Re: [RFC PATCH v2] media: docs-rst: Document m2m stateless video decoder interface

[Hans Verkuil <hverkuil@xs4all.nl>]

On 10/04/2018 10:11 AM, Alexandre Courbot wrote:
> This patch documents the protocol that user-space should follow when
> communicating with stateless video decoders. It is based on the
> following references:
> 
> * The current protocol used by Chromium (converted from config store to
>   request API)
> 
> * The submitted Cedrus VPU driver
> 
> As such, some things may not be entirely consistent with the current
> state of drivers, so it would be great if all stakeholders could point
> out these inconsistencies. :)
> 
> This patch is supposed to be applied on top of the Request API V18 as
> well as the memory-to-memory video decoder interface series by Tomasz
> Figa.
> 
> Changes since V1:
> 
> * Applied fixes received as feedback,
> * Moved controls descriptions to the extended controls file,
> * Document reference frame management and referencing (need Hans' feedback on
>   that).
> 
> Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
> ---
>  .../media/uapi/v4l/dev-stateless-decoder.rst  | 348 ++++++++++++++++++
>  Documentation/media/uapi/v4l/devices.rst      |   1 +
>  .../media/uapi/v4l/extended-controls.rst      |  25 ++
>  .../media/uapi/v4l/pixfmt-compressed.rst      |  54 ++-
>  4 files changed, 424 insertions(+), 4 deletions(-)
>  create mode 100644 Documentation/media/uapi/v4l/dev-stateless-decoder.rst
> 
> diff --git a/Documentation/media/uapi/v4l/dev-stateless-decoder.rst b/Documentation/media/uapi/v4l/dev-stateless-decoder.rst

<snip>

> +Buffer management during decoding
> +=================================
> +Contrary to stateful decoder drivers, a stateless decoder driver does not
> +perform any kind of buffer management. In particular, it guarantees that
> +``CAPTURE`` buffers will be dequeued in the same order as they are queued. This
> +allows user-space to know in advance which ``CAPTURE`` buffer will contain a
> +given frame, and thus to use that buffer ID as the key to indicate a reference
> +frame.
> +
> +This also means that user-space is fully responsible for not queuing a given
> +``CAPTURE`` buffer for as long as it is used as a reference frame. Failure to do
> +so will overwrite the reference frame's data while it is still in use, and
> +result in visual corruption of future frames.
> +
> +Note that this applies to all types of buffers, and not only to
> +``V4L2_MEMORY_MMAP`` ones, as drivers supporting ``V4L2_MEMORY_DMABUF`` will
> +typically maintain a map of buffer IDs to DMABUF handles for reference frame
> +management. Queueing a buffer will result in the map entry to be overwritten
> +with the new DMABUF handle submitted in the :c:func:`VIDIOC_QBUF` ioctl.

The more I think about this, the more I believe that relying on capture buffer
indices is wrong. It's easy enough if there is a straightforward 1-1 relationship,
but what if you have H264 slices as Nicolas mentioned and it becomes a N-1 relationship?

Yes, you can still do this in userspace, but it becomes a lot more complicated.

And what if in the future instead of having one capture buffer per decoded frame
there will be multiple capture buffers per decoded frame, each with a single
slice (for example)?

I would feel much happier if we used a 'cookie' to refer to buffers.

The next problem would be where to put it. I dislike abusing the timestamp field
for this. Part of the reason is that there will be changes there to fix the
year 2038 issue, and I am not entirely sure what that will do to how this field
is handled since there may be conversions from a pre-2038 timeval to a 2038-ready
timeval.

So a union with the timestamp field and a cookie field (+ BUF_FLAG_COOKIE) would
work best IMHO.

Regards,

	Hans