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

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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>
This commit is contained in:
Fabrice Gasnier 2019-09-18 16:54:21 +02:00 committed by Thierry Reding
parent 71523d1812
commit c91e3234c6
1 changed files with 6 additions and 0 deletions
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6
drivers/pwm/pwm-stm32-lp.c
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@ -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++;