drm/i915: avoid brightness overflow when doing scale

Some card's max brightness level is pretty large, e.g. on Acer Aspire 4732Z, the max level is 989910. If user space set a large enough level then the current scale done in intel_panel_set_backlight will cause an integer overflow and the scaled level will be mistakenly small, leaving user with an almost black screen. This patch fixes this problem. Signed-off-by: Aaron Lu <[email protected]> [danvet: Add a comment to explain what's going on.] Signed-off-by: Daniel Vetter <[email protected]>
o11s · Aug 7, 2013 · 22505b8 · 22505b8
1 parent 9dbd8fe
commit 22505b8
Showing 1 changed file with 5 additions and 2 deletions.
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
@@ -497,8 +497,11 @@ void intel_panel_set_backlight(struct drm_device *dev, u32 level, u32 max)
 		goto out;
 	}
 
-	/* scale to hardware */
-	level = level * freq / max;
+	/* scale to hardware, but be careful to not overflow */
+	if (freq < max)
+		level = level * freq / max;
+	else
+		level = freq / max * level;
 
 	dev_priv->backlight.level = level;
 	if (dev_priv->backlight.device)