pwm: iqs620a: Fix overflow and optimize calculations

[ Upstream commit 72d6b24 ]

If state->duty_cycle is 0x100000000000000, the previous calculation of
duty_scale overflows and yields a duty cycle ratio of 0% instead of
100%. Fix this by clamping the requested duty cycle to the maximal
possible duty cycle first. This way it is possible to use a native
integer division instead of a (depending on the architecture) more
expensive 64bit division.

With this change in place duty_scale cannot be bigger than 256 which
allows to simplify the calculation of duty_val.

Fixes: 6f0841a ("pwm: Add support for Azoteq IQS620A PWM generator")
Tested-by: Jeff LaBundy <jeff@labundy.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/pwm/pwm-iqs620a.c

@@ -46,7 +46,8 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 {
 	struct iqs620_pwm_private *iqs620_pwm;
 	struct iqs62x_core *iqs62x;
-	u64 duty_scale;
+	unsigned int duty_cycle;
+	unsigned int duty_scale;
 	int ret;
 
 	if (state->polarity != PWM_POLARITY_NORMAL)
@@ -70,7 +71,8 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	 * For lower duty cycles (e.g. 0), the PWM output is simply disabled to
 	 * allow an external pull-down resistor to hold the GPIO3/LTX pin low.
 	 */
-	duty_scale = div_u64(state->duty_cycle * 256, IQS620_PWM_PERIOD_NS);
+	duty_cycle = min_t(u64, state->duty_cycle, IQS620_PWM_PERIOD_NS);
+	duty_scale = duty_cycle * 256 / IQS620_PWM_PERIOD_NS;
 
 	mutex_lock(&iqs620_pwm->lock);
 
@@ -82,7 +84,7 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	}
 
 	if (duty_scale) {
-		u8 duty_val = min_t(u64, duty_scale - 1, 0xff);
+		u8 duty_val = duty_scale - 1;
 
 		ret = regmap_write(iqs62x->regmap, IQS620_PWM_DUTY_CYCLE,
 				   duty_val);