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SubjectRe: [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@sifive.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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