Re: [Xen-devel] [PATCH v7 3/8] arm64: introduce is_device_dma_coherent

[Grygorii Strashko <grygorii.strashko@xxxxxx>]

Hi Stefano,

On 11/03/2014 01:10 PM, Stefano Stabellini wrote:
> On Mon, 3 Nov 2014, Will Deacon wrote:
>> On Mon, Nov 03, 2014 at 10:46:03AM +0000, Stefano Stabellini wrote:
>>> On Mon, 27 Oct 2014, Stefano Stabellini wrote:
>>>> Introduce a boolean flag and an accessor function to check whether a
>>>> device is dma_coherent. Set the flag from set_arch_dma_coherent_ops.
>>>>
>>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>>>> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
>>>> CC: will.deacon@xxxxxxx
>>>
>>> Will, Catalin,
>>> are you OK with this patch?
>>
>> It would be nicer if the dma_coherent flag didn't have to be duplicated by
>> each architecture in dev_archdata. Is there any reason not to put it in the
>> core code?
> 
> Yes, there is a reason for it: if I added a boolean dma_coherent flag in
> struct device as Catalin initially suggested, what would be the default
> for each architecture? Where would I set it for arch that don't use
> device tree? It is not easy.
> 
> I thought it would be better to introduce is_device_dma_coherent only on
> the architectures where it certainly makes sense to have it. In fact I
> checked and arm and arm64 are the only architectures to define
> set_arch_dma_coherent_ops at the moment. At that point if
> is_device_dma_coherent becomes arch-specific, it makes sense to store
> the flag in dev_archdata instead of struct device.

The proposition from Will looks reasonable for me too, because
there is "small" side-effect of adding such kind of properties to
arch-specific data or even to the core device structure. ;(

There are some sub-systems in kernel which do not create their devices
from DT and instead some host device populates its children devices manually.
 Now, I know at least two cases:
- usb: dwc3 core creates xhci device manually
- pci: adds its client devices

In such, case DMA configuration have to be propagated from host to
child (in our case host device's got DMA configuration from DT), like:
        dma_set_coherent_mask(&xhci->dev, dwc->dev->coherent_dma_mask);

        xhci->dev.parent        = dwc->dev;
        xhci->dev.dma_mask      = dwc->dev->dma_mask;
        xhci->dev.dma_parms     = dwc->dev->dma_parms;

So, once new DMA property is added it has to be propagated from 
host to child device too.

Recently, the new property  dma_pfn_offset was introduced in struct device 
and such kind of problem was observed on keystone 2:
- for usb case it was fixed using Platform Bus notifier (xhci - platform device)
- for pci - the work is in progress, because solution with PCI Bus notifier
  was rejected https://lkml.org/lkml/2014/10/10/308.

In general, if dma_coherent will belong to struct device then
such problems will be possible to fix directly in drivers/subsystems:
xhci->dev.dma_coherent  = dwc->dev->dma_coherent;

But, if it will be arch-specific data then it will be impossible to
set it without introducing proper and arch-specific setters/getters functions.

Also, as an idea, we are thinking about introducing something like:
  void dma_apply_parent_cfg(struct device *dev, struct device *parent)
which will ensure that all DMA configuration properly copied from
parent to children device. Now it should be (as minimum for ARM):
 dma_mask
 coherent_dma_mask
 dma_parms
 dma_pfn_offset
 dev_archdata->dma_ops
 [dma_coherent]?

regards,
-grygorii 


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