drm/dp_mst: Fix fractional DSC bpp handling

The current code does '(bpp << 4) / 16' in the MST PBN
calculation, but that is just the same as 'bpp' so the
DSC codepath achieves absolutely nothing. Fix it up so that
the fractional part of the bpp value is actually used instead
of truncated away. 64*1006 has enough zero lsbs that we can
just shift that down in the dividend and thus still manage
to stick to a 32bit divisor.

And while touching this, let's just make the whole thing more
straightforward by making the passed in bpp value .4 binary
fixed point always, instead of having to pass in different
things based on whether DSC is enabled or not.

v2:
- Fix DSC kunit test cases.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: David Francis <David.Francis@amd.com>
Cc: Mikita Lipski <mikita.lipski@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Fixes: dc48529fb14e ("drm/dp_mst: Add PBN calculation for DSC modes")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[Imre: Fix kunit test cases]
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-3-imre.deak@intel.com

1 files changed, 5 insertions, 15 deletions

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index c490e8befc2f..bc2c9dfe229f 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -4718,13 +4718,12 @@ EXPORT_SYMBOL(drm_dp_check_act_status);
 
 /**
  * drm_dp_calc_pbn_mode() - Calculate the PBN for a mode.
- * @clock: dot clock for the mode
- * @bpp: bpp for the mode.
- * @dsc: DSC mode. If true, bpp has units of 1/16 of a bit per pixel
+ * @clock: dot clock
+ * @bpp: bpp as .4 binary fixed point
  *
  * This uses the formula in the spec to calculate the PBN value for a mode.
  */
-int drm_dp_calc_pbn_mode(int clock, int bpp, bool dsc)
+int drm_dp_calc_pbn_mode(int clock, int bpp)
 {
 	/*
 	 * margin 5300ppm + 300ppm ~ 0.6% as per spec, factor is 1.006
@@ -4735,18 +4734,9 @@ int drm_dp_calc_pbn_mode(int clock, int bpp, bool dsc)
 	 * peak_kbps *= (1006/1000)
 	 * peak_kbps *= (64/54)
 	 * peak_kbps *= 8    convert to bytes
-	 *
-	 * If the bpp is in units of 1/16, further divide by 16. Put this
-	 * factor in the numerator rather than the denominator to avoid
-	 * integer overflow
 	 */
-
-	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),
-				8 * 54 * 1000 * 1000);
+	return DIV_ROUND_UP_ULL(mul_u32_u32(clock * bpp, 64 * 1006 >> 4),
+				1000 * 8 * 54 * 1000);
 }
 EXPORT_SYMBOL(drm_dp_calc_pbn_mode);