linux-kernel - Re: [PATCH v8 4/5] pwm: dwc: use clock rate in hz to avoid rounding issues

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Date:   Sat, 15 Jul 2023 21:42:23 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Ben Dooks <ben.dooks@...ive.com>
Cc:     linux-pwm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, ben.dooks@...ethink.co.uk,
        Thierry Reding <thierry.reding@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Greentime Hu <greentime.hu@...ive.com>,
        jarkko.nikula@...ux.intel.com,
        William Salmon <william.salmon@...ive.com>,
        Jude Onyenegecha <jude.onyenegecha@...ive.com>
Subject: Re: [PATCH v8 4/5] pwm: dwc: use clock rate in hz to avoid rounding
 issues

On Wed, Jun 14, 2023 at 06:14:56PM +0100, Ben Dooks wrote:
> As noted, the clock-rate when not a nice multiple of ns is probably
> going to end up with inacurate caculations, as well as on a non pci

s/caculation/calculations/

> system the rate may change (although we've not put a clock rate
> change notifier in this code yet) so we also add some quick checks
> of the rate when we do any calculations with it.

An externally triggered clock rate change is bad. If you drive a motor
you probably want to prevent an uncontrolled change here. I already
considered to add a call to clk_rate_exclusive_get() in various pwm
drivers for that reason, but didn't come around yet.

> Signed-off-by; Ben Dooks <ben.dooks@...ive.com>
> Reported-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> ---
> v8:
>  - fixup post rename
>  - move to earlier in series
> ---
>  drivers/pwm/pwm-dwc-core.c | 24 +++++++++++++++---------
>  drivers/pwm/pwm-dwc.h      |  2 +-
>  2 files changed, 16 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-dwc-core.c b/drivers/pwm/pwm-dwc-core.c
> index 38cd2163fe01..0f07e26e6c30 100644
> --- a/drivers/pwm/pwm-dwc-core.c
> +++ b/drivers/pwm/pwm-dwc-core.c
> @@ -49,13 +49,14 @@ static int __dwc_pwm_configure_timer(struct dwc_pwm *dwc,
>  	 * periods and check are the result within HW limits between 1 and
>  	 * 2^32 periods.
>  	 */
> -	tmp = DIV_ROUND_CLOSEST_ULL(state->duty_cycle, dwc->clk_ns);
> +	tmp = state->duty_cycle * dwc->clk_rate;
> +	tmp = DIV_ROUND_CLOSEST_ULL(tmp, NSEC_PER_SEC);

New drivers should implement round-down behaviour (i.e. pick the biggest
period (and duty_cycle) that is not bigger than the requested value.
With clk_ns = 10 (which it is the hardcoded value up to now) it doesn't
matter much how you round the division. I suggest to use the opportunity
to align to how new drivers should round. (That would be a separate
patch.)

>  	if (tmp < 1 || tmp > (1ULL << 32))
>  		return -ERANGE;
>  	low = tmp - 1;
>  
> -	tmp = DIV_ROUND_CLOSEST_ULL(state->period - state->duty_cycle,
> -				    dwc->clk_ns);
> +	tmp = (state->period - state->duty_cycle) * dwc->clk_rate;
> +	tmp = DIV_ROUND_CLOSEST_ULL(tmp, NSEC_PER_SEC);
>  	if (tmp < 1 || tmp > (1ULL << 32))
>  		return -ERANGE;
>  	high = tmp - 1;
> @@ -121,11 +122,14 @@ static int dwc_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
>  			     struct pwm_state *state)
>  {
>  	struct dwc_pwm *dwc = to_dwc_pwm(chip);
> +	unsigned long clk_rate;
>  	u64 duty, period;
>  	u32 ctrl, ld, ld2;
>  
>  	pm_runtime_get_sync(chip->dev);
>  
> +	clk_rate = dwc->clk_rate;
> +
>  	ctrl = dwc_pwm_readl(dwc, DWC_TIM_CTRL(pwm->hwpwm));
>  	ld = dwc_pwm_readl(dwc, DWC_TIM_LD_CNT(pwm->hwpwm));
>  	ld2 = dwc_pwm_readl(dwc, DWC_TIM_LD_CNT2(pwm->hwpwm));
> @@ -136,17 +140,19 @@ static int dwc_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
>  	 * based on the timer load-count only.
>  	 */
>  	if (ctrl & DWC_TIM_CTRL_PWM) {
> -		duty = (ld + 1) * dwc->clk_ns;
> -		period = (ld2 + 1)  * dwc->clk_ns;
> +		duty = ld + 1;
> +		period = ld2 + 1;
>  		period += duty;
>  	} else {
> -		duty = (ld + 1) * dwc->clk_ns;
> +		duty = ld + 1;
>  		period = duty * 2;
>  	}
>  
> +	duty *= NSEC_PER_SEC;
> +	period *= NSEC_PER_SEC;
> +	state->period = DIV_ROUND_CLOSEST_ULL(period, clk_rate);
> +	state->duty_cycle = DIV_ROUND_CLOSEST_ULL(duty, clk_rate);
>  	state->polarity = PWM_POLARITY_INVERSED;
> -	state->period = period;
> -	state->duty_cycle = duty;
>  
>  	pm_runtime_put_sync(chip->dev);
>  
> @@ -167,7 +173,7 @@ struct dwc_pwm *dwc_pwm_alloc(struct device *dev)
>  	if (!dwc)
>  		return NULL;
>  
> -	dwc->clk_ns = 10;
> +	dwc->clk_rate = NSEC_PER_SEC / 10;
>  	dwc->chip.dev = dev;
>  	dwc->chip.ops = &dwc_pwm_ops;
>  	dwc->chip.npwm = DWC_TIMERS_TOTAL;
> diff --git a/drivers/pwm/pwm-dwc.h b/drivers/pwm/pwm-dwc.h
> index 64795247c54c..e0a940fd6e87 100644
> --- a/drivers/pwm/pwm-dwc.h
> +++ b/drivers/pwm/pwm-dwc.h
> @@ -42,7 +42,7 @@ struct dwc_pwm_ctx {
>  struct dwc_pwm {
>  	struct pwm_chip chip;
>  	void __iomem *base;
> -	unsigned int clk_ns;
> +	unsigned long clk_rate;
>  	struct dwc_pwm_ctx ctx[DWC_TIMERS_TOTAL];
>  };
>  #define to_dwc_pwm(p)	(container_of((p), struct dwc_pwm, chip))

Best regards
Uwe

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

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