New uAPI for color management proposal and feedback request

Wed May 12 17:59:30 UTC 2021

On Wed, May 12, 2021 at 9:04 AM Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
>
> On Wed, May 12, 2021 at 02:06:56PM +0200, Werner Sembach wrote:
> > Hello,
> >
> > In addition to the existing "max bpc", and "Broadcast RGB/output_csc" drm properties I propose 4 new properties:
> > "preferred pixel encoding", "active color depth", "active color range", and "active pixel encoding"
> >
> >
> > Motivation:
> >
> > Current monitors have a variety pixel encodings available: RGB, YCbCr 4:4:4, YCbCr 4:2:2, YCbCr 4:2:0.
> >
> > In addition they might be full or limited RGB range and the monitors accept different bit depths.
> >
> > Currently the kernel driver for AMD and Intel GPUs automatically configure the color settings automatically with little
> > to no influence of the user. However there are several real world scenarios where the user might disagree with the
> > default chosen by the drivers and wants to set his or her own preference.
> >
> > Some examples:
> >
> > 1. While RGB and YCbCr 4:4:4 in theory carry the same amount of color information, some screens might look better on one
> > than the other because of bad internal conversion. The driver currently however has a fixed default that is chosen if
> > available (RGB for Intel and YCbCr 4:4:4 for AMD). The only way to change this currently is by editing and overloading
> > the edid reported by the monitor to the kernel.
> >
> > 2. RGB and YCbCr 4:4:4 need a higher port clock then YCbCr 4:2:0. Some hardware might report that it supports the higher
> > port clock, but because of bad shielding on the PC, the cable, or the monitor the screen cuts out every few seconds when
> > RGB or YCbCr 4:4:4 encoding is used, while YCbCr 4:2:0 might just work fine without changing hardware. The drivers
> > currently however always default to the "best available" option even if it might be broken.
> >
> > 3. Some screens natively only supporting 8-bit color, simulate 10-Bit color by rapidly switching between 2 adjacent
> > colors. They advertise themselves to the kernel as 10-bit monitors but the user might not like the "fake" 10-bit effect
> > and prefer running at the native 8-bit per color.
> >
> > 4. Some screens are falsely classified as full RGB range wile they actually use limited RGB range. This results in
> > washed out colors in dark and bright scenes. A user override can be helpful to manually fix this issue when it occurs.
> >
> > There already exist several requests, discussion, and patches regarding the thematic:
> >
> > - https://gitlab.freedesktop.org/drm/amd/-/issues/476
> >
> > - https://gitlab.freedesktop.org/drm/amd/-/issues/1548
> >
> > - https://lkml.org/lkml/2021/5/7/695
> >
> > - https://lkml.org/lkml/2021/5/11/416
> >
> >
> > Current State:
> >
> > I only know bits about the Intel i915 and AMD amdgpu driver. I don't know how other driver handle color management
> >
> > - "max bpc", global setting applied by both i915 (only on dp i think?) and amdgpu. Default value is "8". For every
> > resolution + frequency combination the highest possible even number between 6 and max_bpc is chosen. If the range
> > doesn't contain a valid mode the resolution + frequency combination is discarded (but I guess that would be a very
> > special edge case, if existent at all, when 6 doesn't work but 10 would work). Intel HDMI code always checks 8, 12, and
> > 10 and does not check the max_bpc setting.
>
> i915 does limit things below max_bpc for both HDMI and DP.
>
> >
> > - "Broadcast RGB" for i915 and "output_csc" for the old radeon driver (not amdgpu), overwrites the kernel chosen color
> > range setting (full or limited). If I recall correctly Intel HDMI code defaults to full unless this property is set,
> > Intel dp code tries to probe the monitor to find out what to use. amdgpu has no corresponding setting (I don't know how
> > it's decided there).
>
> i915 has the same behaviour for HDMI and DP, as per the CTA-861/DP
> specs. Unfortunately as you already mentioned there are quite a few
> monitors (DP monitors in particular) that don't implemnt the spec
> correctly. IIRC later DP specs even relaxed the wording to say
> that you can basically ignore the spec and do whatever you want.
> Which I supose is just admitting defeat and concluding that there
> is no way to get this right 100% of the time.
>
> >
> > - RGB pixel encoding can be forced by overloading a Monitors edid with one that tells the kernel that only RGB is
> > possible. That doesn't work for YCbCr 4:4:4 however because of the edid specification. Forcing YCbCr 4:2:0 would
> > theoretically also be possible this way. amdgpu has a debugfs switch "force_ycbcr_420" which makes the driver default to
> > YCbCr 4:2:0 on all monitors if possible.
> >
> >
> > Proposed Solution:
> >
> > 1. Add a new uAPI property "preferred pixel encoding", as a per port setting.
> >
> >     - An amdgpu specific implementation was already shared here: https://gitlab.freedesktop.org/drm/amd/-/issues/476
> >
> >     - It also writes back the actually used encoding if the one requested was not possible, overwriting the requested
> > value in the process. I think it would be better to have this feedback channel as a different, read-only property.
> >
> >     - Make this solution vendor agnostic by putting it in the drm_connector_state struct next do max_bpc
> > https://elixir.bootlin.com/linux/v5.13-rc1/source/include/drm/drm_connector.h#L654 and add patches to amdgpu and i915 to
> > respect this setting
> >
> > 2. Convert "Broadcast RGB" to a vendor agnostic setting/replace with a vendor agnostic setting.
> >
> >     - Imho the name is not very fitting, but it pops up in many tutorials throughout the web (some other opinions? how
> > could a rename be handled?".
>
> IIRC there was an attempt to unify this. Not sure what happened to it.

