linux-kernel - Re: [PATCH] drm/amd/display: Expose active display color configurations to userspace

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Message-ID: <20210511110756.7e754c30@eldfell>
Date:   Tue, 11 May 2021 11:07:56 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Alex Deucher <alexdeucher@...il.com>,
        Werner Sembach <wse@...edocomputers.com>
Cc:     Dave Airlie <airlied@...ux.ie>,
        LKML <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        "Leo \(Sunpeng\) Li" <sunpeng.li@....com>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        "Deucher, Alexander" <alexander.deucher@....com>,
        Christian Koenig <christian.koenig@....com>
Subject: Re: [PATCH] drm/amd/display: Expose active display color
 configurations to userspace

On Mon, 10 May 2021 17:47:01 -0400
Alex Deucher <alexdeucher@...il.com> wrote:

> On Fri, May 7, 2021 at 3:27 PM Werner Sembach <wse@...edocomputers.com> wrote:
> >
> > xrandr --prop and other userspace info tools have currently no way of
> > telling which color configuration is used on HDMI and DP ports.
> >
> > The ongoing transsition from HDMI 1.4 to 2.0 and the different bandwidth
> > requirements of YCbCr 4:2:0 and RGB color format raise different
> > incompatibilities. Having these configuration information readily
> > available is a useful tool in debuging washed out colors, color artefacts
> > on small fonts and missing refreshrate options.  
> 
> I think we would ideally want these as generic connector properties
> rather than AMD specific ones since they are not really AMD specific.
> I believe there is already a generic drm property (max_bpc) for the
> color depth.  At this point, I think having a generic RGB vs YCbCr
> property would make sense.  I'm not sure about the color space.

Hi,

I believe that userspace will definitely want to know what exactly is
going on on the monitor cable, so I think exposing more information is
good, but I agree that it needs to be done with generic KMS properties
as much as possible. Userspace is not going to bother having explicit
code for driver-specific properties.

I think a major use case will be Wayland color management, where a
Wayland compositor will want to make sure that the video signal
parameters have not changed since the monitor was last measured
(profiled). If the signal configuration is different, the measured
color profile may be invalid and therefore the end user needs to be
warned. See some ideas in
https://gitlab.freedesktop.org/wayland/weston/-/issues/467
under the heading "Color calibration auditing system".

About the color space: is that something a kernel driver will decide
on its own? I mean in the same sense as the driver will negotiate
HDMI/DP link parameters, perhaps falling back to smaller requirements
if higher requirements signal does not seem to work.

We only need readback properties for things that generic userspace
won't or cannot control explicitly, e.g. because the kernel driver has
room to make a choice rather than fail.

Thanks,
pq

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