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

[Hans Verkuil]

On 07/26/2018 12:20 PM, Tomasz Figa wrote:
> Hi Hans,
> 
> On Wed, Jul 25, 2018 at 8:59 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>>> +
>>> +14. Call :c:func:`VIDIOC_STREAMON` to initiate decoding frames.
>>> +
>>> +Decoding
>>> +========
>>> +
>>> +This state is reached after a successful initialization sequence. In this
>>> +state, client queues and dequeues buffers to both queues via
>>> +:c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`, following standard
>>> +semantics.
>>> +
>>> +Both queues operate independently, following standard behavior of V4L2
>>> +buffer queues and memory-to-memory devices. In addition, the order of
>>> +decoded frames dequeued from ``CAPTURE`` queue may differ from the order of
>>> +queuing coded frames to ``OUTPUT`` queue, due to properties of selected
>>> +coded format, e.g. frame reordering. The client must not assume any direct
>>> +relationship between ``CAPTURE`` and ``OUTPUT`` buffers, other than
>>> +reported by :c:type:`v4l2_buffer` ``timestamp`` field.
>>
>> Is there a relationship between capture and output buffers w.r.t. the timestamp
>> field? I am not aware that there is one.
> 
> I believe the decoder was expected to copy the timestamp of matching
> OUTPUT buffer to respective CAPTURE buffer. Both s5p-mfc and coda seem
> to be implementing it this way. I guess it might be a good idea to
> specify this more explicitly.

What about an output buffer producing multiple capture buffers? Or the case
where the encoded bitstream of a frame starts at one output buffer and ends
at another? What happens if you have B frames and the order of the capture
buffers is different from the output buffers?

In other words, for codecs there is no clear 1-to-1 relationship between an
output buffer and a capture buffer. And we never defined what the 'copy timestamp'
behavior should be in that case or if it even makes sense.

Regards,

	Hans