offset[0]                          | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          plane_offset[1]                          | 24
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          plane_offset[2]                          | 28
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          plane_offset[3]                          | 32
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                           plane_size[0]                           | 36
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                           plane_size[1]                           | 40
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                           plane_size[2]                           | 44
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                           plane_size[3]                           | 48
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |         plane_stride[0]         |         plane_stride[1]         | 52
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |         plane_stride[2]         |         plane_stride[3]         | 56
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 60
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * num_buffers - uint8_t, number of buffers to be used.
>>> + * num_planes - uint8_t, number of planes of the buffer.
>>> + * size - uint32_t, overall size of the buffer including sizes of the
>>> + *   individual planes and padding if applicable.
>>> + * plane_offset - array of uint32_t, offset of the corresponding plane
>>> + *   in octets from the buffer start.
>>> + * plane_size - array of uint32_t, size in octets of the corresponding 
>>> plane
>>> + *   including padding.
>>> + * plane_stride - array of uint32_t, size in octets occupied by the
>> uint32_t -> uint16_t, but see below
>>
>>> + *   corresponding single image line including padding if applicable.
>>> + */
>>> +struct xencamera_buf_request_resp {
>>> +    uint8_t num_buffers;
>>> +    uint8_t num_planes;
>>> +    uint8_t reserved[2];
>>> +    uint32_t size;
>>> +    uint32_t plane_offset[XENCAMERA_MAX_PLANE];
>>> +    uint32_t plane_size[XENCAMERA_MAX_PLANE];
>>> +    uint16_t plane_stride[XENCAMERA_MAX_PLANE];
>> I recommend using uint32_t for the stride. It's more future proof given the
>> ever higher resolutions and colordepths we're seeing.
> Will change to uint32_t
>>> +};
>>> +
>>> +/*
>>> + * Control enumerate response - response for XENCAMERA_OP_CTRL_ENUM:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_CTRL_ENUM  |    reserved    | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               status                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |     index      |      type      |              flags              | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          min low 32-bits                          | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          min high 32-bits                         | 24
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          max low 32-bits                          | 28
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          max high 32-bits                         | 32
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                         step low 32-bits                          | 36
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                         step high 32-bits                         | 40
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                        def_val low 32-bits                        | 44
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                        def_val high 32-bits                       | 48
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 52
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * index - uint8_t, index of the camera control in response.
>>> + * type - uint8_t, type of the control, one of the XENCAMERA_CTRL_XXX.
>>> + * flags - uint16_t, flags of the control, one of the 
>>> XENCAMERA_CTRL_FLG_XXX.
>> Go with a uint32_t for the flags. V4L2 is already using 11 flags...
> ok, will change to uint32_t
>>> + * min - int64_t, minimum value of the control.
>>> + * max - int64_t, maximum value of the control.
>>> + * step - int64_t, minimum size in which control value can be changed.
>>> + * def_val - int64_t, default value of the control.
>>> + */
>>> +struct xencamera_ctrl_enum_resp {
>>> +    uint8_t index;
>>> +    uint8_t type;
>>> +    uint16_t flags;
>>> +    uint8_t reserved[4];
>>> +    int64_t min;
>>> +    int64_t max;
>>> +    int64_t step;
>>> +    int64_t def_val;
>>> +};
>>> +
>>> +/*
>>> + * Get control response - response for XENCAMERA_OP_CTRL_GET:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _OP_CTRL_GET   |    reserved    | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               status                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |       type     |                     reserved                     | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          value low 32-bit                         | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          value high 32-bit                        | 24
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 28
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * type - uint8_t, type of the control, one of the XENCAMERA_CTRL_XXX.
>>> + * value - int64_t, new value of the control.
>>> + */
>>> +
>>> +/*
>>> + *----------------------------------- Events 
>>> ----------------------------------
>>> + *
>>> + * Events are sent via a shared page allocated by the front and propagated 
>>> by
>>> + *   evt-event-channel/evt-ring-ref XenStore entries.
>>> + *
>>> + * All event packets have the same length (64 octets).
>>> + * All event packets have common header:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |      type      |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * id - uint16_t, event id, may be used by front.
>>> + * type - uint8_t, type of the event.
>>> + *
>>> + *
>>> + * Frame captured event - event from back to front when a new captured
>>> + * frame is available:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |_EVT_FRAME_AVAIL|   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |      index     |                     reserved                     | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                              used_sz                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                        seq_num low 32-bits                        | 24
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                        seq_num high 32-bits                       | 28
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * index - uint8_t, index of the buffer that contains new captured frame.
>>> + * used_sz - uint32_t, number of octets this frame has. This can be less
>>> + * than the XENCAMERA_OP_BUF_REQUEST.size (response) for compressed 
>>> formats.
>>> + * seq_num - uint64_t, sequential number of the frame.
>> Must be monotonically increasing, and if there are skips in the number, then
>> it means that frames were skipped. Not all drivers can detect this, so
>> no skips does not necessarily mean there were no skips.
> I'll put a note on this:
>   * seq_num - uint64_t, sequential number of the frame. Must be
>   *   monotonically increasing. If skips are detected in seq_num then that
>   *   means that the frames in-between were dropped.
> 
>>> + */
>>> +struct xencamera_frame_avail_evt {
>>> +    uint8_t index;
>>> +    uint8_t reserved0[3];
>>> +    uint32_t used_sz;
>>> +    uint8_t reserved1[4];
>>> +    uint64_t seq_num;
>>> +};
>>> +
>>> +/*
>>> + * Configuration change - event from back to front when current
>>> + * configuration has changed:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                | _CONFIG_CHANGE |   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                               flags                               | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * flags - uint32_t, set of the XENCAMERA_EVT_CFG_FLG_XXX flags.
>>> + */
>>> +struct xencamera_cfg_change_evt {
>>> +    uint32_t flags;
>>> +};
>> This needs some more work: what should the frontend do when this is received?
>>
>> I would expect it should stop streaming, free all buffers, reread the config
>> and start again.
> That effectively means almost the same as going into
> Closed state and then going back into Connected state
> which can be controlled by the backend with XenBus state machine.
> So, this particular configuration change event can be omitted
> at all. Does this sound reasonable?

