[PATCH v2] fix overflow in idle exit_latency

[Bo Ye <bo.ye@mediatek.com>]

From: C Cheng <C.Cheng@mediatek.com>

In detail:

In C language, when you perform a multiplication operation, if
both operands are of int type, the multiplication operation is
performed on the int type, and then the result is converted to
the target type. This means that if the product of int type
multiplication exceeds the range that int type can represent,
 an overflow will occur even if you store the result in a
variable of int64_t type.

For a multiplication of two int values, it is better to use
mul_u32_u32() rather than s->exit_latency_ns = s->exit_latency *
NSEC_PER_USEC to avoid potential overflow happenning.

Signed-off-by: C Cheng <C.Cheng@mediatek.com>
Signed-off-by: Bo Ye <bo.ye@mediatek.com>
---
Change in v2:
-remove Change-ID
-correct patch author name
-replace multiplication of two int values with mul_u32_u32
-refine commit message
---
 drivers/cpuidle/driver.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
index d9cda7f6ccb9..cf5873cc45dc 100644
--- a/drivers/cpuidle/driver.c
+++ b/drivers/cpuidle/driver.c
@@ -16,6 +16,7 @@
 #include <linux/cpumask.h>
 #include <linux/tick.h>
 #include <linux/cpu.h>
+#include <linux/math64.h>
 
 #include "cpuidle.h"
 
@@ -187,7 +188,7 @@ static void __cpuidle_driver_init(struct cpuidle_driver *drv)
 			s->target_residency = div_u64(s->target_residency_ns, NSEC_PER_USEC);
 
 		if (s->exit_latency > 0)
-			s->exit_latency_ns = s->exit_latency * NSEC_PER_USEC;
+			s->exit_latency_ns = mul_u32_u32(s->exit_latency, NSEC_PER_USEC);
 		else if (s->exit_latency_ns < 0)
 			s->exit_latency_ns =  0;
 		else
-- 
2.18.0