| Message ID | 1568818461-19995-1-git-send-email-fabrice.gasnier@st.com |
|---|---|
| State | Accepted |
| Headers | show |
| Series | [v2] pwm: stm32-lp: add check in case requested period cannot be achieved | expand |
On Wed, Sep 18, 2019 at 04:54:21PM +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@st.com> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> If you are interested to improve the driver further, there are a few things that I would welcome to see fixed in a tested patch: - duty calculation uses requested instead of implemented period. - stm32_pwm_lp_apply calls pwm_get_state - Calculation of prd could be done without a loop - A hint about relevant documentation in the driver's header would be great - Documentation about behaviour of the hardware is missing: - Does the hardware complete the currently running period when STM32_LPTIM_CR is cleared? - Does the output stop in the inactive output level when the PWM is stopped? - clk_get_rate might be called without the clock being enabled. - The driver does: ret = regmap_write(priv->regmap, STM32_LPTIM_CMP, prd - (1 + dty)); That looks wrong. (Consider dty == prd.) Best regards Uwe
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++;
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> --- 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(+)