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SubjectRe: [PATCH 1/2] media: docs-rst: Document memory-to-memory video decoder interface
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On 09/19/2018 12:17 PM, Tomasz Figa wrote:
> Hi Hans,
>
> On Thu, Jul 26, 2018 at 7:20 PM Tomasz Figa <tfiga@chromium.org> wrote:
>>
>> Hi Hans,
>>
>> On Wed, Jul 25, 2018 at 8:59 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>>>
>>> Hi Tomasz,
>>>
>>> Many, many thanks for working on this! It's a great document and when done
>>> it will be very useful indeed.
>>>
>>> Review comments follow...
>>
>> Thanks for review!
>>
>>>
>>> On 24/07/18 16:06, Tomasz Figa wrote:
> [snip]
>>>> +13. Allocate destination (raw format) buffers via :c:func:`VIDIOC_REQBUFS`
>>>> + on the ``CAPTURE`` queue.
>>>> +
>>>> + * **Required fields:**
>>>> +
>>>> + ``count``
>>>> + requested number of buffers to allocate; greater than zero
>>>> +
>>>> + ``type``
>>>> + a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``
>>>> +
>>>> + ``memory``
>>>> + follows standard semantics
>>>> +
>>>> + * **Return fields:**
>>>> +
>>>> + ``count``
>>>> + adjusted to allocated number of buffers
>>>> +
>>>> + * The driver must adjust count to minimum of required number of
>>>> + destination buffers for given format and stream configuration and the
>>>> + count passed. The client must check this value after the ioctl
>>>> + returns to get the number of buffers allocated.
>>>> +
>>>> + .. note::
>>>> +
>>>> + To allocate more than minimum number of buffers (for pipeline
>>>> + depth), use G_CTRL(``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE``) to
>>>> + get minimum number of buffers required, and pass the obtained value
>>>> + plus the number of additional buffers needed in count to
>>>> + :c:func:`VIDIOC_REQBUFS`.
>>>
>>>
>>> I think we should mention here the option of using VIDIOC_CREATE_BUFS in order
>>> to allocate buffers larger than the current CAPTURE format in order to accommodate
>>> future resolution changes.
>>
>> Ack.
>>
>
> I'm about to add a paragraph to describe this, but there is one detail
> to iron out.
>
> The VIDIOC_CREATE_BUFS ioctl accepts a v4l2_format struct. Userspace
> needs to fill in this struct and the specs says that
>
> "Usually this will be done using the VIDIOC_TRY_FMT or VIDIOC_G_FMT
> ioctls to ensure that the requested format is supported by the
> driver."
>
> However, in case of a decoder, those calls would fixup the format to
> match the currently parsed stream, which would likely resolve to the
> current coded resolution (~hardware alignment). How do we get a format
> for the desired maximum resolution?

You would call G_FMT to get the current format/resolution, then update
width and height and call TRY_FMT.

Although to be honest you can also just set pixelformat and width/height
and zero everything else and call TRY_FMT directly, skipping the G_FMT
ioctl.

>
> [snip].
>>>> +
>>>> + * The driver is also allowed to and may not return all decoded frames
> [snip]
>>>> + queued but not decode before the seek sequence was initiated. For
>>>
>>> Very confusing sentence. I think you mean this:
>>>
>>> The driver may not return all decoded frames that where ready for
>>> dequeueing from before the seek sequence was initiated.
>>>
>>> Is this really true? Once decoded frames are marked as buffer_done by the
>>> driver there is no reason for them to be removed. Or you mean something else
>>> here, e.g. the frames are decoded, but the buffers not yet given back to vb2.
>>>
>>
>> Exactly "the frames are decoded, but the buffers not yet given back to
>> vb2", for example, if reordering takes place. However, if one stops
>> streaming before dequeuing all buffers, they are implicitly returned
>> (reset to the state after REQBUFS) and can't be dequeued anymore, so
>> the frames are lost, even if the driver returned them. I guess the
>> sentence was really unfortunate indeed.
>>
>
> Actually, that's not the only case.
>
> The documentation is written from userspace point of view. Queuing an
> OUTPUT buffer is not equivalent to having it decoded (and a CAPTURE
> buffer given back to vb2). If the userspace queues a buffer and then
> stops streaming, the buffer might have been still waiting in the
> queue, for decoding of previous buffers to finish.
>
> So basically by "queued frames" I meant "OUTPUT buffers queued by
> userspace and not sent to the hardware yet" and by "decoded frames" I
> meant "CAPTURE buffers containing matching frames given back to vb2".
>
> How about rewording like this:
>
> * The ``VIDIOC_STREAMOFF`` operation discards any remaining queued
> ``OUTPUT`` buffers, which means that not all of the ``OUTPUT`` buffers
> queued before the seek may have matching ``CAPTURE`` buffers produced.
> For example, [...]

That looks correct.

Regards,

Hans

>
>>>> + example, given an ``OUTPUT`` queue sequence: QBUF(A), QBUF(B),
>>>> + STREAMOFF(OUT), STREAMON(OUT), QBUF(G), QBUF(H), any of the
>>>> + following results on the ``CAPTURE`` queue is allowed: {A’, B’, G’,
>>>> + H’}, {A’, G’, H’}, {G’, H’}.
>>>> +
>>>> + .. note::
>>>> +
>>>> + To achieve instantaneous seek, the client may restart streaming on
>>>> + ``CAPTURE`` queue to discard decoded, but not yet dequeued buffers.
>
> Best regards,
> Tomasz
>

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