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

[Pekka Paalanen]

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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