drm/sun4i: Fix an ulong overflow in the dotclock driver

Boris Brezillon · mripard · commit e84cb605e02f · 2018-10-19T11:50:25.000+02:00
The calculated ideal rate can easily overflow an unsigned long, thus making the best div selection buggy as soon as no ideal match is found before the overflow occurs. Fixes: 4731a72 ("drm/sun4i: request exact rates to our parents") Cc: <stable@vger.kernel.org> Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com> Acked-by: Maxime Ripard <maxime.ripard@bootlin.com> Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com> Link: https://patchwork.freedesktop.org/patch/msgid/20181018100250.12565-1-boris.brezillon@bootlin.com
diff --git a/drivers/gpu/drm/sun4i/sun4i_dotclock.c b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
@@ -81,9 +81,19 @@ static long sun4i_dclk_round_rate(struct clk_hw *hw, unsigned long rate,
 	int i;
 
 	for (i = tcon->dclk_min_div; i <= tcon->dclk_max_div; i++) {
-		unsigned long ideal = rate * i;
+		u64 ideal = (u64)rate * i;
 		unsigned long rounded;
 
+		/*
+		 * ideal has overflowed the max value that can be stored in an
+		 * unsigned long, and every clk operation we might do on a
+		 * truncated u64 value will give us incorrect results.
+		 * Let's just stop there since bigger dividers will result in
+		 * the same overflow issue.
+		 */
+		if (ideal > ULONG_MAX)
+			goto out;
+
 		rounded = clk_hw_round_rate(clk_hw_get_parent(hw),
 					    ideal);
 
