linux-kernel - [PATCH AUTOSEL 5.4 05/10] drm/amd/display: Prevent bandwidth overflow

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Date:   Sat, 19 Dec 2020 22:34:52 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Chris Park <Chris.Park@....com>, Wenjing Liu <Wenjing.Liu@....com>,
        Eryk Brol <eryk.brol@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Sasha Levin <sashal@...nel.org>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org
Subject: [PATCH AUTOSEL 5.4 05/10] drm/amd/display: Prevent bandwidth overflow

From: Chris Park <Chris.Park@....com>

[ Upstream commit 80089dd8410f356d5104496d5ab71a66a4f4646b ]

[Why]
At very high pixel clock, bandwidth calculation exceeds 32 bit size
and overflow value. This causes the resulting selection of link rate
to be inaccurate.

[How]
Change order of operation and use fixed point to deal with integer
accuracy. Also address bug found when forcing link rate.

Signed-off-by: Chris Park <Chris.Park@....com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@....com>
Acked-by: Eryk Brol <eryk.brol@....com>
Signed-off-by: Alex Deucher <alexander.deucher@....com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/gpu/drm/amd/display/dc/core/dc_link.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_link.c b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
index 47cefc05fd3f5..f933791f1fbbb 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc_link.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
@@ -2906,11 +2906,14 @@ uint32_t dc_bandwidth_in_kbps_from_timing(
 {
 	uint32_t bits_per_channel = 0;
 	uint32_t kbps;
+	struct fixed31_32 link_bw_kbps;
 
 #ifdef CONFIG_DRM_AMD_DC_DSC_SUPPORT
 	if (timing->flags.DSC) {
-		kbps = (timing->pix_clk_100hz * timing->dsc_cfg.bits_per_pixel);
-		kbps = kbps / 160 + ((kbps % 160) ? 1 : 0);
+		link_bw_kbps = dc_fixpt_from_int(timing->pix_clk_100hz);
+		link_bw_kbps = dc_fixpt_div_int(link_bw_kbps, 160);
+		link_bw_kbps = dc_fixpt_mul_int(link_bw_kbps, timing->dsc_cfg.bits_per_pixel);
+		kbps = dc_fixpt_ceil(link_bw_kbps);
 		return kbps;
 	}
 #endif
-- 
2.27.0