FAILED: patch "[PATCH] drm/amd/display: Force GAMCOR for subsampled surfaces with PQ/Gamma22/HLG" failed to apply to 6.1-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 2ffaa7c4050605cb27756f5336f778d1b21268b9
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107054051-failed-6.1-2ffaa7c40506@kernel.org' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 2ffaa7c4050605cb27756f5336f778d1b21268b9 Mon Sep 17 00:00:00 2001
From: Harry Wentland <harry.wentland@amd.com>
Date: Fri, 14 Aug 2026 17:18:16 -0400
Subject: [PATCH] drm/amd/display: Force GAMCOR for subsampled surfaces with
 PQ/Gamma22/HLG

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
---
 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 03d88e78165d5..5786ef5787ac6 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)
-- 
2.53.0