Re: [PATCH v1] media: videobuf2: Allow applications customize data offsets of capture buffers

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

Hi Nicolas,

On 3/23/22 16:05, Nicolas Dufresne wrote:
> Hi Dmitry,
> 
> thanks for giving a second look a this issue.
> 
> Le mardi 22 mars 2022 à 16:23 +0300, Dmitry Osipenko a écrit :
>> Use data offsets provided by applications for multi-planar capture
>> buffers. This allows V4L to import and use dma-bufs exported by other
>> subsystems in cases where application wants to customize data offsets
>> of capture buffers in order to meet hardware alignment requirements of
>> both dma-buf exporter and importer.
>>
>> This feature is wanted for providing a better support of media hardware
>> found on Chromebooks. In particular display and camera ISP hardware of
>> Rockchip and MediaTek SoCs require special handling by userspace because
>> display h/w has specific alignment requirements that don't match default
>> alignments expected by V4L and there is a need to customize the data
>> offsets in case of multi-planar formats.
>>
>> Some drivers already have preliminary support for data offsets
>> customization of capture buffers, like NVIDIA Tegra video decoder driver
>> for example, and V4L allows applications to provide data offsets for
>> multi-planar output buffers, let's support such customization for the
>> capture buffers as well.
>>
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
>> ---
>>  Documentation/userspace-api/media/v4l/buffer.rst | 9 ++++++++-
>>  drivers/media/common/videobuf2/videobuf2-v4l2.c  | 7 +++++++
>>  2 files changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
>> index 4638ec64db00..75b1929e2acb 100644
>> --- a/Documentation/userspace-api/media/v4l/buffer.rst
>> +++ b/Documentation/userspace-api/media/v4l/buffer.rst
>> @@ -369,13 +369,20 @@ struct v4l2_plane
>>        - ``data_offset``
>>        - Offset in bytes to video data in the plane. Drivers must set this
>>  	field when ``type`` refers to a capture stream, applications when
>> -	it refers to an output stream.
>> +	it refers to an output or capture stream.
> 
> There is a clear contradiction in this paragraph. Both the driver and the
> application MUST set the data_offset.

I'm not sure where the contradiction is. Application must initialize the
data_offset and driver must set data_offset too, if it's appropriate to
do that for a particular driver.

> Would it be possible to demo your idea by implementing this in a virtual driver
> ? vivid already have data_offset for capture in some cases, you could verify if
> your idea works without any conflict in this scenario.

I actually considered implementing it in the vivid driver, but vivid
driver already sets the data_offset to fixed values [1], so I decided
that not to change it.

But maybe we actually could extend the vivid driver by accepting
data_offset from userspace for the cases where the fixed offset value is
zero in the driver.. not sure.

[1]
https://elixir.bootlin.com/linux/latest/source/drivers/media/test-drivers/vivid/vivid-vid-cap.c#L172

I verified my idea using the NVIDIA Tegra video decoder driver, which
already takes data_offsets for capture planes [3] and libvdpau-tegra
imports DRM dma-bufs into the V4L driver [4][5].

[3]
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/media/platform/nvidia/tegra-vde/v4l2.c#n236
[4]
https://github.com/grate-driver/libvdpau-tegra/blob/master/src/decoder.c#L685
[5]
https://github.com/grate-driver/libvdpau-tegra/blob/master/src/v4l2.c#L359

The plan is to extend RK ISP driver with support of data_offsets next,
once we'll agree that this acceptable approach and we don't actually
need go back to implementing the new VIDIOC_EXT_QBUF ioctl [6].

[6]
https://patchwork.linuxtv.org/project/linux-media/cover/20210114180738.1758707-1-helen.koike@collabora.com/

This patch solves the problem for userspace when it wants to import
buffers into V4L in case of multi-planar formats, but it doesn't cover
all other possible cases that may require offsets customization too. On
the other hand, it's easy to start accepting data_offset for the capture
MPLANES without introducing new UAPIs, so I decided that will be best to
start easy with the simplest solution.