linux-kernel - [PATCH v2] pwm: stm32-lp: add check in case requested period cannot be achieved

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Date:   Wed, 18 Sep 2019 16:54:21 +0200
From:   Fabrice Gasnier <fabrice.gasnier@...com>
To:     <thierry.reding@...il.com>
CC:     <u.kleine-koenig@...gutronix.de>, <alexandre.torgue@...com>,
        <fabrice.gasnier@...com>, <linux-pwm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>
Subject: [PATCH v2] 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@...com>
---
Changes in v2:
- remarks from Uwe: update the comment, use dev_dbg() and print period that
  cannot be reached
---
 drivers/pwm/pwm-stm32-lp.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/pwm/pwm-stm32-lp.c b/drivers/pwm/pwm-stm32-lp.c
index 2211a64..97a9afa 100644
--- a/drivers/pwm/pwm-stm32-lp.c
+++ b/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++;
-- 
2.7.4