Yes.

>>
>> But what if there is no new config because the source was disconnected?
> This is the backend's responsibility to control that,
> e.g. if the source has disconnected then the backend must
> hold virtual camera device via the XenBus state machine in
> closed or any non-operatable state until that source comes back.

I think a lot of this can be resolved by chosing appropriate states
for when something gets connected or disconnected, or is reconfigured
while connected.

This is all a bit of brainstorming, this is all a bit different from what
I'm used to.

>>
>>> +
>>> +/*
>>> + * Control change event- event from back to front when camera control
>>> + * has changed:
>>> + *         0                1                 2               3        
>>> octet
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |               id                |_EVT_CTRL_CHANGE|   reserved     | 4
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |       type     |                     reserved                     | 8
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 12
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 16
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          value low 32-bit                         | 20
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                          value high 32-bit                        | 24
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 28
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
>>> + * +----------------+----------------+----------------+----------------+
>>> + * |                             reserved                              | 64
>>> + * +----------------+----------------+----------------+----------------+
>>> + *
>>> + * type - uint8_t, type of the control, one of the XENCAMERA_CTRL_XXX.
>>> + * value - int64_t, new value of the control.
>>> + */
>> So will this happen for all controls (except write-only)? What about volatile
>> controls? What if the frontend sets a control, does it also get this event?
>> Does it get this event with the current value of a control when it first 
>> connects?
>>
>> What happens if there are multiple quick changes to the same control? Can 
>> the event
>> queue overflow?
>>
>> Sorry, these are all questions we had to answer when we added control event
>> support. We spend a lot of time making the event handling reliable without 
>> losing
>> information (intermediate values can be lost, but never the current value).
>>
>> Since this is relying on a Xen event mechanism you will have to think about 
>> this
>> as well.
> I'll put these notes:
>   * Notes:
>   *  - this event is not sent for write-only controls
>   *  - this event is also sent to the originator of the control change

I actually recommend against this. We don't do this, an application has
to explicitly request this.

The problem is that you don't know if the event you receive is from your
own change, or if an external actor changed it.

So if a frontend receives this event, then it is because another frontend
or the backend changed the control, and not because you did it yourself.

