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From:   Maxime Jourdan <maxi.jourdan@wanadoo.fr>
Date:   Tue, 7 Aug 2018 21:11:04 +0200
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Subject: Re: [PATCH 1/2] media: docs-rst: Document memory-to-memory video
 decoder interface
To:     Hans Verkuil <hverkuil@xs4all.nl>
Cc:     Tomasz Figa <tfiga@chromium.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
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2018-08-07 9:13 GMT+02:00 Hans Verkuil <hverkuil@xs4all.nl>:
> On 07/26/2018 12:20 PM, Tomasz Figa wrote:
>> Hi Hans,
>>
>> On Wed, Jul 25, 2018 at 8:59 PM Hans Verkuil <hverkuil@xs4all.nl> 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

As it is done right now in userspace (FFmpeg, GStreamer) and most (if
not all?) drivers, it's a 1:1 between OUTPUT and CAPTURE. The only
thing that changes is the ordering since OUTPUT buffers are in
decoding order while CAPTURE buffers are in presentation order.

This almost always implies some timestamping kung-fu to match the
OUTPUT timestamps with the corresponding CAPTURE timestamps. It's
often done indirectly by the firmware on some platforms (rpi comes to
mind iirc).

The current constructions also imply one video packet per OUTPUT
buffer. If a video packet is too big to fit in a buffer, FFmpeg will
crop that packet to the maximum buffer size and will discard the
remaining packet data. GStreamer will abort the decoding. This is
unfortunately one of the shortcomings of having fixed-size buffers.
And if they were to split the packet in multiple buffers, then some
drivers in their current state wouldn't be able to handle the
timestamping issues and/or x:1 OUTPUT:CAPTURE buffer numbers.

Maxime