[PATCH] pwm: tegra: Avoid potential overflow for short periods

[PATCH] pwm: tegra: Avoid potential overflow for short periods

[Thierry Reding]

For very short periods, the result of the division might overflow the
unsigned long hz variable (on 32-bit architectures). Avoid that by
making it an unsigned long long. While at it, also remove an unneeded
local variable whose only purpose is to store a temporary computation.

Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
---
 drivers/pwm/pwm-tegra.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
index 9c7f180b9f3a..c040f87ee144 100644
--- a/drivers/pwm/pwm-tegra.c
+++ b/drivers/pwm/pwm-tegra.c
@@ -75,9 +75,8 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 			    int duty_ns, int period_ns)
 {
 	struct tegra_pwm_chip *pc = to_tegra_pwm_chip(chip);
-	unsigned long long c = duty_ns;
-	unsigned long rate, hz;
-	unsigned long long ns100 = NSEC_PER_SEC;
+	unsigned long long c = duty_ns, hz;
+	unsigned long rate;
 	u32 val = 0;
 	int err;
 
@@ -98,9 +97,8 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	rate = clk_get_rate(pc->clk) >> PWM_DUTY_WIDTH;
 
 	/* Consider precision in PWM_SCALE_WIDTH rate calculation */
-	ns100 *= 100;
-	hz = DIV_ROUND_CLOSEST_ULL(ns100, period_ns);
-	rate = DIV_ROUND_CLOSEST(rate * 100, hz);
+	hz = DIV_ROUND_CLOSEST_ULL(100ULL * NSEC_PER_SEC, period_ns);
+	rate = DIV_ROUND_CLOSEST_ULL(100ULL * rate, hz);
 
 	/*
 	 * Since the actual PWM divider is the register's frequency divider
-- 
2.12.0

Re: [PATCH] pwm: tegra: Avoid potential overflow for short periods

[Laxman Dewangan]

On Wednesday 12 April 2017 10:03 PM, Thierry Reding wrote:
> For very short periods, the result of the division might overflow the
> unsigned long hz variable (on 32-bit architectures). Avoid that by
> making it an unsigned long long. While at it, also remove an unneeded
> local variable whose only purpose is to store a temporary computation.
>
> Signed-off-by: Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> ---

Acked-by: Laxman Dewangan <ldewangan-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>