[PATCH AUTOSEL 6.7 18/88] drm/amd/display: Fix MST PBN/X.Y value calculations

Sasha Levin
2024-01-22T14:57:12Z

From: Ilya Bakoulin <ilya.bakoulin@amd.com>

[ Upstream commit 94bbf802efd0a8f13147d6664af6e653637340a8 ]

Changing PBN calculation to be more in line with spec. We don't need to
inflate PBN_NATIVE value by the 1.006 margin, since that is already
taken care of in the get_pbn_per_slot function.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Ilya Bakoulin <ilya.bakoulin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/amd/display/dc/link/link_dpms.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/link/link_dpms.c b/drivers/gpu/drm/amd/display/dc/link/link_dpms.c
index f8e01ca09d96..a3b3aec05d6b 100644
--- a/drivers/gpu/drm/amd/display/dc/link/link_dpms.c
+++ b/drivers/gpu/drm/amd/display/dc/link/link_dpms.c
@@ -1057,18 +1057,21 @@ static struct fixed31_32 get_pbn_from_bw_in_kbps(uint64_t kbps)
uint32_t denominator = 1;

/*
- * margin 5300ppm + 300ppm ~ 0.6% as per spec, factor is 1.006
+ * The 1.006 factor (margin 5300ppm + 300ppm ~ 0.6% as per spec) is not
+ * required when determining PBN/time slot utilization on the link between
+ * us and the branch, since that overhead is already accounted for in
+ * the get_pbn_per_slot function.
+ *
* The unit of 54/64Mbytes/sec is an arbitrary unit chosen based on
* common multiplier to render an integer PBN for all link rate/lane
* counts combinations
* calculate
- * peak_kbps *= (1006/1000)
* peak_kbps *= (64/54)
- * peak_kbps *= 8 convert to bytes
+ * peak_kbps /= (8 * 1000) convert to bytes
*/

- numerator = 64 * PEAK_FACTOR_X1000;
- denominator = 54 * 8 * 1000 * 1000;
+ numerator = 64;
+ denominator = 54 * 8 * 1000;
kbps *= numerator;
peak_kbps = dc_fixpt_from_fraction(kbps, denominator);

--
2.43.0