[RFC PATCH v2 06/17] drm/doc/rfc: Describe why prescriptive color pipeline is needed
Pekka Paalanen
ppaalanen at gmail.com
Fri Nov 10 13:27:18 UTC 2023
On Fri, 10 Nov 2023 11:27:14 +0000
"Shankar, Uma" <uma.shankar at intel.com> wrote:
> > -----Original Message-----
> > From: Pekka Paalanen <ppaalanen at gmail.com>
> > Sent: Thursday, November 9, 2023 5:26 PM
> > To: Shankar, Uma <uma.shankar at intel.com>
> > Cc: Joshua Ashton <joshua at froggi.es>; Harry Wentland
> > <harry.wentland at amd.com>; dri-devel at lists.freedesktop.org; Sebastian Wick
> > <sebastian.wick at redhat.com>; Sasha McIntosh <sashamcintosh at google.com>;
> > Abhinav Kumar <quic_abhinavk at quicinc.com>; Shashank Sharma
> > <shashank.sharma at amd.com>; Xaver Hugl <xaver.hugl at gmail.com>; Hector
> > Martin <marcan at marcan.st>; Liviu Dudau <Liviu.Dudau at arm.com>; Alexander
> > Goins <agoins at nvidia.com>; Michel Dänzer <mdaenzer at redhat.com>; wayland-
> > devel at lists.freedesktop.org; Melissa Wen <mwen at igalia.com>; Jonas Ådahl
> > <jadahl at redhat.com>; Arthur Grillo <arthurgrillo at riseup.net>; Victoria
> > Brekenfeld <victoria at system76.com>; Sima <daniel at ffwll.ch>; Aleix Pol
> > <aleixpol at kde.org>; Naseer Ahmed <quic_naseer at quicinc.com>; Christopher
> > Braga <quic_cbraga at quicinc.com>; Ville Syrjala <ville.syrjala at linux.intel.com>
> > Subject: Re: [RFC PATCH v2 06/17] drm/doc/rfc: Describe why prescriptive color
> > pipeline is needed
> >
> > On Thu, 9 Nov 2023 10:17:11 +0000
> > "Shankar, Uma" <uma.shankar at intel.com> wrote:
> >
> > > > -----Original Message-----
> > > > From: Joshua Ashton <joshua at froggi.es>
> > > > Sent: Wednesday, November 8, 2023 7:13 PM
> > > > To: Shankar, Uma <uma.shankar at intel.com>; Harry Wentland
> > > > <harry.wentland at amd.com>; dri-devel at lists.freedesktop.org
> >
> > ...
> >
> > > > Subject: Re: [RFC PATCH v2 06/17] drm/doc/rfc: Describe why
> > > > prescriptive color pipeline is needed
> > > >
> > > >
> > > >
> > > > On 11/8/23 12:18, Shankar, Uma wrote:
> > > > >
> > > > >
> > > > >> -----Original Message-----
> > > > >> From: Harry Wentland <harry.wentland at amd.com>
> > > > >> Sent: Friday, October 20, 2023 2:51 AM
> > > > >> To: dri-devel at lists.freedesktop.org
> >
> > ...
> >
> > > > >> Subject: [RFC PATCH v2 06/17] drm/doc/rfc: Describe why
> > > > >> prescriptive color pipeline is needed
> >
> > ...
> >
> > > > >> +An example of a drm_colorop object might look like one of these::
> > > > >> +
> > > > >> + /* 1D enumerated curve */
> > > > >> + Color operation 42
> > > > >> + ├─ "TYPE": immutable enum {1D enumerated curve, 1D LUT, 3x3
> > > > >> + matrix, 3x4
> > > > >> matrix, 3D LUT, etc.} = 1D enumerated curve
> > > > >> + ├─ "BYPASS": bool {true, false}
> > > > >> + ├─ "CURVE_1D_TYPE": enum {sRGB EOTF, sRGB inverse EOTF, PQ
> > > > >> + EOTF, PQ
> > > > >> inverse EOTF, …}
> > > > >
> > > > > Having the fixed function enum for some targeted input/output may
> > > > > not be scalable for all usecases. There are multiple colorspaces
> > > > > and transfer functions possible, so it will not be possible to
> > > > > cover all these by any enum definitions. Also, this will depend on
> > > > > the capabilities of
> > > > respective hardware from various vendors.
> > > >
> > > > The reason this exists is such that certain HW vendors such as AMD
> > > > have transfer functions implemented in HW. It is important to take
> > > > advantage of these for both precision and power reasons.
> > >
> > > Issue we see here is that, it will be too usecase and vendor specific.
