Comment 55 for bug 1791312

The following patch seems to work for me:
client@client-LIFEBOOK-AH531 ~/Téléchargements $ diff -u old/linux-4.19.21/drivers/gpu/drm/radeon/radeon_display.c linux-4.19.21/drivers/gpu/drm/radeon/radeon_display.c
--- old/linux-4.19.21/drivers/gpu/drm/radeon/radeon_display.c 2019-02-12 13:47:27.000000000 -0500
+++ linux-4.19.21/drivers/gpu/drm/radeon/radeon_display.c 2019-02-14 15:29:37.743229474 -0500
@@ -921,12 +921,14 @@
  ref_div_max = max(min(100 / post_div, ref_div_max), 1u);

  /* get matching reference and feedback divider */
- *ref_div = min(max(DIV_ROUND_CLOSEST(den, post_div), 1u), ref_div_max);
+ // was: *ref_div = min(max(DIV_ROUND_CLOSEST(den, post_div), 1u), ref_div_max);
+ *ref_div = min(max((den/post_div), 1u), ref_div_max);
  *fb_div = DIV_ROUND_CLOSEST(nom * *ref_div * post_div, den);

  /* limit fb divider to its maximum */
  if (*fb_div > fb_div_max) {
- *ref_div = DIV_ROUND_CLOSEST(*ref_div * fb_div_max, *fb_div);
+ // was: *ref_div = DIV_ROUND_CLOSEST(*ref_div * fb_div_max, *fb_div);
+ *ref_div = (*ref_div * fb_div_max) / (*fb_div);
   *fb_div = fb_div_max;
  }
 }
client@client-LIFEBOOK-AH531 ~/Téléchargements $