Re: [RFC PATCH v2 06/17] drm/doc/rfc: Describe why prescriptive color pipeline is needed

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Fri, 20 Oct 2023 16:22:56 +0200
Sebastian Wick <sebastian.wick@redhat.com> wrote:

> Thanks for continuing to work on this!
> 
> On Thu, Oct 19, 2023 at 05:21:22PM -0400, Harry Wentland wrote:
> > v2:
> >  - Update colorop visualizations to match reality (Sebastian, Alex Hung)
> >  - Updated wording (Pekka)
> >  - Change BYPASS wording to make it non-mandatory (Sebastian)
> >  - Drop cover-letter-like paragraph from COLOR_PIPELINE Plane Property
> >    section (Pekka)
> >  - Use PQ EOTF instead of its inverse in Pipeline Programming example (Melissa)
> >  - Add "Driver Implementer's Guide" section (Pekka)
> >  - Add "Driver Forward/Backward Compatibility" section (Sebastian, Pekka)
> > 

...

> > +Driver Forward/Backward Compatibility
> > +=====================================
> > +
> > +As this is uAPI drivers can't regress color pipelines that have been
> > +introduced for a given HW generation. New HW generations are free to
> > +abandon color pipelines advertised for previous generations.
> > +Nevertheless, it can be beneficial to carry support for existing color
> > +pipelines forward as those will likely already have support in DRM
> > +clients.
> > +
> > +Introducing new colorops to a pipeline is fine, as long as they can be
> > +disabled or are purely informational. DRM clients implementing support
> > +for the pipeline can always skip unknown properties as long as they can
> > +be confident that doing so will not cause unexpected results.
> > +
> > +If a new colorop doesn't fall into one of the above categories
> > +(bypassable or informational) the modified pipeline would be unusable
> > +for user space. In this case a new pipeline should be defined.  
> 
> How can user space detect an informational element? Should we just add a
> BYPASS property to informational elements, make it read only and set to
> true maybe? Or something more descriptive?

Read-only BYPASS set to true would be fine by me, I guess.

I think we also need a definition of "informational".

Counter-example 1: a colorop that represents a non-configurable
YUV<->RGB conversion. Maybe it determines its operation from FB pixel
format. It cannot be set to bypass, it cannot be configured, and it
will alter color values.

Counter-example 2: image size scaling colorop. It might not be
configurable, it is controlled by the plane CRTC_* and SRC_*
properties. You still need to understand what it does, so you can
arrange the scaling to work correctly. (Do not want to scale an image
with PQ-encoded values as Josh demonstrated in XDC.)

Counter-example 3: image sampling colorop. Averages FB originated color
values to produce a color sample. Again do not want to do this with
PQ-encoded values.


Thanks,
pq

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