> > > There will be BT601, BT709, BT2020, SRGB, HDR EOTF and many more. Not
> > > to forget we will need linearization and non-linearization enums for each of
> > these.
> >
> > I don't see that as a problem at all. It's not a combinatorial explosion like
> > input/output combinations in a single enum would be.
> > It's always a curve and its inverse at most.
> >
> > It's KMS properties, not every driver needs to implement every defined enum
> > value but only those values it can and wants to support.
> > Userspace also sees the supported list, it does not need trial and error.
> >
> > This is the only way to actually use hard-wired curves. The alternative would be
> > for userspace to submit a LUT of some type, and the driver needs to start
> > guessing if it matches one of the hard-wired curves the hardware supports, which
> > is just not feasible.
> >
> > Hard-wired curves are an addition, not a replacement, to custom curves defined
> > by parameters or various different LUT representations.
> > Many of these hard-wired curves will emerge as is from common use cases.
>
> Point taken, we can go with this fixed function curve types as long as it represents a
> single mathematical operation, thereby avoiding the combination nightmare.
>
> However, just want to make sure that the same thing can be done with a programmable
> hardware. In the case above, lut tables for the same need to be hardcoded in driver for
> various platforms (depending on its capabilities, precision, number, and distribution of luts etc).
Hi Uma,
you can do that if you want to.
> This is manageable, but driver will get bloated with all kinds of hardcoded lut tables,
> which could have been easily computed by the compositor runtime. Driver cannot compute
> the tables runtime due to the complexity of the floating math involved, so hardcoded
> lut tables will be the only option.
You do not have to do that if you don't want to.
> So we should just ensure that if these enums are not exposed by a driver, but a programmable
> lut block is exposed instead, userspace should fall back to the programmable lut. Having the
> fixed function enum should not become a mandatory norm to implement and expose even for a
> programmable hardware.
I agree.
> With this we will be able to cater to both kinds of hardware with a generic userspace.
> Hope this expectation is ok.
>
> > > Also
> > > a CTM indication to convert colospace.
> >
> > Did someone propose to enumerate matrices? I would not do that, unless you
> > literally have hard-wired matrices in hardware and cannot do custom matrices.
>
> Not currently, but there can be fixed function matrix for certain color space or
> format conversion like BT709->BT2020 etc..
> However, we see this is not proposed currently and if not needed, it's fine and
> don't want to bring another non-problem for discussion.
>
> > > Also, if the underlying hardware block is programmable, its not
> > > limited to be used only for the colorspace management but can be used
> > > for other color enhancements as well by a capable client.
> >
> > Yes, that's why we have other types for curves, the programmable ones.
>
> Got that and agree, it's fine as mentioned above.
>
> > > Hence, we feel that it is bordering on being descriptive with too many
> > > possible combinations (not easy to generalize). So, if hardware is
> > > programmable, lets expose its capability through a blob and be generic.
> >
> > It's not descriptive though. It's a prescription of a mathematical function the
> > hardware implements as fixed-function hardware. The function is a curve. There
> > is no implication that the curve must be used with specific input or output color
> > spaces.
>
> As long as we don’t mix combinations it should be fine. But all hardware's may not
> represent these fixed functions with single mathematical operation level granularity.
> It would be tough to represent such color blocks with a single enum.
If a colorop does not fit for some hardware, then the driver should
not expose that colorop or pipeline.
> > > For any fixed function hardware where Lut etc is stored in ROM and
> > > just a control/enable bit is provided to driver, we can define a
> > > pipeline with a vendor specific color block. This can be identified with a flag
> > (better ways can be discussed).
> >
> > No, there is no need for that. A curve type will do well.
>
> Agree and aligned here.
>
> > A vendor specific colorop needs vendor specific userspace code to program *at
> > all*. A generic curve colorop might list some curve types the userspace does not
> > understand, but also curve types userspace does understand. The understood
> > curve types can still be used by userspace.
>
> Issue is with combination operation in hardware. If it’s a single mathematical operation,
> it would be easy.
>
> > > For example, on some of the Intel platform, we had a fixed function to
> > > convert colorspaces directly with a bit setting. These kinds of things
> > > should be vendor specific and not be part of generic userspace implementation.
> >
> > Why would you forbid generic userspace from making use of them?
>
> Issue is that it was not one single mathematical operation but a combination
> as described below.
>
> > > For reference:
> > > 001b YUV601 to RGB601 YUV BT.601 to RGB BT.601 conversion.
> > > 010b YUV709 to RGB709 YUV BT.709 to RGB BT.709 conversion.
