drm/amd/display: use halving distribution for PQ/sRGB linearizing LUT

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When linearizing, the input is an encoded signal bounded to [0,1] and
PQ/sRGB EOTFs are steepest near 1, requiring more precision near the
bright end.

Take the 8-bit sRGB case as a reference: 256 possible inputs and 256 HW
LUT points line up, so the LUT acts as plain indexing. Float
representations don't land perfectly, but LERP-ing between two HW
entries, when input is within a small epsilon of one of them, doesn't
materially change the result.

Replace the uniform 12-region distribution (16 points each,
192 total, range [2^-12, 1]) with a 9-region halving distribution for
the PQ/sRGB pre-defined EOTF: 128 points in the top region [0.5, 1], 64
in the next, 32 in the next, and so on, down to 1 point in each of the
two darkest regions. Total samples grow from 192 to 256, with uniform
1/256 spacing across [0, 1]. The dark tail below 2^-9 is no longer
sampled separately, which is acceptable for PQ/sRGB.

Suggested-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Tested-by: Matthew Schwartz <matthew.schwartz@linux.dev>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

...

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Melissa Wen 2026-06-23 17:59:01 +02:00 • committed by Alex Deucher

parent 619e5b7e45

commit a71d2b051f

1 changed files with 24 additions and 9 deletions

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drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c

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