Looks like this set could be resurrected:
https://lists.freedesktop.org/archives/dri-devel/2020-April/262153.html

Alex

>
> >
> >     - Also move it from Intel specific structs to the drm_connector_state struct (please let me know if there is a
> > better place)
> >
> > 3. Strive for full implementation of "max bpc"
> >
> >     - I need to double check the Intel HDMI code.
> >
> > 4. Add 3 feedback channels "active color depth", "active color range", and "active pixel encoding" as vendor agnostic
> > settings in the drm_connector_state struct
> >
> >     - Possible values are:
> >
> >         - unknown, undefined, 6-bit, 8-bit, 9-bit, 10-bit, 11-bit, 12-bit, 14-bit, 16-bit (alternatively: an integer
> > from -1 (unknown), 0 (undefined) to 16, let me know what would be more suitable)
> >
> >         - unknown, undefined, full, limited
> >
> >         - unknown, undefined, rgb, ycbcr444, ycbcr422, ycbcr420
> >
> >     - it's the responsibility of the driver to update the values once the port configuration changes
> >
> >     - if the driver does not support the feedback channels they are set to unknown
> >
> >     - if the driver uses a non listed setting it should set the property to undefined
> >
> >     - A more detailed description why I think these feedback channel are important and should be their own read-only
> > property can be found here: https://lkml.org/lkml/2021/5/11/339
> >
> >
> > Adoption:
> >
> > A KDE dev wants to implement the settings in the KDE settings GUI:
> > https://gitlab.freedesktop.org/drm/amd/-/issues/476#note_912370
> >
> > Tuxedo Computers (my employer) wants to implement the settings desktop environment agnostic in Tuxedo Control Center. I
> > will start work on this in parallel to implementing the new kernel code.
>
> I suspect everyone would be happier to accept new uapi if we had
> multiple compositors signed up to implement it.
>
> >
> >
> > Questions:
> >
> > I'm very curious about feedback from the dri-devel community. Would the concept outlaid above be accepted as new uAPI
> > once it's fully implemented?
> >
> > Where would be the best way to store the new vendor agnostic settings? Following the implementation of max_bpc i would
> > put it in the drm_connector_state struct.
> >
> > My way forward would be to implement the feedback channels first, because they can be very useful for debugging the
> > setting properties afterwards.
>
> For debugging we have dmesg/debugfs/etc. If we add new uapi IMO
> it will have to have some real world use cases beyond debugging.
>
> > I will split each of it up it in 3 or 5 patch sets: 1 for the vendor agnostic part, 1 for
> > Intel (or 2 split up between HDMI and DP), and 1 for AMD (or 2 split up between HDMI and DP)
> >
> > Kind regards,
> >
> > Werner Sembach
> >
>
> --
> Ville Syrjälä
> Intel