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

6 files changed, 13 insertions, 25 deletions

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index c6fd34bab358..df3c42bb9274 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -6876,7 +6876,7 @@ static int dm_encoder_helper_atomic_check(struct drm_encoder *encoder,
 								    max_bpc);
 		bpp = convert_dc_color_depth_into_bpc(color_depth) * 3;
 		clock = adjusted_mode->clock;
-		dm_new_connector_state->pbn = drm_dp_calc_pbn_mode(clock, bpp, false);
+		dm_new_connector_state->pbn = drm_dp_calc_pbn_mode(clock, bpp << 4);
 	}
 
 	dm_new_connector_state->vcpi_slots =
diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
index 57230661132b..2afd1bc74978 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
@@ -1636,7 +1636,7 @@ enum dc_status dm_dp_mst_is_port_support_mode(
 	} else {
 		/* check if mode could be supported within full_pbn */
 		bpp = convert_dc_color_depth_into_bpc(stream->timing.display_color_depth) * 3;
-		pbn = drm_dp_calc_pbn_mode(stream->timing.pix_clk_100hz / 10, bpp, false);
+		pbn = drm_dp_calc_pbn_mode(stream->timing.pix_clk_100hz / 10, bpp << 4);
 
 		if (pbn > aconnector->mst_output_port->full_pbn)
 			return DC_FAIL_BANDWIDTH_VALIDATE;
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);
 
diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
index 4625900416e9..6f9c9f3b5b12 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
@@ -107,8 +107,7 @@ static int intel_dp_mst_find_vcpi_slots_for_bpp(struct intel_encoder *encoder,
 			continue;
 
 		crtc_state->pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock,
-						       dsc ? bpp << 4 : bpp,
-						       dsc);
+						       bpp << 4);
 
 		slots = drm_dp_atomic_find_time_slots(state, &intel_dp->mst_mgr,
 						      connector->port,
@@ -982,7 +981,7 @@ intel_dp_mst_mode_valid_ctx(struct drm_connector *connector,
 		return ret;
 
 	if (mode_rate > max_rate || mode->clock > max_dotclk ||
-	    drm_dp_calc_pbn_mode(mode->clock, min_bpp, false) > port->full_pbn) {
+	    drm_dp_calc_pbn_mode(mode->clock, min_bpp << 4) > port->full_pbn) {
 		*status = MODE_CLOCK_HIGH;
 		return 0;
 	}
diff --git a/drivers/gpu/drm/nouveau/dispnv50/disp.c b/drivers/gpu/drm/nouveau/dispnv50/disp.c
index 52f1569ee37c..3d390f5029f6 100644
--- a/drivers/gpu/drm/nouveau/dispnv50/disp.c
+++ b/drivers/gpu/drm/nouveau/dispnv50/disp.c
@@ -982,8 +982,7 @@ nv50_msto_atomic_check(struct drm_encoder *encoder,
 		const int clock = crtc_state->adjusted_mode.clock;
 
 		asyh->or.bpc = connector->display_info.bpc;
-		asyh->dp.pbn = drm_dp_calc_pbn_mode(clock, asyh->or.bpc * 3,
-						    false);
+		asyh->dp.pbn = drm_dp_calc_pbn_mode(clock, asyh->or.bpc * 3 << 4);
 	}
 
 	mst_state = drm_atomic_get_mst_topology_state(state, &mstm->mgr);
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 545beea33e8c..e3c818dfc0e6 100644
--- a/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c
+++ b/drivers/gpu/drm/tests/drm_dp_mst_helper_test.c
@@ -42,13 +42,13 @@ static const struct drm_dp_mst_calc_pbn_mode_test drm_dp_mst_calc_pbn_mode_cases
 		.clock = 332880,
 		.bpp = 24,
 		.dsc = true,
-		.expected = 50
+		.expected = 1191
 	},
 	{
 		.clock = 324540,
 		.bpp = 24,
 		.dsc = true,
-		.expected = 49
+		.expected = 1161
 	},
 };
 
@@ -56,7 +56,7 @@ static void drm_test_dp_mst_calc_pbn_mode(struct kunit *test)
 {
 	const struct drm_dp_mst_calc_pbn_mode_test *params = test->param_value;
 
-	KUNIT_EXPECT_EQ(test, drm_dp_calc_pbn_mode(params->clock, params->bpp, params->dsc),
+	KUNIT_EXPECT_EQ(test, drm_dp_calc_pbn_mode(params->clock, params->bpp << 4),
 			params->expected);
 }