drm/radeon: Prefer lower feedback dividers

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@amd.com>
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/radeon/radeon_display.c b/drivers/gpu/drm/radeon/radeon_display.c
index e0ae911ef427dd028ad3c606dae0f1d61d1c7b7a..7b69d6dfe44a37455ee73147f5f88d78fe409f17 100644 (file)
--- a/drivers/gpu/drm/radeon/radeon_display.c
+++ b/drivers/gpu/drm/radeon/radeon_display.c
@@ -933,7 +933,7 @@ 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) {