pwm: iqs620a: Fix overflow and optimize calculations

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: 6f0841a8197b ("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>

diff --git a/drivers/pwm/pwm-iqs620a.c b/drivers/pwm/pwm-iqs620a.c
index 5ede825..14b18fb 100644 (file)
--- a/drivers/pwm/pwm-iqs620a.c
+++ b/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);