Re: [PATCH v2 5/5] pwm: imx27: wait till the duty cycle is applied

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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On Fri, Sep 25, 2020 at 05:53:30PM +0200, Marco Felsch wrote:
> Currently the driver don't check if the new state was applied or not.

s/don't/doesn't/

> This can cause glitches on the output pin if the new state disables the
> PWM. In this case the PWM clocks are disabled before the new duty cycle
> value gets applied.

Hmm, the problem that is addressed here is that .apply() might turn off
the clock input for the counter before the inactive value is on the pin,
right? So an alternative fix would be to not disable the clock, wouldn't
it?

> The fix is to wait till the desired duty cycle was applied.
> 
> Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
> ---
> v2:
> - new patch
> 
>  drivers/pwm/pwm-imx27.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
> index 07c6a263a39c..ffa00bcd81da 100644
> --- a/drivers/pwm/pwm-imx27.c
> +++ b/drivers/pwm/pwm-imx27.c
> @@ -222,6 +222,26 @@ static int pwm_imx27_get_fifo_slot(struct pwm_chip *chip,
>  	return fifoav;
>  }
>  
> +static int pwm_imx27_wait_till_applied(struct pwm_chip *chip,
> +				       struct pwm_device *pwm)
> +{
> +	unsigned int attempts = 4;
> +	unsigned int period_ms;
> +	int busy_slots;
> +
> +	do {
> +		busy_slots = pwm_imx27_get_fifo_slot(chip, pwm);
> +		if (busy_slots == 0)
> +			return 0;
> +
> +		period_ms = DIV_ROUND_UP(pwm_get_period(pwm),

I was glad you removed the call to pwm_get_state() from .apply(), now it is
back in disguised form here :-\ Also the value shouldn't change over the
iteration of this loop, so determining it once should be enough.

> +					 NSEC_PER_MSEC);
> +		msleep(period_ms);
> +	} while (attempts--);
> +
> +	return -ETIMEDOUT;
> +}
> +
>  static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  			   const struct pwm_state *state)
>  {
> @@ -277,6 +297,11 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  		writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
>  	else
>  		writel(0, imx->mmio_base + MX3_PWMSAR);
> +
> +	ret = pwm_imx27_wait_till_applied(chip, pwm);
> +	if (ret)
> +		goto out;
> +

The framework doesn't define (and this is a problem there) if .apply is
supposed to sleep. OTOH at least sun4i has a similar behaviour.
Thierry, what is your thought on this?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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