[PATCH] pwm: sun4i: Avoid waiting until the next period

[Emil Lenngren]

Well that's one way of "solving it" ;)

But on what hardware do you really need to wait until one full pulse
cycle ends, before a disable command takes effect?

On the hardware I've tested on (GR8 and V3s), it's enough to wait at
most two clock cycles in order for it to take effect before we can
close the gate. And with clock cycle I mean 24 MHz divided by the
prescaler. With prescaler 1, that's 84 nanoseconds. By closing the
gate when the pwm should be disabled, I guess we could save some
nanoampere or microampere (is this important?)

/Emil

2021-05-12 0:00 GMT+02:00, Roman Beranek <roman.beranek at prusa3d.cz>:
> As disabling PWM by clearing the PWM_EN bit doesn't take an effect until
> the last pulse cycle ends, gating the clock too soon may result in the
> output signal getting stuck in an active state. Although the code gives
> an appearance that it takes care of this particular problem by waiting
> for the next period before finally clearing the CLK_GATING and EN bits,
> unless the EN bit has already been cleared by the time the delay begins,
> this measure doesn't achieve anything.
>
> However, even if this detail were to be fixed, there would still remain
> another issue to deal with: if the PWM were to be disabled shortly after
> having its period shortened, the length of the delay might turn out
> insufficient. So instead of waiting for the moment when it becomes safe
> to gate the clock, let's not bother gating it in the first place.
>
> Signed-off-by: Roman Beranek <roman.beranek at prusa3d.com>
> Suggested-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
> ---
>  drivers/pwm/pwm-sun4i.c | 52 +++++++++++------------------------------
>  1 file changed, 13 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> index e01becd10..809163186 100644
> --- a/drivers/pwm/pwm-sun4i.c
> +++ b/drivers/pwm/pwm-sun4i.c
> @@ -89,7 +89,6 @@ struct sun4i_pwm_chip {
>  	void __iomem *base;
>  	spinlock_t ctrl_lock;
>  	const struct sun4i_pwm_data *data;
> -	unsigned long next_period[2];
>  };
>
>  static inline struct sun4i_pwm_chip *to_sun4i_pwm_chip(struct pwm_chip
> *chip)
> @@ -235,26 +234,15 @@ static int sun4i_pwm_apply(struct pwm_chip *chip,
> struct pwm_device *pwm,
>  	struct pwm_state cstate;
>  	u32 ctrl, duty = 0, period = 0, val;
>  	int ret;
> -	unsigned int delay_us, prescaler = 0;
> -	unsigned long now;
> +	unsigned int prescaler = 0;
>  	bool bypass;
>
>  	pwm_get_state(pwm, &cstate);
>
> -	if (!cstate.enabled) {
> -		ret = clk_prepare_enable(sun4i_pwm->clk);
> -		if (ret) {
> -			dev_err(chip->dev, "failed to enable PWM clock\n");
> -			return ret;
> -		}
> -	}
> -
>  	ret = sun4i_pwm_calculate(sun4i_pwm, state, &duty, &period, &prescaler,
>  				  &bypass);
>  	if (ret) {
>  		dev_err(chip->dev, "period exceeds the maximum value\n");
> -		if (!cstate.enabled)
> -			clk_disable_unprepare(sun4i_pwm->clk);
>  		return ret;
>  	}
>
> @@ -284,8 +272,6 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct
> pwm_device *pwm,
>
>  	val = (duty & PWM_DTY_MASK) | PWM_PRD(period);
>  	sun4i_pwm_writel(sun4i_pwm, val, PWM_CH_PRD(pwm->hwpwm));
> -	sun4i_pwm->next_period[pwm->hwpwm] = jiffies +
> -		nsecs_to_jiffies(cstate.period + 1000);
>
>  	if (state->polarity != PWM_POLARITY_NORMAL)
>  		ctrl &= ~BIT_CH(PWM_ACT_STATE, pwm->hwpwm);
> @@ -296,34 +282,12 @@ static int sun4i_pwm_apply(struct pwm_chip *chip,
> struct pwm_device *pwm,
>
>  	if (state->enabled)
>  		ctrl |= BIT_CH(PWM_EN, pwm->hwpwm);
> +	else
> +		ctrl &= ~BIT_CH(PWM_EN, pwm->hwpwm);
>
>  	sun4i_pwm_writel(sun4i_pwm, ctrl, PWM_CTRL_REG);
> -
> -	spin_unlock(&sun4i_pwm->ctrl_lock);
> -
> -	if (state->enabled)
> -		return 0;
> -
> -	/* We need a full period to elapse before disabling the channel. */
> -	now = jiffies;
> -	if (time_before(now, sun4i_pwm->next_period[pwm->hwpwm])) {
> -		delay_us = jiffies_to_usecs(sun4i_pwm->next_period[pwm->hwpwm] -
> -					   now);
> -		if ((delay_us / 500) > MAX_UDELAY_MS)
> -			msleep(delay_us / 1000 + 1);
> -		else
> -			usleep_range(delay_us, delay_us * 2);
> -	}
> -
> -	spin_lock(&sun4i_pwm->ctrl_lock);
> -	ctrl = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> -	ctrl &= ~BIT_CH(PWM_CLK_GATING, pwm->hwpwm);
> -	ctrl &= ~BIT_CH(PWM_EN, pwm->hwpwm);
> -	sun4i_pwm_writel(sun4i_pwm, ctrl, PWM_CTRL_REG);
>  	spin_unlock(&sun4i_pwm->ctrl_lock);
>
> -	clk_disable_unprepare(sun4i_pwm->clk);
> -
>  	return 0;
>  }
>
> @@ -457,6 +421,13 @@ static int sun4i_pwm_probe(struct platform_device
> *pdev)
>  		goto err_bus;
>  	}
>
> +	ret = clk_prepare_enable(pwm->clk);
> +	if (ret) {
> +		dev_err(&pdev->dev, "failed to prepare and enable PWM clock %pe\n",
> +			ERR_PTR(ret));
> +		goto err_clk;
> +	}
> +
>  	pwm->chip.dev = &pdev->dev;
>  	pwm->chip.ops = &sun4i_pwm_ops;
>  	pwm->chip.npwm = pwm->data->npwm;
> @@ -476,6 +447,8 @@ static int sun4i_pwm_probe(struct platform_device
> *pdev)
>  	return 0;
>
>  err_pwm_add:
> +	clk_disable_unprepare(pwm->clk);
> +err_clk:
>  	clk_disable_unprepare(pwm->bus_clk);
>  err_bus:
>  	reset_control_assert(pwm->rst);
> @@ -492,6 +465,7 @@ static int sun4i_pwm_remove(struct platform_device
> *pdev)
>  	if (ret)
>  		return ret;
>
> +	clk_disable_unprepare(pwm->clk);
>  	clk_disable_unprepare(pwm->bus_clk);
>  	reset_control_assert(pwm->rst);
>
> --
> 2.31.1
>
>