[PATCH] watchdog: dw_wdt: fix signedness bug in dw_wdt_top_in_seconds()

[Jisheng Zhang]

On 64bit platforms, "(1 << (16 + top)) / clk_get_rate(dw_wdt.clk)" is
sign-extended to 64bit then converted to unsigned 64bit, finally divide
the clk rate. If the top is the maximum TOP i.e 15, "(1 << (16 +15))"
will be sign-extended to 0xffffffff80000000, then converted to unsigned
0xffffffff80000000, which is a huge number, thus the final result is
wrong.

We fix this issue by giving usigned value(1U in this case) at first.

Let's assume clk rate is 25MHZ,
Before the patch:
dw_wdt_top_in_seconds(15) = -864612050

After the patch:
dw_wdt_top_in_seconds(15) = 85

Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxxx>
---
 drivers/watchdog/dw_wdt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
index 6ea0634..8fefa4ad 100644
--- a/drivers/watchdog/dw_wdt.c
+++ b/drivers/watchdog/dw_wdt.c
@@ -81,7 +81,7 @@ static inline int dw_wdt_top_in_seconds(unsigned top)
 	 * There are 16 possible timeout values in 0..15 where the number of
 	 * cycles is 2 ^ (16 + i) and the watchdog counts down.
 	 */
-	return (1 << (16 + top)) / clk_get_rate(dw_wdt.clk);
+	return (1U << (16 + top)) / clk_get_rate(dw_wdt.clk);
 }
 
 static int dw_wdt_get_top(void)
-- 
2.6.4

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