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

[Hans Verkuil]

On 1/31/19 1:30 PM, Hans Verkuil wrote:
> On 1/31/19 11:45 AM, Hans Verkuil wrote:
>> On 1/24/19 11:04 AM, 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 DÃsseldorf. 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@xxxxxxxxxxxx>
>>> ---
>>>  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
>>>
>>
>> <snip>
>>
>>> +4.  **This step only applies to coded formats that contain resolution information
>>> +    in the stream.** Continue queuing/dequeuing bitstream buffers to/from the
>>> +    ``OUTPUT`` queue via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`. The
>>> +    buffers will be processed and returned to the client in order, until
>>> +    required metadata to configure the ``CAPTURE`` queue are found. This is
>>> +    indicated by the decoder sending a ``V4L2_EVENT_SOURCE_CHANGE`` event with
>>> +    ``V4L2_EVENT_SRC_CH_RESOLUTION`` source change type.
>>> +
>>> +    * It is not an error if the first buffer does not contain enough data for
>>> +      this to occur. Processing of the buffers will continue as long as more
>>> +      data is needed.
>>> +
>>> +    * If data in a buffer that triggers the event is required to decode the
>>> +      first frame, it will not be returned to the client, until the
>>> +      initialization sequence completes and the frame is decoded.
>>> +
>>> +    * If the client sets width and height of the ``OUTPUT`` format to 0,
>>> +      calling :c:func:`VIDIOC_G_FMT`, :c:func:`VIDIOC_S_FMT`,
>>> +      :c:func:`VIDIOC_TRY_FMT` or :c:func:`VIDIOC_REQBUFS` on the ``CAPTURE``
>>> +      queue will return the ``-EACCES`` error code, until the decoder
>>> +      configures ``CAPTURE`` format according to stream metadata.
>>
>> I think this should also include the G/S_SELECTION ioctls, right?
> 
> I've started work on adding compliance tests for codecs to v4l2-compliance and
> I quickly discovered that this 'EACCES' error code is not nice at all.
> 
> The problem is that it is really inconsistent with V4L2 behavior: the basic
> rule is that there always is a format defined, i.e. G_FMT will always return
> a format.
> 
> Suddenly returning an error is actually quite painful to handle because it is
> a weird exception just for the capture queue of a stateful decoder if no
> output resolution is known.
> 
> Just writing that sentence is painful.

Note that adding EACCES support in vicodec was just as painful as adding
v4l2-compliance support for it. So it is also easier to handle this in
the driver if you can just set up a default format.

Regards,

	Hans

> 
> Why not just return some default driver defined format? It will automatically
> be updated once the decoder parsed the bitstream and knows the new resolution.
> 
> It really is just the same behavior as with a resolution change.
> 
> It is also perfectly fine to request buffers for the capture queue for that
> default format. It's pointless, but not a bug.
> 
> Unless I am missing something I strongly recommend changing this behavior.
> 
> Regards,
> 
> 	Hans
>