linux-kernel - [PATCH 4.19 44/88] drm/radeon: Prefer lower feedback dividers

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Message-Id: <20200908152223.324574445@linuxfoundation.org>
Date:   Tue,  8 Sep 2020 17:25:45 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Christian König <christian.koenig@....com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Alex Deucher <alexander.deucher@....com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.19 44/88] drm/radeon: Prefer lower feedback dividers

From: Kai-Heng Feng <kai.heng.feng@...onical.com>

[ Upstream commit fc8c70526bd30733ea8667adb8b8ffebea30a8ed ]

Commit 2e26ccb119bd ("drm/radeon: prefer lower reference dividers")
fixed screen flicker for HP Compaq nx9420 but breaks other laptops like
Asus X50SL.

Turns out we also need to favor lower feedback dividers.

Users confirmed this change fixes the regression and doesn't regress the
original fix.

Fixes: 2e26ccb119bd ("drm/radeon: prefer lower reference dividers")
BugLink: https://bugs.launchpad.net/bugs/1791312
BugLink: https://bugs.launchpad.net/bugs/1861554
Reviewed-by: Christian König <christian.koenig@....com>
Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
Signed-off-by: Alex Deucher <alexander.deucher@....com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/gpu/drm/radeon/radeon_display.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/radeon/radeon_display.c b/drivers/gpu/drm/radeon/radeon_display.c
index 3f0f3a578ddf0..c28d76d898fdb 100644
--- a/drivers/gpu/drm/radeon/radeon_display.c
+++ b/drivers/gpu/drm/radeon/radeon_display.c
@@ -926,7 +926,7 @@ static void avivo_get_fb_ref_div(unsigned nom, unsigned den, unsigned post_div,
 
 	/* get matching reference and feedback divider */
 	*ref_div = min(max(den/post_div, 1u), ref_div_max);
-	*fb_div = DIV_ROUND_CLOSEST(nom * *ref_div * post_div, den);
+	*fb_div = max(nom * *ref_div * post_div / den, 1u);
 
 	/* limit fb divider to its maximum */
 	if (*fb_div > fb_div_max) {
-- 
2.25.1