linux-kernel - Re: [v5 2/2] pwm: sifive: change the PWM controlled LED algorithm

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Date:   Mon, 11 Dec 2023 21:49:50 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Nylon Chen <nylon.chen@...ive.com>
Cc:     linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        conor+dt@...nel.org, paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, thierry.reding@...il.com,
        emil.renner.berthing@...onical.com, vincent.chen@...ive.com,
        greentime.hu@...ive.com, zong.li@...ive.com, nylon7717@...il.com
Subject: Re: [v5 2/2] pwm: sifive: change the PWM controlled LED algorithm

Hello Nylon,

On Tue, Oct 24, 2023 at 06:19:02PM +0800, Nylon Chen wrote:
> The `frac` variable represents the pulse inactive time, and the result
> of this algorithm is the pulse active time. Therefore, we must reverse the result.
> 
> The reference is SiFive FU740-C000 Manual[0]
> 
> Link: https://sifive.cdn.prismic.io/sifive/1a82e600-1f93-4f41-b2d8-86ed8b16acba_fu740-c000-manual-v1p6.pdf [0]
> 
> Signed-off-by: Nylon Chen <nylon.chen@...ive.com>
> Signed-off-by: Vincent Chen <vincent.chen@...ive.com>
> ---
>  drivers/pwm/pwm-sifive.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
> index eabddb7c7820..353c2342fbf1 100644
> --- a/drivers/pwm/pwm-sifive.c
> +++ b/drivers/pwm/pwm-sifive.c
> @@ -101,7 +101,7 @@ static void pwm_sifive_update_clock(struct pwm_sifive_ddata *ddata,
>  
>  	/* As scale <= 15 the shift operation cannot overflow. */
>  	num = (unsigned long long)NSEC_PER_SEC << (PWM_SIFIVE_CMPWIDTH + scale);
> -	ddata->real_period = div64_ul(num, rate);
> +	ddata->real_period = DIV_ROUND_UP_ULL(num, rate);

It's unclear to me, why you changed that.

>  	dev_dbg(ddata->chip.dev,
>  		"New real_period = %u ns\n", ddata->real_period);
>  }
> @@ -121,13 +121,14 @@ static int pwm_sifive_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
>  		state->enabled = false;
>  
>  	state->period = ddata->real_period;
> +
> +	duty = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - duty;

I would have placed that directly after

	duty = readl(...);

which then also influences

	state->enabled = duty > 0;

(as it should?).

>  	state->duty_cycle =
>  		(u64)duty * ddata->real_period >> PWM_SIFIVE_CMPWIDTH;
> -	state->polarity = PWM_POLARITY_INVERSED;
> +	state->polarity = PWM_POLARITY_NORMAL;
>  
>  	return 0;
>  }
> -
>  static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  			    const struct pwm_state *state)
>  {
> @@ -139,7 +140,7 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	int ret = 0;
>  	u32 frac;
>  
> -	if (state->polarity != PWM_POLARITY_INVERSED)
> +	if (state->polarity != PWM_POLARITY_NORMAL)
>  		return -EINVAL;
>  
>  	cur_state = pwm->state;
> @@ -158,6 +159,7 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	num = (u64)duty_cycle * (1U << PWM_SIFIVE_CMPWIDTH);
>  	frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
>  	/* The hardware cannot generate a 100% duty cycle */
> +	frac = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - frac;
>  	frac = min(frac, (1U << PWM_SIFIVE_CMPWIDTH) - 1);

frac can only be > (1U << PWM_SIFIVE_CMPWIDTH) - 1 if an overflow
happend the line above. Is that what you want here?

>  	mutex_lock(&ddata->lock);

Best regards
Uwe

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

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