Re: [V7 43/45] drm/amd/display: Add AMD color pipeline doc

[Harry Wentland <harry.wentland@amd.com>]

On 2025-01-17 04:19, Pekka Paalanen wrote:
> On Thu, 19 Dec 2024 21:33:49 -0700
> Alex Hung <alex.hung@amd.com> wrote:
> 
>> From: Harry Wentland <harry.wentland@amd.com>
>>
>> Add kernel doc for AMD color pipeline.
>>
>> Signed-off-by: Alex Hung <alex.hung@amd.com>
>> Signed-off-by: Harry Wentland <harry.wentland@amd.com>
>> ---
>>  .../amd/display/amdgpu_dm/amdgpu_dm_color.c   | 122 +++++++++++++++---
>>  1 file changed, 102 insertions(+), 20 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
>> index 5e8c5c0657c4..56fb0870a2fc 100644
>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
> 
> ...
> 
>> + * AMD plane color pipeline
>> + * ------------------------
>> + *
>> + * The AMD &drm_plane color pipeline is advertised for DCN generations
>> + * 3.0 and newer. It exposes these elements in this order:
>> + *
>> + * 1. 1D curve colorop
>> + * 2. Multiplier
>> + * 3. 3x4 CTM
>> + * 4. 1D curve colorop
>> + * 5. 1D LUT
>> + * 6. 3D LUT
>> + * 7. 1D curve colorop
>> + * 8. 1D LUT
>> + *
>> + * The multiplier (#2) is a simple multiplier that is applied to all
>> + * channels.
>> + *
>> + * The 3x4 CTM (#3) is a simple 3x4 matrix.
> 
> Hi,
> 
> I'm curious of why does the multiplier exist? Under what conditions is
> it not feasible to precompute the multiplier into the CTM coefficients
> and must instead be programmed as a separate colorop?
> 

AMD HW only supports S2.13 and S3.13 for the CTM coefficients [1] and
our driver only ever picks S2.13. The multiplier is programmed as a
floating point value instead.

[1] https://gitlab.freedesktop.org/agd5f/linux/-/blob/amd-staging-drm-next/drivers/gpu/drm/amd/display/dc/dc_hw_types.h#L1089

Harry

> 
> Thanks,
> pq
> 
>>   *
>> - * The pipe blending also happens after these blocks so we don't actually
>> - * support any CRTC props with correct blending with multiple planes - but we
>> - * can still support CRTC color management properties in DM in most single
>> - * plane cases correctly with clever management of the DC interface in DM.
>> + * #1, and #7 are non-linear to linear curves. #4 is a linear to
>> + * non-linear curve. They support sRGB, PQ, and BT.709/BT.2020 EOTFs or
>> + * their inverse.
>>   *
>> - * As per DRM documentation, blocks should be in hardware bypass when their
>> - * respective property is set to NULL. A linear DGM/RGM LUT should also
>> - * considered as putting the respective block into bypass mode.
>> + * The 1D LUTs (#5 and #8) are plain 4096 entry LUTs.
>>   *
>> - * This means that the following
>> - * configuration is assumed to be the default:
>> + * The 3DLUT (#6) is a tetrahedrally interpolated 17 cube LUT.
>>   *
>> - * Plane DGM Bypass -> Plane CTM Bypass -> Plane RGM Bypass -> ...
>> - * CRTC DGM Bypass -> CRTC CTM Bypass -> CRTC RGM Bypass
>>   */
>>  
>>  #define MAX_DRM_LUT_VALUE 0xFFFF
>