linux-kernel - Re: [PATCH v2 2/2] pwm: sifive: change the PWM controlled LED algorithm

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Date:   Mon, 30 Jan 2023 11:17:07 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Nylon Chen <nylon.chen@...ive.com>
Cc:     aou@...s.berkeley.edu, conor@...nel.org,
        emil.renner.berthing@...onical.com, geert+renesas@...der.be,
        heiko@...ech.de, krzysztof.kozlowski+dt@...aro.org,
        palmer@...belt.com, paul.walmsley@...ive.com, robh+dt@...nel.org,
        thierry.reding@...il.com, devicetree@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, nylon7717@...il.com,
        zong.li@...ive.com, greentime.hu@...ive.com,
        vincent.chen@...ive.com
Subject: Re: [PATCH v2 2/2] pwm: sifive: change the PWM controlled LED
 algorithm

On Mon, Jan 30, 2023 at 05:32:29PM +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].
> 
> [0]: https://sifive.cdn.prismic.io/sifive/1a82e600-1f93-4f41-b2d8-86ed8b16acba_fu740-c000-manual-v1p6.pdf
> 
> Signed-off-by: Nylon Chen <nylon.chen@...ive.com>
> ---
>  drivers/pwm/pwm-sifive.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
> index 62b6acc6373d..a5eda165d071 100644
> --- a/drivers/pwm/pwm-sifive.c
> +++ b/drivers/pwm/pwm-sifive.c
> @@ -158,6 +158,7 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
>  	/* The hardware cannot generate a 100% duty cycle */
>  	frac = min(frac, (1U << PWM_SIFIVE_CMPWIDTH) - 1);
> +	frac = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - frac;

The same problem exists in pwm_sifive_get_state(), doesn't it?

As fixing this is an interruptive change anyhow, this is the opportunity
to align the driver to the rules tested by PWM_DEBUG.

The problems I see in the driver (only checked quickly, so I might be
wrong):

 - state->period != ddata->approx_period isn't necessarily a problem. If
   state->period > ddata->real_period that's fine and the driver should
   continue

 - frac = DIV64_U64_ROUND_CLOSEST(num, state->period);
   is wrong for two reasons:
   it should round down and use the real period.

Best regards
Uwe

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

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