[RFC] Plane color pipeline KMS uAPI

[Simon Ser]

On Friday, May 5th, 2023 at 15:30, Joshua Ashton <joshua at froggi.es> wrote:

> > > AMD would expose the following objects and properties:
> > >
> > >     Plane 10
> > >     ├─ "type": immutable enum {Overlay, Primary, Cursor} = Primary
> > >     └─ "color_pipeline": enum {0, 42} = 0
> > >     Color operation 42 (input CSC)
> > >     ├─ "type": enum {Bypass, Matrix} = Matrix
> > >     ├─ "matrix_data": blob
> > >     └─ "next": immutable color operation ID = 43
> > >     Color operation 43
> > >     ├─ "type": enum {Scaling} = Scaling
> > >     └─ "next": immutable color operation ID = 44
> > >     Color operation 44 (DeGamma)
> > >     ├─ "type": enum {Bypass, 1D curve} = 1D curve
> > >     ├─ "1d_curve_type": enum {sRGB, PQ, …} = sRGB
> > >     └─ "next": immutable color operation ID = 45
> 
> Some vendors have per-tap degamma and some have a degamma after the sample.
> How do we distinguish that behaviour?
> It is important to know.

Can you elaborate? What is "per-tap" and "sample"? Is the "Scaling" color
operation above not enough to indicate where in the pipeline the hw performs
scaling?

> > >     Color operation 45 (gamut remap)
> > >     ├─ "type": enum {Bypass, Matrix} = Matrix
> > >     ├─ "matrix_data": blob
> > >     └─ "next": immutable color operation ID = 46
> > >     Color operation 46 (shaper LUT RAM)
> > >     ├─ "type": enum {Bypass, 1D curve} = 1D curve
> > >     ├─ "1d_curve_type": enum {LUT} = LUT
> > >     ├─ "lut_size": immutable range = 4096
> > >     ├─ "lut_data": blob
> > >     └─ "next": immutable color operation ID = 47
> > >     Color operation 47 (3D LUT RAM)
> > >     ├─ "type": enum {Bypass, 3D LUT} = 3D LUT
> > >     ├─ "lut_size": immutable range = 17
> > >     ├─ "lut_data": blob
> > >     └─ "next": immutable color operation ID = 48
> > >     Color operation 48 (blend gamma)
> > >     ├─ "type": enum {Bypass, 1D curve} = 1D curve
> > >     ├─ "1d_curve_type": enum {LUT, sRGB, PQ, …} = LUT
> > >     ├─ "lut_size": immutable range = 4096
> > >     ├─ "lut_data": blob
> > >     └─ "next": immutable color operation ID = 0
> > >
> > > To configure the pipeline for an HDR10 PQ plane (path at the top) and a HDR
> > > display, gamescope would perform an atomic commit with the following property
> > > values:
> > >
> > >     Plane 10
> > >     └─ "color_pipeline" = 42
> > >     Color operation 42 (input CSC)
> > >     └─ "matrix_data" = PQ → scRGB (TF)
> 
> ^
> Not sure what this is.
> We don't use an input CSC before degamma.
> 
> > >     Color operation 44 (DeGamma)
> > >     └─ "type" = Bypass
> 
> ^
> If we did PQ, this would be PQ -> Linear / 80
> If this was sRGB, it'd be sRGB -> Linear
> If this was scRGB this would be just treating it as it is. So... Linear / 80.
> 
> > >     Color operation 45 (gamut remap)
> > >     └─ "matrix_data" = scRGB (TF) → PQ
> 
> ^
> This is wrong, we just use this to do scRGB primaries (709) to 2020.
> 
> We then go from scRGB -> PQ to go into our shaper + 3D LUT.
> 
> > >     Color operation 46 (shaper LUT RAM)
> > >     └─ "lut_data" = PQ → Display native
> 
> ^
> "Display native" is just the response curve of the display.
> In HDR10, this would just be PQ -> PQ
> If we were doing HDR10 on SDR, this would be PQ -> Gamma 2.2 (mapped
> from 0 to display native luminance) [with a potential bit of headroom
> for tonemapping in the 3D LUT]
> For SDR on HDR10 this would be Gamma 2.2 -> PQ (Not intending to start
> an sRGB vs G2.2 argument here! :P)
> 
> > >     Color operation 47 (3D LUT RAM)
> > >     └─ "lut_data" = Gamut mapping + tone mapping + night mode
> > >     Color operation 48 (blend gamma)
> > >     └─ "1d_curve_type" = PQ
> 
> ^
> This is wrong, this should be Display Native -> Linearized Display Referred

In the HDR case, isn't this the inverse of PQ?

> > You cannot do a TF with a matrix, and a gamut remap with a matrix on
> > electrical values is certainly surprising, so the example here is a
> > bit odd, but I don't think that hurts the intention of demonstration.
> 
> I have done some corrections inline.
> 
> You can see our fully correct color pipeline here:
> https://raw.githubusercontent.com/ValveSoftware/gamescope/master/src/docs/Steam%20Deck%20Display%20Pipeline.png
> 
> Please let me know if you have any more questions about our color pipeline.

As expected, I got the gamescope part wrong. I'm pretty confident that the
proposed API would still work since the AMD vendor-specific props would just
be exposed as color operation objects. Can you confirm we can make the
gamescope pipeline work with the AMD color pipeline outlined above?