Re: New uAPI for color management proposal and feedback request

[Werner Sembach <wse@tuxedocomputers.com>]

Am 12.05.21 um 19:59 schrieb Alex Deucher:
> On Wed, May 12, 2021 at 9:04 AM Ville Syrjälä
> <ville.syrjala@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
Thanks, I will look into it.
>
>>>     - 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