On Wed, May 12, 2021 at 9:04 AM Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> 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
