linux-kernel - Re: [PATCH] drm/tegra: Enable cmu for Tegra186 and Tegra194

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Message-ID: <72llskwvuwyllvz24zoex4ad6v6t5skiehmwylj7exoh7bmzjo@xq3v7vja5w62>
Date: Tue, 4 Nov 2025 10:14:00 +0100
From: Thierry Reding <thierry.reding@...il.com>
To: Aaron Kling <webgeek1234@...il.com>
Cc: Mikko Perttunen <mperttunen@...dia.com>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Jonathan Hunter <jonathanh@...dia.com>, dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/tegra: Enable cmu for Tegra186 and Tegra194

On Mon, Nov 03, 2025 at 12:39:57PM -0600, Aaron Kling wrote:
> On Mon, Nov 3, 2025 at 5:54 AM Thierry Reding <thierry.reding@...il.com> wrote:
> >
> > On Sat, Nov 01, 2025 at 06:15:17PM -0500, Aaron Kling via B4 Relay wrote:
> > > From: Aaron Kling <webgeek1234@...il.com>
> > >
> > > Without the cmu, nvdisplay will display colors that are notably darker
> > > than intended. The vendor bootloader and the downstream display driver
> > > enable the cmu and sets a sRGB table. Loading that table here results in
> > > the intended colors.
> > >
> > > Signed-off-by: Aaron Kling <webgeek1234@...il.com>
> > > ---
> > >  drivers/gpu/drm/tegra/dc.h  |  13 +++
> > >  drivers/gpu/drm/tegra/sor.c | 206 ++++++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 219 insertions(+)
> >
> > What does "darker than intended" mean? Who defines the intention? How do
> > we know what the intention is? What this patch ultimately seems to be
> > doing is define sRGB to be the default colorspace. Is that always the
> > right default choice? What if people want to specify a different
> > colorspace?
> 
> I reported this issue almost a month ago. See kernel lore [0] and
> freedesktop issue [1]. The pictures in the latter show what nvdisplay
> looks like right now. It's nigh unusably dark. When booted into
> Android with a tv launcher that has a black background, as is default
> for LineageOS, it is really hard to read anything. Is it correct as a
> default? Well, cboot hardcodes this, so... presumably? It would be
> more ideal to expose this and csc to userspace, but I'm not sure if
> drm has a standardized interface for that or if tegra would have to
> make something vendor specific. I think that would be a separate
> change concept compared to setting this default, though.

The reason I'm asking is because I don't recall ever seeing "broken"
colors like you do. So I suspect that this may also be related to what
display is connected, or the mode that we're setting. It could perhaps
also be related to what infoframes we're sending and how these are
supported/interpreted by the attached display.

All of that is to say that maybe this looks broken on the particular
setup that you have but may works fine on other setups. Changing the
default may fix your setup and break others.

Thierry

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