[PATCH 1/2] hwmon: pwm-fan: Ensure that calculation doesn't discard big period values

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

With MAX_PWM being defined to 255 the code

	unsigned long period;
	...
	period = ctx->pwm->args.period;
	state.duty_cycle = DIV_ROUND_UP(pwm * (period - 1), MAX_PWM);

calculates a too small value for duty_cycle if the configured period is
big (either by discarding the 64 bit value ctx->pwm->args.period or by
overflowing the multiplication). As this results in a too slow fan and
so maybe an overheating machine better be safe than sorry and error out
in .probe.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
---
 drivers/hwmon/pwm-fan.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
index 1f63807c0399..ec171f2b684a 100644
--- a/drivers/hwmon/pwm-fan.c
+++ b/drivers/hwmon/pwm-fan.c
@@ -324,8 +324,18 @@ static int pwm_fan_probe(struct platform_device *pdev)
 
 	ctx->pwm_value = MAX_PWM;
 
-	/* Set duty cycle to maximum allowed and enable PWM output */
 	pwm_init_state(ctx->pwm, &state);
+	/*
+	 * __set_pwm assumes that MAX_PWM * (period - 1) fits into an unsigned
+	 * long. Check this here to prevent the fan running at a too low
+	 * frequency.
+	 */
+	if (state.period > ULONG_MAX / MAX_PWM + 1) {
+		dev_err(dev, "Configured period too big\n");
+		return -EINVAL;
+	}
+
+	/* Set duty cycle to maximum allowed and enable PWM output */
 	state.duty_cycle = ctx->pwm->args.period - 1;
 	state.enabled = true;
 

base-commit: 2c85ebc57b3e1817b6ce1a6b703928e113a90442
-- 
2.29.2