[PATCH 6.18.y] drm/amd/display: Fix fast updates on DCE 8.1

[Sasha Levin <sashal@kernel.org>]

From: Timur Kristóf <timur.kristof@gmail.com>

[ Upstream commit 687d31ea9ac76b299817060fa1e5582d6c752ae0 ]

DCE8 should use legacy fast updates.
This was already set for DCE 8.0 and 8.3, but it seems
that DCE 8.1 (found in Kaveri chips) was forgotten.

This fix should be backported to all kernel versions that
have "Refactor fast update to use new HWSS build sequence",
but this patch won't apply cleanly to older kernels due to
another refactor in "Remove dc param from check_update".

Fixes: 0baae6246307 ("drm/amd/display: Refactor fast update to use new HWSS build sequence")
Fixes: 9ec11bb842b6 ("drm/amd/display: Remove dc param from check_update")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patch.msgid.link/20260923122033.1046651-2-timur.kristof@gmail.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 57605379559e3b5044c8274a1010ec658e5e73af)
Cc: stable@vger.kernel.org
[ sashal: omitted dc->check_config initialization because enable_legacy_fast_update remains in dc->debug. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/amd/display/dc/resource/dce80/dce80_resource.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/amd/display/dc/resource/dce80/dce80_resource.c b/drivers/gpu/drm/amd/display/dc/resource/dce80/dce80_resource.c
index b645c0c8ec3c5..9874d2cb632ce 100644
--- a/drivers/gpu/drm/amd/display/dc/resource/dce80/dce80_resource.c
+++ b/drivers/gpu/drm/amd/display/dc/resource/dce80/dce80_resource.c
@@ -1121,6 +1121,7 @@ static bool dce81_construct(
 	dc->caps.max_cursor_size = 128;
 	dc->caps.min_horizontal_blanking_period = 80;
 	dc->caps.is_apu = true;
+	dc->debug = debug_defaults;
 
 	/*************************************************
 	 *  Create resources                             *
-- 
2.53.0