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

[Tomasz Figa <tfiga@chromium.org>]

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?

[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, [...]

> > > +       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