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From: Sasha Levin <sashal@kernel.org>
To: linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Cc: Ilya Bakoulin <ilya.bakoulin@amd.com>,
	Daniel Wheeler <daniel.wheeler@amd.com>,
	Wenjing Liu <wenjing.liu@amd.com>,
	Rodrigo Siqueira <rodrigo.siqueira@amd.com>,
	Alex Deucher <alexander.deucher@amd.com>,
	Sasha Levin <sashal@kernel.org>,
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Subject: [PATCH AUTOSEL 6.7 18/88] drm/amd/display: Fix MST PBN/X.Y value calculations
Date: Mon, 22 Jan 2024 09:50:51 -0500
Message-ID: <20240122145608.990137-18-sashal@kernel.org>
In-Reply-To: <20240122145608.990137-1-sashal@kernel.org>
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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