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

[Ian Arkver <ian.arkver.dev@gmail.com>]

Hi Hans,

On 08/08/18 07:43, Hans Verkuil wrote:
> On 08/08/2018 05:11 AM, Tomasz Figa wrote:
>> On Tue, Aug 7, 2018 at 4:13 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>>>
>>> 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.
>>
>> You're perfectly right. There is no 1:1 relationship, but it doesn't
>> prevent copying timestamps. It just makes it possible for multiple
>> CAPTURE buffers to have the same timestamp or some OUTPUT timestamps
>> not to be found in any CAPTURE buffer.
> 
> We need to document the behavior. Basically there are three different
> corner cases that need documenting:
> 
> 1) one OUTPUT buffer generates multiple CAPTURE buffers
> 2) multiple OUTPUT buffers generate one CAPTURE buffer
> 3) the decoding order differs from the presentation order (i.e. the
>     CAPTURE buffers are out-of-order compared to the OUTPUT buffers).
> 
> For 1) I assume that we just copy the same OUTPUT timestamp to multiple
> CAPTURE buffers.

I'm not sure how this interface would handle something like a temporal
scalability layer, but conceivably this assumption might be invalid in
that case.

Regards,
Ian.

> 
> For 2) we need to specify if the CAPTURE timestamp is copied from the first
> or last OUTPUT buffer used in creating the capture buffer. Using the last
> OUTPUT buffer makes more sense to me.
> 
> And 3) implies that timestamps can be out-of-order. This needs to be
> very carefully documented since it is very unexpected.
> 
> This should probably be a separate patch, adding text to the v4l2_buffer
> documentation (esp. the V4L2_BUF_FLAG_TIMESTAMP_COPY documentation).
> 
> Regards,
> 
> 	Hans
>