[RFC PATCH 1/3] drm/color: Add RGB Color encodings

[Harry Wentland]

On 2021-04-26 2:07 p.m., Ville Syrjälä wrote:
> On Mon, Apr 26, 2021 at 01:38:50PM -0400, Harry Wentland wrote:
>> From: Bhawanpreet Lakha <Bhawanpreet.Lakha at amd.com>
>>
>> Add the following color encodings
>> - RGB versions for BT601, BT709, BT2020
>> - DCI-P3: Used for digital movies
>>
>> Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha at amd.com>
>> Signed-off-by: Harry Wentland <harry.wentland at amd.com>
>> ---
>>   drivers/gpu/drm/drm_color_mgmt.c | 4 ++++
>>   include/drm/drm_color_mgmt.h     | 4 ++++
>>   2 files changed, 8 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
>> index bb14f488c8f6..a183ebae2941 100644
>> --- a/drivers/gpu/drm/drm_color_mgmt.c
>> +++ b/drivers/gpu/drm/drm_color_mgmt.c
>> @@ -469,6 +469,10 @@ static const char * const color_encoding_name[] = {
>>   	[DRM_COLOR_YCBCR_BT601] = "ITU-R BT.601 YCbCr",
>>   	[DRM_COLOR_YCBCR_BT709] = "ITU-R BT.709 YCbCr",
>>   	[DRM_COLOR_YCBCR_BT2020] = "ITU-R BT.2020 YCbCr",
>> +	[DRM_COLOR_RGB_BT601] = "ITU-R BT.601 RGB",
>> +	[DRM_COLOR_RGB_BT709] = "ITU-R BT.709 RGB",
>> +	[DRM_COLOR_RGB_BT2020] = "ITU-R BT.2020 RGB",
>> +	[DRM_COLOR_P3] = "DCI-P3",
> 
> These are a totally different thing than the YCbCr stuff.
> The YCbCr stuff just specifies the YCbCr<->RGB converison matrix,
> whereas these are I guess supposed to specify the primaries/whitepoint?
> But without specifying what we're converting *to* these mean absolutely
> nothing. Ie. I don't think they belong in this property.
> 

If this is the intention I don't see it documented.

I might have overlooked something but do we have a way to explicitly 
specify today what *to* format the YCbCr color encodings convert into? 
Would that be a combination of the output color encoding specified via 
colorspace_property and the color space encoded in the primaries and 
whitepoint of the hdr_output_metadata?

Fundamentally I don't see how the use of this property differs, whether 
you translate from YCbCr or from RGB. In either case you're converting 
from the defined input color space and pixel format to an output color 
space and pixel format.

> The previous proposals around this topic have suggested a new
> property to specify a conversion matrix either explicitly, or
> via a separate enum (which would specify both the src and dst
> colorspaces). I've always argued the enum approach is needed
> anyway since not all hardware has a programmable matrix for
> this. But a fully programmable matrix could be nice for tone
> mapping purposes/etc, so we may want to make sure both are
> possible.
> 
> As for the transfer func, the proposals so far have mostly just
> been to expose a programmable degamma/gamma LUTs for each plane.
> But considering how poor the current gamma uapi is we've thrown
> around some ideas how to allow the kernel to properly expose the
> hw capabilities. This is one of those ideas:
> https://lists.freedesktop.org/archives/dri-devel/2019-April/212886.html>> I think that basic idea could be also be extended to allow fixed
> curves in case the hw doesn't have a fully programmable LUT. But
> dunno if that's relevant for your hw.
> 

The problem with exposing gamma, whether per-plane or per-crtc, is that 
it is hard to define an API that works for all the HW out there. The 
capabilities for different HW differ a lot, not just between vendors but 
also between generations of a vendor's HW.

Another reason I'm proposing to define the color space (and gamma) of a 
plane is to make this explicit. Up until the color space and gamma of a 
plane or framebuffer are not well defined, which leads to drivers 
assuming the color space and gamma of a buffer (for blending and other 
purposes) and might lead to sub-optimal outcomes.

Harry