Hi,

On Mon, 2018-08-06 at 10:32 +0200, Hans Verkuil wrote:
> On 08/06/2018 10:16 AM, Paul Kocialkowski wrote:
> > On Sat, 2018-08-04 at 15:50 +0200, Hans Verkuil wrote:
> > > Regarding point 3: I think this should be documented next to the pixel format. I.e.
> > > the MPEG-2 Slice format used by the stateless cedrus codec requires the request API
> > > and that two MPEG-2 controls (slice params and quantization matrices) must be present
> > > in each request.
> > > 
> > > I am not sure a control flag (e.g. V4L2_CTRL_FLAG_REQUIRED_IN_REQ) is needed here.
> > > It's really implied by the fact that you use a stateless codec. It doesn't help
> > > generic applications like v4l2-ctl or qv4l2 either since in order to support
> > > stateless codecs they will have to know about the details of these controls anyway.
> > > 
> > > So I am inclined to say that it is not necessary to expose this information in
> > > the API, but it has to be documented together with the pixel format documentation.
> > 
> > I think this is affected by considerations about codec profile/level
> > support. More specifically, some controls will only be required for
> > supporting advanced codec profiles/levels, so they can only be
> > explicitly marked with appropriate flags by the driver when the target
> > profile/level is known. And I don't think it would be sane for userspace
> > to explicitly set what profile/level it's aiming at. As a result, I
> > don't think we can explicitly mark controls as required or optional.
> > 
> > I also like the idea that it should instead be implicit and that the
> > documentation should detail which specific stateless metadata controls
> > are required for a given profile/level.
> > 
> > As for controls validation, the approach followed in the Cedrus driver
> > is to check that the most basic controls are filled and allow having
> > missing controls for those that match advanced profiles.
> > 
> > Since this approach feels somewhat generic enough to be applied to all
> > stateless VPU drivers, maybe this should be made a helper in the
> > framework?
> 
> Sounds reasonable. Not sure if it will be in the first version, but it is
> easy to add later.

Definitely, I don't think this is such a high priority for now either.

> > In addition, I see a need for exposing the maximum profile/level that
> > the driver supports for decoding. I would suggest reusing the already-
> > existing dedicated controls used for encoding for this purpose. For
> > decoders, they would be used to expose the (read-only) maximum
> > profile/level that is supported by the hardware and keep using them as a
> > settable value in a range (matching the level of support) for encoders.
> > 
> > This is necessary for userspace to determine whether a given video can
> > be decoded in hardware or not. Instead of half-way decoding the video
> > (ending up in funky results), this would easily allow skipping hardware
> > decoding and e.g. falling back on software decoding.
> 
> I think it might be better to expose this through new read-only bitmask
> controls: i.e. a bitmask containing the supported profiles and levels.

It seems that this is more or less what the coda driver is doing for
decoding actually, although it uses a menu control between min/max
supported profile/levels, with a mask to "blacklist" the unsupported
values. Then, the V4L2_CTRL_FLAG_READ_ONLY flag is set to keep the
control read-only.

> Reusing the existing controls for a decoder is odd since there is not
> really a concept of a 'current' value since you just want to report what
> is supported. And I am not sure if all decoders can report the profile
> or level that they detect.

Is that really a problem when the READ_ONLY flag is set? I thought it
was designed to fit this specific case, when the driver reports a value
that userspace cannot affect.

Otherwise, I agree that having a bitmask type would be a better fit, but
I think it would be beneficial to keep the already-defined control and
associated values, which implies using the menu control type for both
encoders and decoders.

If this is not an option, I would be in favour of adding per-codec read-
only bitmask controls (e.g. for H264 something like
V4L2_CID_MPEG_VIDEO_H264_PROFILE_SUPPORT) that expose the already-
existing profile/level definitions as bit identifiers (a bit like coda
is using them to craft a mask for the menu items to blacklist) for
decoding only.

What do you think?

Cheers,

Paul

-- 
Paul Kocialkowski, Bootlin (formerly Free Electrons)
Embedded Linux and kernel engineering
https://bootlin.com