The ROM early-return checks for PQ, Gamma 2.2, and HLG in
mod_color_calculate_degamma_params() do not consider the map_user_ramp
parameter. When map_user_ramp is true (indicating a subsampled surface
that requires post-scaler degamma via GAMCOR), the function still takes
the ROM path, programming PRE_DEGAM instead.
For subsampled formats the chroma channels must be upsampled by the
scaler before degamma is applied. PRE_DEGAM sits before the scaler in
the pipeline (CNVC domain), while GAMCOR sits after it (CM domain).
Using the ROM for subsampled surfaces means degamma is applied to
subsampled chroma before upsampling, producing incorrect results.
Fix by gating the dcn_arch ROM capability checks on !map_user_ramp,
forcing PQ/Gamma22/HLG through the GAMCOR LUT path when the surface
is subsampled.
Fixes: a8bf71649088 ("drm/amd/display: Internal refactoring to abstract color
caps")
Cc: [email protected]
Assisted-by: Copilot:claude-opus-4.6
Signed-off-by: Harry Wentland <[email protected]>
---
drivers/gpu/drm/amd/display/modules/color/color_gamma.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
b/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
index 03d88e78165d..5786ef5787ac 100644
--- a/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
+++ b/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
@@ -1696,7 +1696,8 @@ bool mod_color_calculate_degamma_params(struct
dc_color_caps *dc_caps,
return true;
if (dc_caps != NULL &&
- dc_caps->dpp.dcn_arch == 1) {
+ dc_caps->dpp.dcn_arch == 1 &&
+ !map_user_ramp) {
if (input_tf->tf == TRANSFER_FUNCTION_PQ &&
dc_caps->dpp.dgam_rom_caps.pq == 1)
--
2.54.0
