Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 04/18/2018 01:55 PM, Roger Pau Monné wrote:
> On Wed, Apr 18, 2018 at 01:39:35PM +0300, Oleksandr Andrushchenko wrote:
> > On 04/18/2018 01:18 PM, Paul Durrant wrote:
> > > > -----Original Message-----
> > > > From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf
> > > > Of Roger Pau Monné
> > > > Sent: 18 April 2018 11:11
> > > > To: Oleksandr Andrushchenko <andr2000@xxxxxxxxx>
> > > > Cc: jgross@xxxxxxxx; Artem Mygaiev <Artem_Mygaiev@xxxxxxxx>;
> > > > Dongwon Kim <dongwon.kim@xxxxxxxxx>; airlied@xxxxxxxx;
> > > > Oleksandr_Andrushchenko@xxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; dri-
> > > > devel@xxxxxxxxxxxxxxxxxxxxx; Potrola, MateuszX
> > > > <mateuszx.potrola@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> > > > daniel.vetter@xxxxxxxxx; boris.ostrovsky@xxxxxxxxxx; Matt Roper
> > > > <matthew.d.roper@xxxxxxxxx>
> > > > Subject: Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy
> > > > helper DRM driver
> > > >
> > > > On Wed, Apr 18, 2018 at 11:01:12AM +0300, Oleksandr Andrushchenko
> > > > wrote:
> > > > > On 04/18/2018 10:35 AM, Roger Pau Monné wrote:
> > > > After speaking with Oleksandr on IRC, I think the main usage of the
> > > > gntdev extension is to:
> > > >
> > > > 1. Create a dma-buf from a set of grant references.
> > > > 2. Share dma-buf and get a list of grant references.
> > > >
> > > > I think this set of operations could be broken into:
> > > >
> > > > 1.1 Map grant references into user-space using the gntdev.
> > > > 1.2 Create a dma-buf out of a set of user-space virtual addresses.
> > > >
> > > > 2.1 Map a dma-buf into user-space.
> > > > 2.2 Get grefs out of the user-space addresses where the dma-buf is
> > > >       mapped.
> > > >
> > > > So it seems like what's actually missing is a way to:
> > > >
> > > >    - Create a dma-buf from a list of user-space virtual addresses.
> > > >    - Allow to map a dma-buf into user-space, so it can then be used with
> > > >      the gntdev.
> > > >
> > > > I think this is generic enough that it could be implemented by a
> > > > device not tied to Xen. AFAICT the hyper_dma guys also wanted
> > > > something similar to this.
> > Ok, so just to summarize, xen-zcopy/hyper-dmabuf as they are now,
> > are no go from your POV?
> My opinion is that there seems to be a more generic way to implement
> this, and thus I would prefer that one.
>
> > Instead, we have to make all that fancy stuff
> > with VAs <-> device-X and have that device-X driver live out of drivers/xen
> > as it is not a Xen specific driver?
> That would be my preference if feasible, simply because it can be
> reused by other use-cases that need to create dma-bufs in user-space.
There is a use-case I have: a display unit on my target has a DMA
controller which can't do scatter-gather, e.g. it only expects a
single starting address of the buffer.
In order to create a dma-buf from grefs in this case
I allocate memory with dma_alloc_xxx and then balloon pages of the
buffer and finally map grefs onto this DMA buffer.
This way I can give this shared buffer to the display unit as its bus
addresses are contiguous.

With the proposed solution (gntdev + device-X) I won't be able to 
achieve this,
as I have no control over from where gntdev/balloon drivers get the pages
(even more, those can easily be out of DMA address space of the display 
unit).
Thus, even if implemented, I can't use this approach.
> In any case I just knew about dma-bufs this morning, there might be
> things that I'm missing.
>
> Roger.

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