pwm: lpss: Avoid potential overflow of base_unit

The resolution of base_unit is derived from base_unit_bits and thus must be
equal to (2^base_unit_bits - 1). Otherwise frequency and therefore base_unit
might potentially overflow.

Prevent the above by substracting 1 in all cases where base_unit_bits or
derivative is used.
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-lpss.c

@@ -102,7 +102,7 @@ static int pwm_lpss_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	 * The equation is:
 	 * base_unit = round(base_unit_range * freq / c)
 	 */
-	base_unit_range = BIT(lpwm->info->base_unit_bits);
+	base_unit_range = BIT(lpwm->info->base_unit_bits) - 1;
 	freq *= base_unit_range;
 
 	base_unit = DIV_ROUND_CLOSEST_ULL(freq, c);
@@ -117,8 +117,8 @@ static int pwm_lpss_config(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	ctrl = pwm_lpss_read(pwm);
 	ctrl &= ~PWM_ON_TIME_DIV_MASK;
-	ctrl &= ~((base_unit_range - 1) << PWM_BASE_UNIT_SHIFT);
-	base_unit &= (base_unit_range - 1);
+	ctrl &= ~(base_unit_range << PWM_BASE_UNIT_SHIFT);
+	base_unit &= base_unit_range;
 	ctrl |= (u32) base_unit << PWM_BASE_UNIT_SHIFT;
 	ctrl |= on_time_div;
 	pwm_lpss_write(pwm, ctrl);