[RFC PATCH 01/10] drm/doc/rfc: Describe why prescriptive color pipeline is needed

Wed Nov 8 16:19:12 UTC 2023

On Wed, 8 Nov 2023 09:31:17 -0500
Harry Wentland <harry.wentland at amd.com> wrote:

> On 2023-11-08 06:40, Sebastian Wick wrote:
> > On Wed, Nov 8, 2023 at 11:16 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:  
> >>
> >> On Tue, 7 Nov 2023 11:58:26 -0500
> >> Harry Wentland <harry.wentland at amd.com> wrote:
> >>  
> >>> On 2023-11-07 04:55, Pekka Paalanen wrote:  
> >>>> On Mon, 6 Nov 2023 11:19:27 -0500
> >>>> Harry Wentland <harry.wentland at amd.com> wrote:
> >>>>  
> >>>>> On 2023-10-20 06:36, Pekka Paalanen wrote:  
> >>>>>> On Thu, 19 Oct 2023 10:56:40 -0400
> >>>>>> Harry Wentland <harry.wentland at amd.com> wrote:
> >>>>>>  
> >>>>>>> On 2023-10-10 12:13, Melissa Wen wrote:  
> >>>>>>>> O 09/08, Harry Wentland wrote:  
> >>>>>>>>> Signed-off-by: Harry Wentland <harry.wentland at amd.com>  
> >>>>>>
> >>>>>> ...
> >>>>>>  
> >>>>>>>> Also, with this new plane API in place, I understand that we will
> >>>>>>>> already need think on how to deal with the mixing between old drm color
> >>>>>>>> properties (color encoding and color range) and these new way of setting
> >>>>>>>> plane color properties. IIUC, Pekka asked a related question about it
> >>>>>>>> when talking about CRTC automatic RGB->YUV (?)
> >>>>>>>>  
> >>>>>>>
> >>>>>>> We'll still need to confirm whether we'll want to deprecate these
> >>>>>>> existing properties. If we do that we'd want a client prop. Things
> >>>>>>> should still work without deprecating them, if drivers just pick up
> >>>>>>> after the initial encoding and range CSC.
> >>>>>>>
> >>>>>>> But realistically it might be better to deprecate them and turn them
> >>>>>>> into explicit colorops.  
> >>>>>>
> >>>>>> The existing properties would need to be explicitly reflected in the
> >>>>>> new pipelines anyway, otherwise there would always be doubt at which
> >>>>>> point of a pipeline the old properties apply, and they might even
> >>>>>> need to change positions between pipelines.
> >>>>>>
> >>>>>> I think it is simply easier to just hide all old color related
> >>>>>> properties when userspace sets the client-cap to enable pipelines. The
> >>>>>> problem is to make sure to hide all old properties on all drivers that
> >>>>>> support the client-cap.
> >>>>>>
> >>>>>> As a pipeline must be complete (describe everything that happens to
> >>>>>> pixel values), it's going to be a flag day per driver.
> >>>>>>
> >>>>>> Btw. the plane FB YUV->RGB conversion needs a colorop in every pipeline
> >>>>>> as well. Maybe it's purely informative and non-configurable, keyed by
> >>>>>> FB pixel format, but still.
> >>>>>>
> >>>>>> We also need a colorop to represent sample filtering, e.g. bilinear,
> >>>>>> like I think Sebastian may have mentioned in the past. Everything
> >>>>>> before the sample filter happens "per tap" as Joshua Ashton put it, and
> >>>>>> everything after it happens on the sample that was computed as a
> >>>>>> weighted average of the filter tap inputs (texels).
> >>>>>>
> >>>>>> There could be colorops other than sample filtering that operate on
> >>>>>> more than one sample at a time, like blur or sharpness. There could
> >>>>>> even be colorops that change the image size like adding padding that
> >>>>>> the following colorop hardware requires, and then yet another colorop
> >>>>>> that clips that padding away. For an example, see
> >>>>>> https://lists.freedesktop.org/archives/dri-devel/2023-October/427015.html
> >>>>>>
> >>>>>> If that padding and its color can affect the pipeline results of the
> >>>>>> pixels near the padding (e.g. some convolution is applied with them,
> >>>>>> which may be the reason why padding is necessary to begin with), then
> >>>>>> it would be best to model it.
> >>>>>>  
> >>>>>
> >>>>> I hear you but I'm also somewhat shying away from defining this at this point.  
> >>>>
> >>>> Would you define them before the new UAPI is released though?
> >>>>
> >>>> I agree there is no need to have them in this patch series, but I think
> >>>> we'd hit the below problems if the UAPI is released without them.
> >>>>  
> >>>>> There are already too many things that need to happen and I will focus on the
> >>>>> actual color blocks (LUTs, matrices) first. We'll always be able to add a new
> >>>>> (read-only) colorop type to define scaling and tap behavior at any point and
> >>>>> a client is free to ignore a color pipeline if it doesn't find any tap/scale
> >>>>> info in it.  
> >>>>
> >>>> How would userspace know to look for tap/scale info, if there is no
> >>>> upstream definition even on paper?
> >>>>  
> >>>
> >>> So far OSes did not care about this. Whether that's good or bad is
> >>> something everyone can answer for themselves.
> >>>
> >>> If you write a compositor and really need this you can just ignore
> >>> color pipelines that don't have this, i.e., you'll probably want
> >>> to wait with implementing color pipeline support until you have what
> >>> you need from DRM/KMS.
> >>>
> >>> This is not to say I don't want to have support for Weston. But I'm
> >>> wondering if we place too much importance on getting every little
> >>> thing figured out whereas we could be making forward progress and
> >>> address more aspects of a color pipeline in the future. There is a
> >>> reason gamescope has made a huge difference in driving the color
> >>> management work forward.
> >>>  
> >>>> And the opposite case, if someone writes userspace without tap/scale
> >>>> colorops, and then drivers add those, and there is no pipeline without
> >>>> them, because they always exist. Would that userspace disregard all
> >>>> those pipelines because it does not understand tap/scale colorops,
> >>>> leaving no usable pipelines? Would that not be kernel regressing
> >>>> userspace?
> >>>>  
> >>>
> >>> The simple solution is to leave previously advertised pipelines
> >>> untouched and add a new version that does include scaling information.
> >>>  
> >>>> If the kernel keeps on exposing pipelines without the colorops, it
> >>>> fails the basic promise of the whole design: that all pixel value
> >>>> affecting operations are at least listed if not controllable.
> >>>>
> >>>> How will we avoid painting ourselves in a corner?
> >>>>
> >>>> Maybe we need a colorop for "here be dragons" documented as having
> >>>> unknown and unreliable effects, until driver authors are sure that
> >>>> everything has been modelled in the pipeline and there are no unknowns?
> >>>> Or a flag on the pipelines, if we can have that. Then we can at least
> >>>> tell when the pipeline does not fulfil the basic promise.
> >>>>  
> >>>
> >>> The will always be dragons at some level.  
> >>
> >> Do I understand right that the goal of fully understood color pipelines
> >> is a lost cause?
> >>
> >> That every pipeline might always have something unknown and there is no
> >> way for userspace to know if it does? Maybe because driver developers
> >> don't know either?
> >>
> >> By something unknown I refer to anything outside of basic precision
> >> issues. Doing interpolation or mixing of inputs on the wrong side of a
> >> known non-linear colorop, for example.  
> > 
> > I don't think that's the case. Hardware vendors should understand the
> > hardware and exposing everything that affects the values is the goal
> > here. There will be a transitional period where the pipelines might
> > not expose every detail yet but that's fine. It's better than what we
> > have now and should become even better with time. It would maybe be
> > helpful in the future to have a cap, or property, or whatever, to
> > indicate that the pipelines are "complete" descriptions of what
> > happens to the values but we can discuss it when it becomes relevant.
> >   
> 
> I agree, for the most part. But how do you then define "complete" if
> you exclude precision issues?

