[RFC] Plane color pipeline KMS uAPI

Dave Airlie airlied at gmail.com
Tue May 9 19:53:08 UTC 2023

On Wed, 10 May 2023 at 00:31, Harry Wentland <harry.wentland at amd.com> wrote:
> On 5/7/23 19:14, Dave Airlie wrote:
> > On Sat, 6 May 2023 at 08:21, Sebastian Wick <sebastian.wick at redhat.com> wrote:
> >>
> >> On Fri, May 5, 2023 at 10:40 PM Dave Airlie <airlied at gmail.com> wrote:
> >>>
> >>> On Fri, 5 May 2023 at 01:23, Simon Ser <contact at emersion.fr> wrote:
> >>>>
> >>>> Hi all,
> >>>>
> >>>> The goal of this RFC is to expose a generic KMS uAPI to configure the color
> >>>> pipeline before blending, ie. after a pixel is tapped from a plane's
> >>>> framebuffer and before it's blended with other planes. With this new uAPI we
> >>>> aim to reduce the battery life impact of color management and HDR on mobile
> >>>> devices, to improve performance and to decrease latency by skipping
> >>>> composition on the 3D engine. This proposal is the result of discussions at
> >>>> the Red Hat HDR hackfest [1] which took place a few days ago. Engineers
> >>>> familiar with the AMD, Intel and NVIDIA hardware have participated in the
> >>>> discussion.
> >>>>
> >>>> This proposal takes a prescriptive approach instead of a descriptive approach.
> >>>> Drivers describe the available hardware blocks in terms of low-level
> >>>> mathematical operations, then user-space configures each block. We decided
> >>>> against a descriptive approach where user-space would provide a high-level
> >>>> description of the colorspace and other parameters: we want to give more
> >>>> control and flexibility to user-space, e.g. to be able to replicate exactly the
> >>>> color pipeline with shaders and switch between shaders and KMS pipelines
> >>>> seamlessly, and to avoid forcing user-space into a particular color management
> >>>> policy.
> >>>
> >>> I'm not 100% sold on the prescriptive here, let's see if someone can
> >>> get me over the line with some questions later.
> >>>
> >>> My feeling is color pipeline hw is not a done deal, and that hw
> >>> vendors will be revising/evolving/churning the hw blocks for a while
> >>> longer, as there is no real standards in the area to aim for, all the
> >>> vendors are mostly just doing whatever gets Windows over the line and
> >>> keeps hw engineers happy. So I have some concerns here around forwards
> >>> compatibility and hence the API design.
> >>>
> >>> I guess my main concern is if you expose a bunch of hw blocks and
> >>> someone comes up with a novel new thing, will all existing userspace
> >>> work, without falling back to shaders?
> >>> Do we have minimum guarantees on what hardware blocks have to be
> >>> exposed to build a useable pipeline?
> >>> If a hardware block goes away in a new silicon revision, do I have to
> >>> rewrite my compositor? or will it be expected that the kernel will
> >>> emulate the old pipelines on top of whatever new fancy thing exists.
> >>
> >> I think there are two answers to those questions.
> >
> > These aren't selling me much better :-)
> >>
> >> The first one is that right now KMS already doesn't guarantee that
> >> every property is supported on all hardware. The guarantee we have is
> >> that properties that are supported on a piece of hardware on a
> >> specific kernel will be supported on the same hardware on later
> >> kernels. The color pipeline is no different here. For a specific piece
> >> of hardware a newer kernel might only change the pipelines in a
> >> backwards compatible way and add new pipelines.
> >>
> >> So to answer your question: if some hardware with a novel pipeline
> >> will show up it might not be supported and that's fine. We already
> >> have cases where some hardware does not support the gamma lut property
> >> but only the CSC property and that breaks night light because we never
> >> bothered to write a shader fallback. KMS provides ways to offload work
> >> but a generic user space always has to provide a fallback and this
> >> doesn't change. Hardware specific user space on the other hand will
> >> keep working with the forward compatibility guarantees we want to
> >> provide.
> >
> > In my mind we've screwed up already, isn't a case to be made for
> > continue down the same path.
> >
> > The kernel is meant to be a hardware abstraction layer, not just a
> > hardware exposure layer. The kernel shouldn't set policy and there are
> > cases where it can't act as an abstraction layer (like where you need
> > a compiler), but I'm not sold that this case is one of those yet. I'm
> > open to being educated here on why it would be.
> >
> Thanks for raising these points. When I started out looking at color
> management I favored the descriptive model. Most other HW vendors
> I've talked to also tell me that they think about descriptive APIs
> since that allows HW vendors to map that to whatever their HW supports.
> Sebastian, Pekka, and others managed to change my mind about this
> but I still keep having difficult questions within AMD.
> Sebastian, Pekka, and Jonas have already done a good job to describe
> our reasoning behind the prescriptive model. It might be helpful to
> see how different the results of different tone-mapping operators
> can look:
> http://helgeseetzen.com/wp-content/uploads/2017/06/HS1.pdf
> According to my understanding all other platforms that have HDR now
> have a single compositor. At least that's true for Windows. This allows
> driver developers to tune their tone-mapping algorithm to match the
> algorithm used by the compositor when offloading plane composition.
> This is not true on Linux, where we have a myriad of compositors for
> good reasons, many of which have a different view of how they want color
> management to look like. Even if we would come up with an API that lets
> compositors define their input, output, scaling, and blending space in
> detail it would still not be feasible to describe the minutia of
> the tone-mapping algorithms, hence leading to differences in output
> when KMS color management is used.
> I am debating whether we need to be serious about a userspace library
> (or maybe a user-mode driver) to provide an abstraction from the
> descriptive to the prescriptive model. HW vendors need a way to provide
> timely support for new HW generations without requiring updates to a
> large number of compositors.

There are also other vendor side effects to having this in userspace.

Will the library have a loader?
Will it allow proprietary plugins?
Will it allow proprietary reimplementations?
What will happen when a vendor wants distros to ship *their*
proprietary fork of said library?

How would NVIDIA integrate this with their proprietary stack?


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