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

[Sebastian Wick]

On Fri, Oct 27, 2023 at 10:59:25AM +0200, Michel Dänzer wrote:
> On 10/26/23 21:25, Alex Goins wrote:
> > On Thu, 26 Oct 2023, Sebastian Wick wrote:
> >> On Thu, Oct 26, 2023 at 11:57:47AM +0300, Pekka Paalanen wrote:
> >>> On Wed, 25 Oct 2023 15:16:08 -0500 (CDT)
> >>> Alex Goins <agoins at nvidia.com> wrote:
> >>>
> >>>> Despite being programmable, the LUTs are updated in a manner that is less
> >>>> efficient as compared to e.g. the non-static "degamma" LUT. Would it be helpful
> >>>> if there was some way to tag operations according to their performance,
> >>>> for example so that clients can prefer a high performance one when they
> >>>> intend to do an animated transition? I recall from the XDC HDR workshop
> >>>> that this is also an issue with AMD's 3DLUT, where updates can be too
> >>>> slow to animate.
> >>>
> >>> I can certainly see such information being useful, but then we need to
> >>> somehow quantize the performance.
> > 
> > Right, which wouldn't even necessarily be universal, could depend on the given
> > host, GPU, etc. It could just be a relative performance indication, to give an
> > order of preference. That wouldn't tell you if it can or can't be animated, but
> > when choosing between two LUTs to animate you could prefer the higher
> > performance one.
> > 
> >>>
> >>> What I was left puzzled about after the XDC workshop is that is it
> >>> possible to pre-load configurations in the background (slow), and then
> >>> quickly switch between them? Hardware-wise I mean.
> > 
> > This works fine for our "fast" LUTs, you just point them to a surface in video
> > memory and they flip to it. You could keep multiple surfaces around and flip
> > between them without having to reprogram them in software. We can easily do that
> > with enumerated curves, populating them when the driver initializes instead of
> > waiting for the client to request them. You can even point multiple hardware
> > LUTs to the same video memory surface, if they need the same curve.
> > 
> >>
> >> We could define that pipelines with a lower ID are to be preferred over
> >> higher IDs.
> > 
> > Sure, but this isn't just an issue with a pipeline as a whole, but the
> > individual elements within it and how to use them in a given context.
> > 
> >>
> >> The issue is that if programming a pipeline becomes too slow to be
> >> useful it probably should just not be made available to user space.
> > 
> > It's not that programming the pipeline is overall too slow. The LUTs we have
> > that are relatively slow to program are meant to be set infrequently, or even
> > just once, to allow the scaler and tone mapping operator to operate in fixed
> > point PQ space. You might still want the tone mapper, so you would choose a
> > pipeline that includes them, but when it comes to e.g. animating a night light,
> > you would want to choose a different LUT for that purpose.
> > 
> >>
> >> The prepare-commit idea for blob properties would help to make the
> >> pipelines usable again, but until then it's probably a good idea to just
> >> not expose those pipelines.
> > 
> > The prepare-commit idea actually wouldn't work for these LUTs, because they are
> > programmed using methods instead of pointing them to a surface. I'm actually not
> > sure how slow it actually is, would need to benchmark it. I think not exposing
> > them at all would be overkill, since it would mean you can't use the preblending
> > scaler or tonemapper, and animation isn't necessary for that.
> > 
> > The AMD 3DLUT is another example of a LUT that is slow to update, and it would
> > obviously be a major loss if that wasn't exposed. There just needs to be some
> > way for clients to know if they are going to kill performance by trying to
> > change it every frame.
> 
> Might a first step be to require the ALLOW_MODESET flag to be set when changing the values for a colorop which is too slow to be updated per refresh cycle?
> 
> This would tell the compositor: You can use this colorop, but you can't change its values on the fly.

I argued before that changing any color op to passthrough should never
require ALLOW_MODESET and while this is really hard to guarantee from a
driver perspective I still believe that it's better to not expose any
feature requiring ALLOW_MODESET or taking too long to program to be
useful for per-frame changes.

When user space has ways to figure out if going back to a specific state
(in this case setting everything to bypass) without ALLOW_MODESET we can
revisit this decision, but until then, let's keep things simple and only
expose things that work reliably without ALLOW_MODESET and fast enough
to work for per-frame changes.

Harry, Pekka: Should we document this? It obviously restricts what can
be exposed but exposing things that can't be used by user space isn't
useful.

> 
> -- 
> Earthling Michel Dänzer            |                  https://redhat.com
> Libre software enthusiast          |         Mesa and Xwayland developer
>