If someone can develop some kind of precision indication in the UAPI,
we might be able to answer that question then.

> >> An incremental UAPI development approach is fine by me, meaning that
> >> pipelines might not be complete at first, but I believe that requires
> >> telling userspace whether the driver developers consider the pipeline
> >> complete (no undescribed operations that would significantly change
> >> results from the expected results given the UAPI exposed pipeline).
> >>
> >> The prime example of what I would like to know is that if a FB
> >> contains PQ-encoded image and I use a color pipeline to scale that
> >> image up, will the interpolation happen before or after the non-linear
> >> colorop that decodes PQ. That is a significant difference as pointed
> >> out by Joshua.
> >>  
> 
> That's fair and I want to give that to you. My concern stems from
> the sentiment that I hear that any pipeline that doesn't explicitly
> advertise this is useless. I don't agree there. Let's not let perfect
> be the enemy of good.

It's up to the use case. The policy of what is sufficient should reside
in userspace.

What about matching compositor shader composition with KMS?

Can we use that as a rough precision threshold? If userspace implements
the exact same color pipeline as the KMS UAPI describes, then that and
the KMS composited result should be indistinguishable in side-by-side
or alternating visual inspection on any monitor in isolation.

Did this whole effort not start from wanting to off-load things to
display hardware but still maintain visual equivalence to software/GPU
composition?

Thanks,
pq
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