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SubjectRe: [PATCH v9 14/27] pwm: jz4740: Improve algorithm of clock calculation
On Thu, Dec 27, 2018 at 07:13:06PM +0100, Paul Cercueil wrote:
> The previous algorithm hardcoded details about how the TCU clocks work.
> The new algorithm will use clk_round_rate to find the perfect clock rate
> for the PWM channel.
>
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> ---
>
> Notes:
> v9: New patch
>
> drivers/pwm/pwm-jz4740.c | 26 +++++++++++++++-----------
> 1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/pwm/pwm-jz4740.c b/drivers/pwm/pwm-jz4740.c
> index c6136bd4434b..dd80a2cf6528 100644
> --- a/drivers/pwm/pwm-jz4740.c
> +++ b/drivers/pwm/pwm-jz4740.c
> @@ -110,23 +110,27 @@ static int jz4740_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> struct jz4740_pwm_chip *jz4740 = to_jz4740(pwm->chip);
> struct clk *clk = jz4740->clks[pwm->hwpwm],
> *parent_clk = clk_get_parent(clk);
> - unsigned long rate, period, duty;
> + unsigned long rate, new_rate, period, duty;
> unsigned long long tmp;
> - unsigned int prescaler = 0;
>
> rate = clk_get_rate(parent_clk);
> - tmp = (unsigned long long)rate * state->period;
> - do_div(tmp, 1000000000);
> - period = tmp;
>
> - while (period > 0xffff && prescaler < 6) {
> - period >>= 2;
> - rate >>= 2;
> - ++prescaler;
> + for (;;) {
> + tmp = (unsigned long long)rate * state->period;
> + do_div(tmp, 1000000000);

NSEC_PER_SEC?

> +
> + if (tmp <= 0xffff)
> + break;
> +
> + new_rate = clk_round_rate(clk, rate - 1);
> +
> + if (new_rate < rate)
> + rate = new_rate;
> + else
> + return -EINVAL;

You are assuming stuff here about the parent clk which isn't guaranteed
(AFAICT) by the clk framework: If you call clk_round_rate(clk, rate - 1)
this might well return rate even if the clock could run slower than
rate.

Wouldn't it make sense to start iterating with rate = 0xffff * 1e9 /
period? Otherwise you get bad configurations if rate is considerable
slower than necessary.

Best regards
Uwe

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

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