> > > 011b YUV2020 to RGB2020 YUV BT.2020 to RGB BT.2020 conversion.
> > > 100b RGB709 to RGB2020 RGB BT.709 to RGB BT.2020 conversion.
> >
> > This is nothing like the curves we talked about above.
> > Anyway, you can expose these fixed-function operations with a colorop that has
> > an enum choosing the conversion. There is no need to make it vendor-specific at
> > all. It's possible that only specific chips from Intel support it, but nothing stops
> > anyone else from implementing or emulating the colorop if they can construct a
> > hardware configuration achieving the same result.
> >
> > It seems there are already problems in exploding the number of pipelines to
> > expose, so it's best to try to avoid single-use colorops and use enums in more
> > generic colorops instead.
>
> Yeah, this is how hardware will implement and it involves multiple mathematical operations,
> controlled with one programmable bit to enable the same. These will be tough to generalize.
> What should be the type of color op for these would be an open.
>
> It would be great if we can address this generically.
We would need to know what those four things actually do. Your
description is very vague. Are there curves involved?
> > > > Additionally, not every vendor implements bucketed/segemented LUTs
> > > > the same way, so it's not feasible to expose that in a way that's
> > > > particularly useful or not vendor-specific.
> >
> > Joshua, I see no problem here really. They are just another type of LUT for a curve
> > colorop, with a different configuration blob that can be defined in the UAPI.
>
> Yeah, agree.
> And the programmable hardware can be easily exposed and generalize for all vendors,
> so it should not be a concern.
>
> > > If the underlying hardware is programmable, the structure which we
> > > propose to advertise the capability of the block to userspace will be sufficient to
> > compute the LUT coefficients.
> > > The caps can be :
> > > 1. Number of segments in Lut
> > > 2. Precision of lut
> > > 3. Starting and ending point of the segment 4. Number of samples in
> > > the segment.
> > > 5. Any other flag which could be useful in this computation.
> > >
> > > This way we can compute LUT's generically and send to driver. This
> > > will be scalable for all colorspaces, configurations and vendors.
> >
> > Drop the mention of colorspaces, and I hope so. :-)
> >
> > Color spaces don't quite exist in a prescriptive pipeline definition.
>
> Yeah. For driver it's just a LUT for programmable hardware, OR mathematical
> operation for fixed function hardware defined via enum 😊
>
> > > > Thus we decided to have a regular 1D LUT modulated onto a known curve.
> > > > This is the only real cross-vendor solution here that allows HW
> > > > curve implementations to be taken advantage of and also works with
> > > > bucketing/segemented LUTs.
> > > > (Including vendors we are not aware of yet).
> > > >
> > > > This also means that vendors that only support HW curves at some
> > > > stages without an actual LUT are also serviced.
> > >
> > > Any fixed function vendor implementation should be supported but with
> > > a vendor specific color block. Trying to come up with enums which
> > > aligns with some underlying hardware may not be scalable.
> >
> > I disagree with both of you.
> >
> > Who said there could be only one "degamma" block on a plane's pipeline?
> >
> > If hardware is best modelled as a fixed-function selectable curve followed by a
> > custom curve, then expose exactly those two generic colorops. Nothing stops a
> > pipeline from having two curve colorops in sequence with a disjoint set of
> > supported types or features. If some hardware does not have one of the curve
> > colorops, then just don't add the missing one in a pipeline.
>
> Agree, I think we are aligned now here.
Awesome!
Thanks,
pq
> > > > You are right that there *might* be some usecase not covered by this
> > > > right now, and that it would need kernel churn to implement new
> > > > curves, but unfortunately that's the compromise that we (so-far)
> > > > have decided on in order to ensure everyone can have good, precise, power-
> > efficient support.
> > >
> > > Yes, we are aligned on this. But believe programmable hardware should
> > > be able to expose its caps. Fixed function hardware should be non-generic and
> > vendor specific.
> > >
> > > > It is always possible for us to extend the uAPI at a later date for
> > > > other curves, or other properties that might expose a generic
> > > > segmented LUT interface (such as what you have proposed for a while) for
> > vendors that can support it.
> > > > (With the whole color pipeline thing, we can essentially do 'versioning'
> > > > with that, if we wanted a new 1D LUT type.)
> > >
> > > Most of the hardware vendors have programmable luts (including AMD),
> > > so it would be good to have this as a default generic compositor
> > > implementation. And yes, any new color block with a type can be added
> > > to the existing API's as the need arises without breaking compatibility.
> > >
> > > Regards,
> > > Uma Shankar
> > >
> > > >
> > > > Thanks!
> > > > - Joshie 🐸✨
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