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

[Xaver Hugl]

I'm afraid that would not be very useful. It indeed depends on the refresh
rate, but also on how close to vblank the compositor does its commits / on
what the latency requirements for the currently shown content are.
When the compositor presents a fullscreen video with frames that are queued
up in advance, needing a full frame to program the atomic commit could be
acceptable, but when the user moves the cursor or plays a game, the
compositor needs to do the commits as close to vblank as possible. Without
a known upper bound on the time that it takes to program the hardware
that's not doable.

Am Fr., 27. Okt. 2023 um 14:01 Uhr schrieb Pekka Paalanen <
ppaalanen at gmail.com>:

> On Fri, 27 Oct 2023 12:01:32 +0200
> Sebastian Wick <sebastian.wick at redhat.com> wrote:
>
> > 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.
>
> In an ideal world... but in real world, I don't know.
>
> Would it help if there was a list collected, with all the things in
> various hardware that is known to be too heavy to reprogram every
> refresh? Maybe that would allow a more educated decision?
>
> I bet that depends also on the refresh rate.
>
> I would probably be fine with some sort of update cost classification
> on colorops, and the kernel keeping track of blobs: if userspace sets
> the same blob on the same colorop that is already there (by blob ID, no
> need to compare contents), then it's a no-op change.
>
>
> Anyway, I really like reading Alex Goins' reply, it seems we are very
> much on the same page here. :-)
>
>
> Thanks,
> pq
>
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