pwm: stm32-lp: Add check in case requested period cannot be achieved

LPTimer can use a 32KHz clock for counting. It depends on clock tree
configuration. In such a case, PWM output frequency range is limited.
Although unlikely, nothing prevents user from requesting a PWM frequency
above counting clock (32KHz for instance):
- This causes (prd - 1) = 0xffff to be written in ARR register later in
the apply() routine.
This results in badly configured PWM period (and also duty_cycle).
Add a check to report an error is such a case.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-stm32-lp.c

@@ -59,6 +59,12 @@ static int stm32_pwm_lp_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	/* Calculate the period and prescaler value */
 	div = (unsigned long long)clk_get_rate(priv->clk) * state->period;
 	do_div(div, NSEC_PER_SEC);
+	if (!div) {
+		/* Clock is too slow to achieve requested period. */
+		dev_dbg(priv->chip.dev, "Can't reach %u ns\n",	state->period);
+		return -EINVAL;
+	}
+
 	prd = div;
 	while (div > STM32_LPTIM_MAX_ARR) {
 		presc++;