Re: [PATCH v3 1/2] media: docs-rst: Document memory-to-memory video decoder interface

[Hans Verkuil <hverkuil@xs4all.nl>]

On 4/9/19 6:57 PM, Philipp Zabel wrote:
> On Fri, 2019-04-05 at 13:21 +0200, Hans Verkuil wrote:
>> On 4/5/19 12:59 PM, Philipp Zabel wrote:
>>> On Thu, 2019-01-24 at 19:04 +0900, Tomasz Figa wrote:
>>>> Due to complexity of the video decoding process, the V4L2 drivers of
>>>> stateful decoder hardware require specific sequences of V4L2 API calls
>>>> to be followed. These include capability enumeration, initialization,
>>>> decoding, seek, pause, dynamic resolution change, drain and end of
>>>> stream.
>>>>
>>>> Specifics of the above have been discussed during Media Workshops at
>>>> LinuxCon Europe 2012 in Barcelona and then later Embedded Linux
>>>> Conference Europe 2014 in Düsseldorf. The de facto Codec API that
>>>> originated at those events was later implemented by the drivers we already
>>>> have merged in mainline, such as s5p-mfc or coda.
>>>>
>>>> The only thing missing was the real specification included as a part of
>>>> Linux Media documentation. Fix it now and document the decoder part of
>>>> the Codec API.
>>>>
>>>> Signed-off-by: Tomasz Figa <tfiga@chromium.org>
>>>> ---
>>>>  Documentation/media/uapi/v4l/dev-decoder.rst  | 1076 +++++++++++++++++
>>>>  Documentation/media/uapi/v4l/dev-mem2mem.rst  |    5 +
>>>>  Documentation/media/uapi/v4l/pixfmt-v4l2.rst  |    5 +
>>>>  Documentation/media/uapi/v4l/v4l2.rst         |   10 +-
>>>>  .../media/uapi/v4l/vidioc-decoder-cmd.rst     |   40 +-
>>>>  Documentation/media/uapi/v4l/vidioc-g-fmt.rst |   14 +
>>>>  6 files changed, 1135 insertions(+), 15 deletions(-)
>>>>  create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst
>>>>
>>>> diff --git a/Documentation/media/uapi/v4l/dev-decoder.rst b/Documentation/media/uapi/v4l/dev-decoder.rst
>>>> new file mode 100644
>>>> index 000000000000..b7db2352ad41
>>>> --- /dev/null
>>>> +++ b/Documentation/media/uapi/v4l/dev-decoder.rst
>>>> @@ -0,0 +1,1076 @@
>>>> +.. -*- coding: utf-8; mode: rst -*-
>>>> +
>>>> +.. _decoder:
>>>> +
>>>> +*************************************************
>>>> +Memory-to-memory Stateful Video Decoder Interface
>>>> +*************************************************
>>>> +
>>>
>>> [...]
>>>> +
>>>> +Drain
>>>> +=====
>>>> +
>>>
>>> [...]
>>>> +
>>>> +3. Once all the ``OUTPUT`` buffers queued before the ``V4L2_DEC_CMD_STOP`` call
>>>> +   are dequeued and the last ``CAPTURE`` buffer is dequeued, the decoder is
>>>> +   stopped and it will accept, but not process, any newly queued ``OUTPUT``
>>>> +   buffers until the client issues any of the following operations:
>>>> +
>>>
>>> [...]
>>>> +
>>>> +.. note::
>>>> +
>>>> +   Once the drain sequence is initiated, the client needs to drive it to
>>>> +   completion, as described by the steps above, unless it aborts the process by
>>>> +   issuing :c:func:`VIDIOC_STREAMOFF` on any of the ``OUTPUT`` or ``CAPTURE``
>>>> +   queues.  The client is not allowed to issue ``V4L2_DEC_CMD_START`` or
>>>> +   ``V4L2_DEC_CMD_STOP`` again while the drain sequence is in progress and they
>>>> +   will fail with -EBUSY error code if attempted.
>>>
>>> Should the V4L2_DEC_CMD_STOP documentation in
>>> Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst:
>>>
>>>   * - ``V4L2_DEC_CMD_STOP``
>>>     - 1
>>>     - Stop the decoder. When the decoder is already stopped, this
>>>       command does nothing.
>>>
>>> be updated to mention that for mem2mem decoders this not only does
>>> nothing, but returns an -EBUSY error return value?
>>
>> Yes. Basically in this case the STOP is still in progress, so returning
>> EBUSY makes sense and should be documented.
>>
>>>
>>> Also, if V4L2_DEC_CMD_STOP_IMMEDIATELY is set, should the decoder drop
>>> all pending buffers and signal EOS immediately? Should this be mentioned
>>> here?
>>
>> Hmm, the decoder spec says that flags should be set to 0, and current codec
>> drivers explicitly disallow a non-0 flags value.
>>
>> I'm not sure if there is a practical use-case for V4L2_DEC_CMD_STOP_IMMEDIATELY
>> in the context of a decoder. If you want to stop immediately, then just call
>> streamoff on the OUTPUT queue.
> 
> v4l2-compliance currently complains if V4L2_DEC_CMD_STOP_IMMEDIATELY is
> not supported:
> 
>                 fail: v4l2-test-codecs.cpp(104): ret != 0
>         test VIDIOC_(TRY_)DECODER_CMD: FAIL
> 
> Should testDecoder be changed to set the V4L2_DEC_CMD_STOP_IMMEDIATELY
> flag only if node->codec_mask does not have the STATEFUL_DECODER bit
> set?

Yes. I see that the vicodec driver doesn't handle the DECODER_CMD ioctl
correctly, which is why I haven't noticed this v4l2-compliance issue before.

Regards,

	Hans

> 
> regards
> Philipp
>