>   *  - this event is not sent when frontend first connects, e.g. initial
>   *    control state must be explicitly queried
> 
>>> +
>>> +
>>> +struct xencamera_req {
>>> +    uint16_t id;
>>> +    uint8_t operation;
>>> +    uint8_t reserved[5];
>>> +    union {
>>> +        struct xencamera_config config;
>>> +        struct xencamera_buf_request buf_request;
>>> +        struct xencamera_buf_create_req buf_create;
>>> +        struct xencamera_index index;
>>> +        struct xencamera_ctrl_value ctrl_value;
>>> +        struct xencamera_get_ctrl_req get_ctrl;
>>> +        uint8_t reserved[56];
>>> +    } req;
>>> +};
>>> +
>>> +struct xencamera_resp {
>>> +    uint16_t id;
>>> +    uint8_t operation;
>>> +    uint8_t reserved;
>>> +    int32_t status;
>>> +    union {
>>> +        struct xencamera_config config;
>>> +        struct xencamera_buf_request_resp buf_request;
>>> +        struct xencamera_ctrl_enum_resp ctrl_enum;
>>> +        struct xencamera_ctrl_value ctrl_value;
>>> +        uint8_t reserved1[56];
>>> +    } resp;
>>> +};
>>> +
>>> +struct xencamera_evt {
>>> +    uint16_t id;
>>> +    uint8_t type;
>>> +    uint8_t reserved[5];
>>> +    union {
>>> +        struct xencamera_frame_avail_evt frame_avail;
>>> +        struct xencamera_cfg_change_evt cfg_change;
>>> +        struct xencamera_ctrl_value ctrl_value;
>>> +        uint8_t reserved[56];
>>> +    } evt;
>>> +};
>>> +
>>> +DEFINE_RING_TYPES(xen_cameraif, struct xencamera_req, struct 
>>> xencamera_resp);
>>> +
>>> +/*
>>> + 
>>> ******************************************************************************
>>> + *                        Back to front events delivery
>>> + 
>>> ******************************************************************************
>>> + * In order to deliver asynchronous events from back to front a shared 
>>> page is
>>> + * allocated by front and its granted reference propagated to back via
>>> + * XenStore entries (evt-ring-ref/evt-event-channel).
>>> + * This page has a common header used by both front and back to synchronize
>>> + * access and control event's ring buffer, while back being a producer of 
>>> the
>>> + * events and front being a consumer. The rest of the page after the header
>>> + * is used for event packets.
>>> + *
>>> + * Upon reception of an event(s) front may confirm its reception
>>> + * for either each event, group of events or none.
>>> + */
>>> +
>>> +struct xencamera_event_page {
>>> +    uint32_t in_cons;
>>> +    uint32_t in_prod;
>>> +    uint8_t reserved[56];
>>> +};
>>> +
>>> +#define XENCAMERA_EVENT_PAGE_SIZE 4096
>>> +#define XENCAMERA_IN_RING_OFFS (sizeof(struct xencamera_event_page))
>>> +#define XENCAMERA_IN_RING_SIZE (XENCAMERA_EVENT_PAGE_SIZE - 
>>> XENCAMERA_IN_RING_OFFS)
>>> +#define XENCAMERA_IN_RING_LEN (XENCAMERA_IN_RING_SIZE / sizeof(struct 
>>> xencamera_evt))
>>> +#define XENCAMERA_IN_RING(page) \
>>> +    ((struct xencamera_evt *)((char *)(page) + XENCAMERA_IN_RING_OFFS))
>>> +#define XENCAMERA_IN_RING_REF(page, idx) \
>>> +    (XENCAMERA_IN_RING((page))[(idx) % XENCAMERA_IN_RING_LEN])
>>> +
>>> +#endif /* __XEN_PUBLIC_IO_CAMERAIF_H__ */
>>> +
>>> +/*
>>> + * Local variables:
>>> + * mode: C
>>> + * c-file-style: "BSD"
>>> + * c-basic-offset: 4
>>> + * tab-width: 4
>>> + * indent-tabs-mode: nil
>>> + * End:
>>> + */
>>>
>> Regards,
>>
>>      Hans
> Thank you,
> Oleksandr
> 
> [1] https://www.fourcc.org/
> [2] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/videodev2.h
> 

Regards,

        Hans

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