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@xxxxxxxxxx> 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 >
