Re: [media] uvcvideo: support for contiguous DMA buffers

[Vincent ABRIOU <vincent.abriou@st.com>]

Hi Sakari,

On 01/11/2017 12:03 PM, Sakari Ailus wrote:
> Hi Vincent,
>
> On Mon, Jan 09, 2017 at 03:49:00PM +0000, Vincent ABRIOU wrote:
>>
>>
>> On 01/09/2017 04:37 PM, Laurent Pinchart wrote:
>>> Hi Vincent,
>>>
>>> Thank you for the patch.
>>>
>>> On Monday 03 Oct 2016 13:27:16 Vincent Abriou wrote:
>>>> Allow uvcvideo compatible devices to allocate their output buffers using
>>>> contiguous DMA buffers.
>>>
>>> Why do you need this ? If it's for buffer sharing with a device that requires
>>> dma-contig, can't you allocate the buffers on the other device and import them
>>> on the UVC side ?
>>>
>>
>> Hi Laurent,
>>
>> I need this using Gstreamer simple pipeline to connect an usb webcam
>> (v4l2src) with a display (waylandsink) activating the zero copy path.
>>
>> The waylandsink plugin does not have any contiguous memory pool to
>> allocate contiguous buffer. So it is up to the upstream element, here
>> v4l2src, to provide such contiguous buffers.
>
> Do you need (physically) contiguous memory?
>

Yes, drm driver that does not have mmu needs to have contiguous buffers.

> The DMA-BUF API does help sharing the buffers but it, at least currently,
> does not help allocating memory or specifying a common format so that all
> the devices the buffer needs to be accessible can actually make use of it.
>
> Instead of hacking drivers to make use of different memory allocation
> strategies required by unrelated devices, we should instead fix these
> problems in a proper, scalable way.
>

Scalable way? Like central allocator?

Vincent