[PATCH 5.10.y] drm/amd/display: Fix swap in median3() for DSC rate control

[Sasha Levin <sashal@kernel.org>]

From: Keita Morisaki <kmta1236@gmail.com>

[ Upstream commit 7b23bf82ccbdb4cf0752f2b8686e7319a7ce497f ]

median3() is meant to return the median of three values, but its last
compare-and-swap step swaps b and c instead of a and b. When c is
smaller than both a and b, it returns max(a, b) instead of min(a, b).
For example, median3(2, 3, 1) returns 3 instead of 2.

_do_calc_rc_params() always passes 0 as the first argument, and the
second argument is never below 12. So whenever the third argument is
negative, first_line_bpg_offset (and second_line_bpg_offset for 4:2:0)
is set to the second argument instead of 0.

Fix this by swapping a and b in the last step, as intended.

Note that this change does not affect valid DSC configurations. The
third argument is negative only when bits per pixel exceeds the
uncompressed rate of the colour format:

 - 3 * bpc for 4:4:4, 3 * bpc + 2 for RGB

 - 2 * bpc for 4:2:2

 - 1.5 * bpc for 4:2:0

Fixes: 97bda0322b8a ("drm/amd/display: Add DSC support for Navi (v2)")
Signed-off-by: Keita Morisaki <kmta1236@gmail.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 1ddc4bc97a62fc89d9d3681413d9f977e1fa1a83)
Cc: stable@vger.kernel.org
[ sashal: Adjusted the target source location because median3() has not been relocated. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/amd/display/dc/dsc/rc_calc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/dsc/rc_calc.c b/drivers/gpu/drm/amd/display/dc/dsc/rc_calc.c
index 4da21966ddcec..c860e536e98e2 100644
--- a/drivers/gpu/drm/amd/display/dc/dsc/rc_calc.c
+++ b/drivers/gpu/drm/amd/display/dc/dsc/rc_calc.c
@@ -169,7 +169,7 @@ static int median3(int a, int b, int c)
 	if (b > c)
 		swap(b, c);
 	if (a > b)
-		swap(b, c);
+		swap(a, b);
 
 	return b;
 }
-- 
2.53.0