[PATCH 09/17] drm/dp_mst: Fix fractional bpp scaling in drm_dp_calc_pbn_mode()

[Imre Deak]

For fractional bpp values passed to the function in a .4 fixed point
format, the fractional part is currently ignored due to scaling bpp too
early. Fix this by scaling the overhead factor instead and to avoid an
overflow multiplying bpp with the overhead factor instead of the clock
rate.

While at it simplify the formula, and pass the expected fixed point bpp
values in the kunit tests.

Signed-off-by: Imre Deak <imre.deak at intel.com>
---
 drivers/gpu/drm/display/drm_dp_mst_topology.c  | 7 ++-----
 drivers/gpu/drm/tests/drm_dp_mst_helper_test.c | 8 ++++----
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index ed96cfcfa3040..bd0f35a0ea5fb 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -4712,12 +4712,9 @@ int drm_dp_calc_pbn_mode(int clock, int bpp, bool dsc)
 	 * factor in the numerator rather than the denominator to avoid
 	 * integer overflow
 	 */
+	u32 bpp_m = (dsc ? 64 / 16 : 64) * 1006 * bpp;
 
-	if (dsc)
-		return DIV_ROUND_UP_ULL(mul_u32_u32(clock * (bpp / 16), 64 * 1006),
-					8 * 54 * 1000 * 1000);
-
-	return DIV_ROUND_UP_ULL(mul_u32_u32(clock * bpp, 64 * 1006),
+	return DIV_ROUND_UP_ULL(mul_u32_u32(clock, bpp_m),
 				8 * 54 * 1000 * 1000);
 }
 EXPORT_SYMBOL(drm_dp_calc_pbn_mode);
diff --git a/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c b/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c
index 545beea33e8c7..ea2182815ebe8 100644
--- a/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c
+++ b/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c
@@ -40,15 +40,15 @@ static const struct drm_dp_mst_calc_pbn_mode_test drm_dp_mst_calc_pbn_mode_cases
 	},
 	{
 		.clock = 332880,
-		.bpp = 24,
+		.bpp = 24 << 4,
 		.dsc = true,
-		.expected = 50
+		.expected = 1191
 	},
 	{
 		.clock = 324540,
-		.bpp = 24,
+		.bpp = 24 << 4,
 		.dsc = true,
-		.expected = 49
+		.expected = 1161
 	},
 };
 
-- 
2.37.2