linux-kernel - Re: [PATCH v8 2/3] pwm: sifive: change the PWM controlled LED algorithm

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Message-ID: <4kqkazromkzyhic2mgyyjrh4jlnp6djfuotu37btdfolqp5e2o@6jkbjnahvrbo>
Date: Mon, 5 Feb 2024 19:07:13 +0100
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Nylon Chen <nylon.chen@...ive.com>
Cc: linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org, conor@...nel.org, robh+dt@...nel.org, 
	krzysztof.kozlowski+dt@...aro.org, palmer@...belt.com, paul.walmsley@...ive.com, 
	aou@...s.berkeley.edu, thierry.reding@...il.com, vincent.chen@...ive.com, 
	zong.li@...ive.com, nylon7717@...il.com
Subject: Re: [PATCH v8 2/3] pwm: sifive: change the PWM controlled LED
 algorithm

Hello,

Regarding the Subject: The patch has nothing to do with an LED, has it?

On Fri, Jan 26, 2024 at 03:40:44PM +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.

Please break lines at 75 columns in the commit log.

> 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]

I looked at Figure 29 in this document (version v1p6, pdf page 148). Not
sure I understand that correctly, but I expect that the output of the
">=?" node below pwmcmp0 to become 1 if pwms has reached pwmcmp0, is
that right? In that case this output is zero when pwmcount is zero and
then pwmcmp0ip is zero, too. So a period starts with the inactive part
and so it's inversed polarity.

What made you think that the current driver implementation is wrong?

> Co-developed-by: Zong Li <zong.li@...ive.com>
> Signed-off-by: Zong Li <zong.li@...ive.com>
> Co-developed-by: Vincent Chen <vincent.chen@...ive.com>
> Signed-off-by: Vincent Chen <vincent.chen@...ive.com>
> Signed-off-by: Nylon Chen <nylon.chen@...ive.com>
> ---
>  drivers/pwm/pwm-sifive.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
> index eabddb7c7820..b07c8598bb21 100644
> --- a/drivers/pwm/pwm-sifive.c
> +++ b/drivers/pwm/pwm-sifive.c
> @@ -113,6 +113,7 @@ static int pwm_sifive_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
>  	u32 duty, val;
>  
>  	duty = readl(ddata->regs + PWM_SIFIVE_PWMCMP(pwm->hwpwm));
> +	duty = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - duty;

I find it irritating that both values are assigned to duty. I'd spend
another variable and make this:

	inactive = readl(ddata->regs + PWM_SIFIVE_PWMCMP(pwm->hwpwm));
	duty = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - inactive;


>  
>  	state->enabled = duty > 0;
>  
> @@ -123,11 +124,10 @@ static int pwm_sifive_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
>  	state->period = ddata->real_period;
>  	state->duty_cycle =
>  		(u64)duty * ddata->real_period >> PWM_SIFIVE_CMPWIDTH;
> -	state->polarity = PWM_POLARITY_INVERSED;
> +	state->polarity = PWM_POLARITY_NORMAL;
>  
>  	return 0;
>  }
> -

Please keep this empty line between functions.

>  static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  			    const struct pwm_state *state)
>  {

Best regards
Uwe

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

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