[PATCH 6.1.y] drm/amd/display: Force GAMCOR for subsampled surfaces with PQ/Gamma22/HLG

[Sasha Levin <sashal@kernel.org>]

From: Harry Wentland <harry.wentland@amd.com>

[ Upstream commit 2ffaa7c4050605cb27756f5336f778d1b21268b9 ]

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: stable@vger.kernel.org
Assisted-by: Copilot:claude-opus-4.6
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Tested-by: Robert Mader <robert.mader@collabora.com>
Link: https://patch.msgid.link/20260814211816.1219597-11-harry.wentland@amd.com
[ sashal: Used the existing mapUserRamp parameter name instead of map_user_ramp in the added condition. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 447a0ec9cbe21..b6ba7d14bd8cc 100644
--- a/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
+++ b/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
@@ -1900,7 +1900,8 @@ 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 &&
+			!mapUserRamp) {
 
 			if (input_tf->tf == TRANSFER_FUNCTION_PQ &&
 					dc_caps->dpp.dgam_rom_caps.pq == 1)
-- 
2.53.0