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

From: Uwe Kleine-König
Date: Tue Sep 17 2019 - 16:29:00 EST


On Tue, Sep 17, 2019 at 03:51:50PM +0200, Fabrice Gasnier wrote:
> 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@xxxxxx>
> ---
> 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..5c2c728 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) {
> + /* Fall here in case source clock < period */

Does "clock < period" make sense? I'd just write: "Clock is too slow to
achieve period."

> + dev_err(priv->chip.dev, "Can't reach expected period\n");

IMHO this is little helpful. If a consumer requests such an
unsatisfiable state several times your log is spammed and you don't even
see the what was requested. I'd drop the message completely (or make it
a dev_debug).

> + return -EINVAL;
> + }
> +
> prd = div;
> while (div > STM32_LPTIM_MAX_ARR) {
> presc++;

Best regards
Uwe

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