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

[Tomasz Figa]

On Mon, Oct 8, 2018 at 9:22 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>
> On 09/19/2018 12:17 PM, Tomasz Figa wrote:
> > Hi Hans,
> >
> > On Thu, Jul 26, 2018 at 7:20 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
> >>
> >> Hi Hans,
> >>
> >> On Wed, Jul 25, 2018 at 8:59 PM Hans Verkuil <hverkuil@xxxxxxxxx> 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.
>

Wouldn't TRY_FMT adjust the width and height back to match current stream?

Best regards,
Tomasz