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

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: 4731a72df273 ("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

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);