Re: [PATCH v2 2/2] media: docs-rst: Document memory-to-memory video encoder interface

[Tomasz Figa]

On Fri, Jan 25, 2019 at 5:14 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote:
>
> Le mercredi 23 janvier 2019 Ã 14:04 +0100, Hans Verkuil a Ãcrit :
> > > > Does this return the same set of formats as in the 'Querying Capabilities' phase?
> > > >
> > >
> > > It's actually an interesting question. At this point we wouldn't have
> > > the OUTPUT resolution set yet, so that would be the same set as in the
> > > initial query. If we set the resolution (with some arbitrary
> > > pixelformat), it may become a subset...
> >
> > But doesn't setting the capture format also set the resolution?
> >
> > To quote from the text above:
> >
> > "The encoder will derive a new ``OUTPUT`` format from the ``CAPTURE`` format
> >  being set, including resolution, colorimetry parameters, etc."
> >
> > So you set the capture format with a resolution (you know that), then
> > ENUM_FMT will return the subset for that codec and resolution.
> >
> > But see also the comment at the end of this email.
>
> I'm thinking that the fact that there is no "unset" value for pixel
> format creates a certain ambiguity. Maybe we could create a new pixel
> format, and all CODEC driver could have that set by default ? Then we
> can just fail STREAMON if that format is set.

The state on the CAPTURE queue is actually not "unset". The queue is
simply not ready (yet) and any operations on it will error out.

Once the application sets the coded resolution on the OUTPUT queue or
the decoder parses the stream information, the CAPTURE queue becomes
ready and one can do the ioctls on it.

>
> That being said, in GStreamer, I have split each elements per CODEC,
> and now only enumerate the information "per-codec". That makes me think
> this "global" enumeration was just a miss-use of the API / me learning
> to use it. Not having to implement this rather complex thing in the
> driver would be nice. Notably, the new Amlogic driver does not have
> this "Querying Capabilities" phase, and with latest GStreamer works
> just fine.

What do you mean by "doesn't have"? Does it lack an implementation of
VIDIOC_ENUM_FMT and VIDIOC_ENUM_FRAMESIZES?

Best regards,
Tomasz