AMD display drivers handling DRM CRTC color mgmt props

Fri Apr 22 14:28:11 UTC 2022

On 04/21, Harry Wentland wrote:
> 
> 
> On 2022-04-21 15:20, Melissa Wen wrote:
> > On 04/21, Harry Wentland wrote:
> > > 
> > > 
> > > On 2022-04-21 10:37, Melissa Wen wrote:
> > > > Hi all,
> > > > 
> > > > I'm examining how DRM color management properties (degamma, ctm, gamma)
> > > > are applied to AMD display drivers. As far I could understand thanks
> > > > Nicholas documentation on amdgpu_dm/amdgpu_dm_color, DC drivers have
> > > > per-plane color correction features:
> > > > 
> > Hi Harry,
> > 
> > Wow, thanks so much for all details!
> > > 
> > > DC programs some of the color correction features pre-blending but
> > > DRM/KMS has not per-plane color correction properties.
> > > 
> > > See this series from Uma Shankar for an RFC on how to introduce those
> > > properties for 1D LUTs and CSC matrix:
> > > https://patchwork.freedesktop.org/series/90826/
> > > 
> > > Bhanuprakash has a series of IGT tests for these properties:
> > > https://patchwork.freedesktop.org/series/96895/
> > > 
> > > I've rebased these on amd-staging-drm-next and maintain a kernel and IGT
> > > branch with these patches:
> > > https://gitlab.freedesktop.org/hwentland/linux/-/tree/color-and-hdr
> > > https://gitlab.freedesktop.org/hwentland/igt-gpu-tools/-/tree/color-and-hdr
> > > 
> > > We've had many discussions with Weston guys on this. In order to merge the
> > > kernel properties we need a canonical userspace implementation that are
> > > using it. Weston guys are working towards that but if you want to suggest a
> > > different userspace to serve as that vehicle I'd be all ears. :)
> > > 
> > > Note that in order to show this all working we also need a Wayland Protocol
> > > update.
> > > 
> > > See
> > > https://gitlab.freedesktop.org/pq/color-and-hdr
> > > https://gitlab.freedesktop.org/swick/wayland-protocols
> > > https://gitlab.freedesktop.org/wayland/weston/-/issues/467
> > 
> > So, I've followed these discussions (until the issue on naming) because
> > initially I considered it addresses our current goals for color
> > correction. But after some discussions, what we are targeting is a 3D
> > LUT after blending (per-CRTC). I found past proposals on dri-devel
> > [1][2] to extend the DRM CRTC color management properties, but they
> > didn't move forward and were never applied.
> > 
> 
> They're stuck in limbo until we have an upstream userspace
> implementation that's making use of them.

Yes... afaiu, the basic requirements for all of these changes are IGT
tests + open userspace usage, right?

> 
> > > 
> > > > * - Input gamma LUT (de-normalized)
> > > > * - Input CSC (normalized)
> > > > * - Surface degamma LUT (normalized)
> > > > * - Surface CSC (normalized)
> > > > * - Surface regamma LUT (normalized)
> > > > * - Output CSC (normalized)
> > > > so DM is "adapting" those DRM per-CRTC properties to fit into three of
> > > > these color correction stages, which I guess are the surface stages:
> > > > 
> > > > * - Surface degamma LUT (normalized)
> > > > * - Surface CSC (normalized)
> > > > * - Surface regamma LUT (normalized)
> > > > 
> > > > I'm trying to understand what this mapping is doing. A comment mentions
> > > > that is not possible to do these color corrections after blending, so,
> > > > the same color correction pipe is performed on every plane before
> > > > blending?  (is the surface the plane?) Does this adaptation affect the
> > > > expected output?  Moreover, is there something that I misunderstood? :)
> > > > 
> > > 
> > > What's possible to do before and after blending has changed quite a bit
> > > between DCN generations. We program the CRTC Gamma and CTM after blending.
> > > See attached picture for a view relating the color bits between the DRM
> > > interface, DC interface and DCN 3.0 HW blocks.
> > 
> > This picture is really enlightening, thanks!
> > You said it changes between generations, therefore, I can't consider the
> > DCN 2.x family follow the same mapping, right? If so, can you share the
> > main differences for a DCN 2.x regarding per-CRTC properties?
> > 
> 
> See attached diagram for DCN 2.0.

