linux-kernel - Re: [PATCH v4 03/17] drm/uAPI: Add "active bpc" as feedback channel for "max bpc" drm property

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Date:   Thu, 15 Jul 2021 12:10:02 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Werner Sembach <wse@...edocomputers.com>
Cc:     sunpeng.li@....com, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        airlied@...ux.ie, amd-gfx@...ts.freedesktop.org,
        tzimmermann@...e.de, rodrigo.vivi@...el.com,
        alexander.deucher@....com, christian.koenig@....com
Subject: Re: [PATCH v4 03/17] drm/uAPI: Add "active bpc" as feedback channel
 for "max bpc" drm property

On Wed, 14 Jul 2021 20:18:57 +0200
Werner Sembach <wse@...edocomputers.com> wrote:

> Am 01.07.21 um 13:30 schrieb Werner Sembach:
> > Am 01.07.21 um 09:42 schrieb Pekka Paalanen:  
> >> On Wed, 30 Jun 2021 11:42:10 +0200
> >> Werner Sembach <wse@...edocomputers.com> wrote:
> >>  
> >>> Am 30.06.21 um 10:21 schrieb Pekka Paalanen:  
> >>>> On Tue, 29 Jun 2021 13:02:05 +0200
> >>>> Werner Sembach <wse@...edocomputers.com> wrote:
> >>>>     
> >>>>> Am 28.06.21 um 19:03 schrieb Werner Sembach:  
> >>>>>> Am 18.06.21 um 11:11 schrieb Werner Sembach:  
> >>>>>>> Add a new general drm property "active bpc" which can be used by graphic
> >>>>>>> drivers to report the applied bit depth per pixel back to userspace.
> >>>>>>>
> >>>>>>> While "max bpc" can be used to change the color depth, there was no way to
> >>>>>>> check which one actually got used. While in theory the driver chooses the
> >>>>>>> best/highest color depth within the max bpc setting a user might not be
> >>>>>>> fully aware what his hardware is or isn't capable off. This is meant as a
> >>>>>>> quick way to double check the setup.
> >>>>>>>
> >>>>>>> In the future, automatic color calibration for screens might also depend on
> >>>>>>> this information being available.
> >>>>>>>
> >>>>>>> Signed-off-by: Werner Sembach <wse@...edocomputers.com>
> >>>>>>> ---
> >>>>>>>    drivers/gpu/drm/drm_connector.c | 51 +++++++++++++++++++++++++++++++++
> >>>>>>>    include/drm/drm_connector.h     |  8 ++++++
> >>>>>>>    2 files changed, 59 insertions(+)

> New idea: Instead of the "active"-properties with various if cases in 
> the kernel code, there could just be blob properties exposing the hdmi 
> infoframes, hdmi general control packages, dp misc0 and misc1 and dp vsc 
> sdp.
> 
> Combined they have all the color information and it is made sure that 
> it's what is actually send to the monitor (I would consider sending 
> something differed then what is told in the infoframes a bug).
> 
> They also have built in version numbers, if in the future they contain 
> more information.
> 
> Only disadvantage: We leave parsing for human readable output to the 
> userspace.
> 
> Alternatively keep the "active"-properties but fill them from the 
> infoframes.
> 
> I'm not entirely sure where to do that on amd, because there the 
> infoframes are directly created in the dc code shortly before writing 
> them to the hardware registers and immediately forgotten afterwards. But 
> you still have access to the connector struct from that code so the 
> property could be updated directly there.
> 

Hi,

I'm not fundamentally against that as long as we have a common
userspace library to parse those blobs. In libdrm perhaps? Or a new
library?

But I also don't know about the technical feasibility, is it a good
idea.

OTOH, that could be the best thing for testing drivers vs. KMS UAPI
when you don't have a hardware HDMI/DP grabber to inspect the
infoframes. So maybe kernel CI would like that?


Thanks,
pq

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