From: jsg <jsg@openbsd.org>
Date: Tue, 6 Feb 2024 03:14:23 +0000 (+0000)
Subject: drm/amd/display: Fix MST PBN/X.Y value calculations
X-Git-Url: http://artulab.com/gitweb/?a=commitdiff_plain;h=1ad3b714919eade51a4d8c5b926be30b91cb0035;p=openbsd

drm/amd/display: Fix MST PBN/X.Y value calculations

From Ilya Bakoulin
01e7578c7cd3d8815fae130ac74b1303c056bd2d in linux-6.6.y/6.6.16
94bbf802efd0a8f13147d6664af6e653637340a8 in mainline linux
---

diff --git a/sys/dev/pci/drm/amd/display/dc/link/link_dpms.c b/sys/dev/pci/drm/amd/display/dc/link/link_dpms.c
index 87a2f12aeb4..4450fbf788a 100644
--- a/sys/dev/pci/drm/amd/display/dc/link/link_dpms.c
+++ b/sys/dev/pci/drm/amd/display/dc/link/link_dpms.c
@@ -1059,18 +1059,21 @@ 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);