Thanks again!

> 
> > > 
> > > > That said, if the DRM color mgmt supports per-CRTC 3D LUT as the last
> > > 
> > > Where do you see 3D LUT support in DRM? Is there a new proposal that I've
> > > missed?
> > 
> > So, it's exactly what I aim to work: a proposal to add 3D LUT to the
> > current range of DRM per-CRTC color properties. But I also need to
> > understand how this property will be mapped to AMD display once it
> > exists in the DRM framework.
> > 
> 
> Ah, nice to see. :)
> 
> > One of the things that caught my attention after seeing the attached
> > picture is the position of 3D LUT. I was expecting to see the 3D LUT
> > correction after gamma correction. Is this position a particularity of
> > DCN 3.0 (that varies between hw) or was I expecting a wrong color
> > correction pipeline at all?
> > 
> 
> Before DCN 3.0 there was no 3D LUT after blending.
>
By comparing these diagrams, I'm curious: in case we have a per-CRTC 3D
LUT support on DRM, DCN 2.0 generations would initially map this
property as a pre-blending property on DPP (currently the same approach
for CTM, for example), right? But after we also have a per-plane color
management property, those per-CRTC property would be ignored? And how
about degamma for both generations? No problem if there isn't an answer
yet (many if's), but it may help me to think of a more generic solution.

> Note in the diagram that our HW (and DC interface) have a Shaper LUT
> available before the 3D LUT. You could expose if you want to shape your
> content post-blending before applying the 3D LUT.
> 
> The 3D LUT is most effective when it's in non-linear space. Currently
> DRM has no way to specify a way for drm_plane to be linearized (see notes
> (1) and (2)) so it is assumed that you're blending in non-linear space and
> therefore your pixels would already be non-linear going into your 3D LUT.
> 
> (1) unless you use the drm_plane PWL API that was proposed
> (2) amdgpu_dm is currently setting the drm_crtc degamma LUT on the
>     DC plane. This might lead to unexpected behavior when using
>     multiple planes (though I believe gamescope is making use of
>     this behavior).

Thanks for raising these points. In fact, I was considering unexpected
behavior when I saw this DRM <-> DC mapping.
> 
> Have you looked at [1] yet? It might provide a good example on how to
> define a 3D LUT. For AMD HW you'll want a 17x17x17 LUT.
> 
> [1] http://intel.github.io/libva/structVAProcFilterParameterBuffer3DLUT.html

Not yet, but it seems helpful. I'll take as a reference... until now,
I've only examined details on DC drivers.

Thanks,

Melissa

> 
> Harry
> 
> > Melissa
> > 
> > [1] https://lore.kernel.org/all/20201221015730.28333-1-laurent.pinchart+renesas@ideasonboard.com/
> > [2] https://github.com/vsyrjala/linux/commit/4d28e8ddf2a076f30f9e5bdc17cbb4656fe23e69
> > > 
> > > I'm thinking of putting a 3D LUT proposal together but haven't gotten around
> > > to it yet. We'll want a pre-blending 3D LUT, and possible a programmable
> > > post-blending one as well.
> > > 
> > > Thanks,
> > > Harry
> > > 
> > > > step of color correction, I don't see how to accommodate it in the
> > > > mapping above, but I see DC already supports programming 3D LUT on DPP.
> > > > Once DRM has the 3D LUT interface and DM mapped it as a DPP property,
> > > > the 3D LUT will be at the end of the color correction pipeline? Is there
> > > > anything I need to worry about mapping DRM 3D LUT support? Or any
> > > > advice?
> > > > 
> > > > Thanks in advance,
> > > > 
> > > > Melissa
> > 
> > 

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