New uAPI for color management proposal and feedback request

[Werner Sembach]

Am 19.05.21 um 15:49 schrieb Ville Syrjälä:
> On Wed, May 19, 2021 at 12:34:05PM +0300, Pekka Paalanen wrote:
>> On Wed, 12 May 2021 16:04:16 +0300
>> 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
>>>>
>> ...
>>
>>>> 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.
>> I think having Weston support for these would be good, but for now it
>> won't be much of an UI: just weston.ini to set, and the log to see what
>> happened.
>>
>> However, knowing what happened is going to be important for color
>> calibration auditing:
>> https://gitlab.freedesktop.org/wayland/weston/-/issues/467
>>
>> Yes, please, very much for read-only properties for the feedback part.
>> Properties that both userspace and kernel will write are hard to deal
>> with in general.
>>
>> Btw. "max bpc" I can kind of guess that conversion from framebuffer
>> format to the wire bpc happens automatically and only as the final
>> step,
> Well, there could be dithering and whatnot also involved. So it's
> not super well specified atm either.
>
>> but "Broadcast RGB" is more complicated: is the output from the
>> abstract pixel pipeline sent as-is and "Broadcast RGB" is just another
>> inforframe bit to the monitor, or does "Broadcast RGB" setting actually
>> change what happens in the pixel pipeline *and* set infoframe bits?
> It does indeed compress the actual pixel data. There was once a patch
> porposed to introduce a new enum value that only sets the infoframe and
> thus would allow userspace to pass through already limited range data.
> Shouldn't be hard to resurrect that if needed.

For the time being I try to keep the functionality of Broadcast RGB the same and just port it over to AMDGPU, but i
haven't looked into it in detail yet.

>
>> My vague recollection is that framebuffer was always assumed to be in
>> full range, and then if "Broadcast RGB" was set to limited range, the
>> driver would mangle the pixel pipeline to convert from full to limited
>> range. This means that it would be impossible to have limited range
>> data in a framebuffer, or there might be a double-conversion by
>> userspace programming a LUT for limited->full and then the driver
>> adding full->limited. I'm also confused how full/limited works when
>> framebuffer is in RGB/YCbCr and the monitor wire format is in RGB/YCbCr
>> and there may be RGB->YCbCR or YCbCR->RGB conversions going on - or
>> maybe even FB YCbCR -> RGB -> DEGAMMA -> CTM -> GAMMA -> YCbCR.
>>
>> I wish someone drew a picture of the KMS abstract pixel pipeline with
>> all the existing KMS properties in it. :-)
> Here's an ugly one for i915:
>
>     (input RGB vs. YCbCr?)
> [FB] -> [YCbCr?] -> [YCbCr->RGB conversion     ] -> [plane blending] -> ...
>       |             [YCbCr color range/encoding] |
>       \ [RGB?] ----------------------------------/
>
>                                            (output RGB limited vs. RGB full vs. YCbCr?)
> ... -> [DEGAMMA_LUT] -> [CTM] -> [GAMMA_LUT] -> [YCbCr?] -> [RGB->YCbCr conversion      ] -> [to port]
>                                               |             [always BT.709/limited range]
>                                               \ [RGB?] -> ...
>
> ... -> [RGB passthrough             ] -> [to port]
>      | [Broadcast RGB=full or       ]
>      | [Broadcast RGB=auto + IT mode]
>      |
>      \ [RGB full->limited conversion] -> [to port]
>        [Broadcast RGB=limited or    ]
>        [Broadcast RGB=auto + CE mode]
>
> I guess having something like that in the docs would be nice. Not sure
> if there's a way to make something that looks decent for html/etc.
>