per-plane LUTs and CSCs?

[Alex Deucher]

On Thu, Sep 10, 2020 at 4:29 AM Simon Ser <contact at emersion.fr> wrote:
>
> On Thursday, September 10, 2020 10:18 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
>
> > On Thu, Sep 10, 2020 at 07:50:59AM +0000, Simon Ser wrote:
> >
> > > On Wednesday, September 9, 2020 12:57 PM, Laurentiu Palcu laurentiu.palcu at oss.nxp.com wrote:
> > >
> > > > Hi all,
> > > > I was wondering whether you could give me an advise on how to proceed further
> > > > with the following issue as I'm preparing to upstream the next set of patches
> > > > for the iMX8MQ display controller(DCSS). The display controller has 3 planes,
> > > > each with 2 CSCs and one degamma LUT. The CSCs are in front and after the LUT
> > > > respectively. Then the output from those 3 pipes is blended together and then
> > > > gamma correction is applied using a linear-to-nonlinear LUT and another CSC, if
> > > > needed.
> > > > Currently, downstream, we have all those CSCs and LUTs hard-coded into a header
> > > > file. Based on the colorspace, range, gamut selected for the output and/or
> > > > plane input, we pick up the right CSCs and LUTs from that header file to
> > > > configure our pipes... I guess this solution does the job, userspace doesn't
> > > > need to care much about how to generate those tables. But, it's also not very
> > > > flexible in case there is an app smart enough to know and actually generate
> > > > their own custom tables. :/
> > > > Looking through the dri-devel archives, I've seen that there was a tentative to
> > > > implement a more or less generic per-plane LUT/CSC solution but it didn't make
> > > > it in due to lack of userspace consumers...
> > >
> > > Apart from full color management mentioned by Pekka, are there other
> > > use-cases for these per-plane props?
> > > One thing I can think of is that some drivers annoyingly only apply the
> > > per-CRTC gamma LUT to the primary plane. I think it would make more
> > > sense to not attach a gamma prop to the CRTC and instead only attach it
> > > to the primary plane to make that clear. But I guess that would also
> > > break existing user-space?
> >
> > Uh, which drivers? This would be a driver bug (and there's been plenty of
> > those where the cursor has the wrong color temp and fun stuff like that).
> > We need to fix these driver issues instead of lamenting in userspace that
> > it's all kinda broken :-)
>
> Apparently this is bug with the old radeon driver [1]. It works fine on
> all i915 setups I've tried, and also works fine on amdgpu (with DC).
>
> I've opened a radeon bug.
>
> [1]: https://github.com/swaywm/wlroots/issues/968

This is a hardware limitation.  I suspend all hardware of a certain
age had this same limitation.  Old stuff didn't have multiple planes.
It had a primary plane and a cursor and gamma didn't apply to the
cursor.  The hardware supported by radeon goes back more than 20
years.

Alex


>
> > > The gamma LUT is currently used by some compositors to implement "night
> > > light"/redshift-like features.
> > >
> > > > Adding CSC and degamma LUT properties for each plane as well as a gamma
> > > > LUT and CSC for CRTC, would help get rid of the LUT/CSC header and rely
> > > > entirely on userspace to fill in those tables. But userspace has to know
> > > > how to generate those LUTs and colorspace conversion matrices in the
> > > > first place...
> > >
> > > So it wouldn't be enough for user-space to fill these gamma LUTs with
> > > linear ramps and get colors that look fine? Would the process be more
> > > involved? Would user-space need to know about the driver and generate
> > > gamma LUTs depending on the driver?
> > > That would be a show-stopper.
> >
> > Yeah we can't expose such an uapi, instead what I have in mind that the
> > in-kernel lut/csc for yuv/broadcast rgb and whatever else are mixed in
> > with what userspace provides. That means a slight loss of precision and
> > we'll have to have a nice fixed point math library for this stuff in the
> > kernel. But color correction hw is too special between vendors that I
> > don't think there's any other solution. E.g. even on intel we sometimes
> > have dedicated register bits/hw functions for yuv/rgb broadcast value
> > range and whatever else, and on other platforms we need to do that by
> > adjusting the lut/csc.
>
> Yeah, that makes sense to me.
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