drm/rect: Round above 1 << 16 upwards to correct scale calculation functions.

When calculating limits we want to be as pessimistic as possible,
so we have to explicitly say whether we want to round up or down
to accurately calculate whether we are below min_scale or above
max_scale.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Fix wording in documentation. (Ville)]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503112217.37292-2-maarten.lankhorst@linux.intel.com

drivers/gpu/drm/drm_rect.c

@@ -106,7 +106,10 @@ static int drm_calc_scale(int src, int dst)
 	if (dst == 0)
 		return 0;
 
-	scale = src / dst;
+	if (src > (dst << 16))
+		return DIV_ROUND_UP(src, dst);
+	else
+		scale = src / dst;
 
 	return scale;
 }
@@ -121,6 +124,10 @@ static int drm_calc_scale(int src, int dst)
  * Calculate the horizontal scaling factor as
  * (@src width) / (@dst width).
  *
+ * If the scale is below 1 << 16, round down. If the scale is above
+ * 1 << 16, round up. This will calculate the scale with the most
+ * pessimistic limit calculation.
+ *
  * RETURNS:
  * The horizontal scaling factor, or errno of out of limits.
  */
@@ -152,6 +159,10 @@ EXPORT_SYMBOL(drm_rect_calc_hscale);
  * Calculate the vertical scaling factor as
  * (@src height) / (@dst height).
  *
+ * If the scale is below 1 << 16, round down. If the scale is above
+ * 1 << 16, round up. This will calculate the scale with the most
+ * pessimistic limit calculation.
+ *
  * RETURNS:
  * The vertical scaling factor, or errno of out of limits.
  */
@@ -189,6 +200,10 @@ EXPORT_SYMBOL(drm_rect_calc_vscale);
  * If the calculated scaling factor is above @max_vscale,
  * decrease the height of rectangle @src to compensate.
  *
+ * If the scale is below 1 << 16, round down. If the scale is above
+ * 1 << 16, round up. This will calculate the scale with the most
+ * pessimistic limit calculation.
+ *
  * RETURNS:
  * The horizontal scaling factor.
  */
@@ -239,6 +254,10 @@ EXPORT_SYMBOL(drm_rect_calc_hscale_relaxed);
  * If the calculated scaling factor is above @max_vscale,
  * decrease the height of rectangle @src to compensate.
  *
+ * If the scale is below 1 << 16, round down. If the scale is above
+ * 1 << 16, round up. This will calculate the scale with the most
+ * pessimistic limit calculation.
+ *
  * RETURNS:
  * The vertical scaling factor.
  */