Date:   Mon, 10 Sep 2018 14:49:45 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Alexandre Courbot <acourbot@...omium.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Pawel Osciak <posciak@...omium.org>,
        linux-media@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] media: docs-rst: Document m2m stateless video decoder
 interface

On 09/10/2018 01:57 PM, Hans Verkuil wrote:
> On 09/10/2018 01:25 PM, Hans Verkuil wrote:
>>> +
>>> +Decoding
>>> +========
>>> +
>>> +For each frame, the client is responsible for submitting a request to which the
>>> +following is attached:
>>> +
>>> +* Exactly one frame worth of encoded data in a buffer submitted to the
>>> +  ``OUTPUT`` queue,
>>> +* All the controls relevant to the format being decoded (see below for details).
>>> +
>>> +``CAPTURE`` buffers must not be part of the request, but must be queued
>>> +independently. The driver will pick one of the queued ``CAPTURE`` buffers and
>>> +decode the frame into it. Although the client has no control over which
>>> +``CAPTURE`` buffer will be used with a given ``OUTPUT`` buffer, it is guaranteed
>>> +that ``CAPTURE`` buffers will be returned in decode order (i.e. the same order
>>> +as ``OUTPUT`` buffers were submitted), so it is trivial to associate a dequeued
>>> +``CAPTURE`` buffer to its originating request and ``OUTPUT`` buffer.
>>> +
>>> +If the request is submitted without an ``OUTPUT`` buffer or if one of the
>>> +required controls are missing, then :c:func:`MEDIA_REQUEST_IOC_QUEUE` will return
>>> +``-EINVAL``.
>>
>> Not entirely true: if buffers are missing, then ENOENT is returned. Missing required
>> controls or more than one OUTPUT buffer will result in EINVAL. This per the latest
>> Request API changes.
>>
>>  Decoding errors are signaled by the ``CAPTURE`` buffers being
>>> +dequeued carrying the ``V4L2_BUF_FLAG_ERROR`` flag.
>>
>> Add here that if the reference frame had an error, then all other frames that refer
>> to it should also set the ERROR flag. It is up to userspace to decide whether or
>> not to drop them (part of the frame might still be valid).
>>
>> I am not sure whether this should be documented, but there are some additional
>> restrictions w.r.t. reference frames:
>>
>> Since decoders need access to the decoded reference frames there are some corner
>> cases that need to be checked:
>>
>> 1) V4L2_MEMORY_USERPTR cannot be used for the capture queue: the driver does not
>>    know when a malloced but dequeued buffer is freed, so the reference frame
>>    could suddenly be gone.
>>
>> 2) V4L2_MEMORY_DMABUF can be used, but drivers should check that the dma buffer is
>>    still available AND increase the dmabuf refcount while it is used by the HW.
>>
>> 3) What to do if userspace has requeued a buffer containing a reference frame,
>>    and you want to decode a B/P-frame that refers to that buffer? We need to
>>    check against that: I think that when you queue a capture buffer whose index
>>    is used in a pending request as a reference frame, than that should fail with
>>    an error. And trying to queue a request referring to a buffer that has been
>>    requeued should also fail.
>>
>> We might need to add some support for this in v4l2-mem2mem.c or vb2.
>>
>> We will have similar (but not quite identical) issues with stateless encoders.
> 
> Related to this is the question whether buffer indices that are used to refer
> to reference frames should refer to the capture or output queue.
> 
> Using capture indices works if you never queue more than one request at a time:
> you know exactly what the capture buffer index is of the decoded I-frame, and
> you can use that in the following requests.
> 
> But if multiple requests are queued, then you don't necessarily know to which
> capture buffer an I-frame will be decoded, so then you can't provide this index
> to following B/P-frames. This puts restrictions on userspace: you can only
> queue B/P-frames once you have decoded the I-frame. This might be perfectly
> acceptable, though.
> 
> Using output buffer indices will work (the driver will know to which capture
> buffer index the I-frames mapped), but it requires that the output buffer that
> contained a reference frame isn't requeued, since that means that the driver
> will lose this mapping. I think this will make things too confusing, though.
> 
> A third option is that you don't refer to reference frames by buffer index,
> but instead by some other counter (sequence counter, perhaps?).

Definitely not sequence number, since it has the same problem as buffer index.

This could be a value relative to the frame you are trying to decode: i.e. 'use
the capture buffer from X frames ago'. This makes it index independent and is
still easy to keep track of inside the driver and application.

Regards,

	Hans

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