Messages in this thread | | | Date | Sun, 26 May 2019 21:41:20 +0200 | From | Uwe Kleine-König <> | Subject | Re: [PATCH 10/14] pwm: meson: simplify the calculation of the pre-divider and count |
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On Sat, May 25, 2019 at 08:11:29PM +0200, Martin Blumenstingl wrote: > Replace the loop to calculate the pre-divider and count with two > separate div64_u64() calculations. This makes the code easier to read > and improves the precision. > > Two example cases: > 1) 32.768kHz LPO clock for the SDIO wifi chip on Khadas VIM > clock input: 500MHz (FCLK_DIV4) > period: 30518ns > duty cycle: 15259ns > old algorithm: pre_div=0, cnt=15259 > new algorithm: pre_div=0, cnt=15259 > (no difference in calculated values) > > 2) PWM LED on Khadas VIM > clock input: 24MHz (XTAL) > period: 7812500ns > duty cycle: 7812500ns > old algorithm: pre_div=2, cnt=62004 > new algorithm: pre_div=2, cnt=62500 > Using a scope (24MHz sampling rate) shows the actual difference: > - old: 7753000ns, off by -59500ns (0.7616%) > - new: 7815000ns, off by +2500ns (0.032%) > > Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> > Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com> > --- > drivers/pwm/pwm-meson.c | 25 ++++++++++--------------- > 1 file changed, 10 insertions(+), 15 deletions(-) > > diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c > index 27915d6475e3..9afa1e5aaebf 100644 > --- a/drivers/pwm/pwm-meson.c > +++ b/drivers/pwm/pwm-meson.c > @@ -12,6 +12,7 @@ > #include <linux/err.h> > #include <linux/io.h> > #include <linux/kernel.h> > +#include <linux/math64.h> > #include <linux/module.h> > #include <linux/of.h> > #include <linux/of_device.h> > @@ -145,7 +146,6 @@ static int meson_pwm_calc(struct meson_pwm *meson, struct pwm_device *pwm, > struct meson_pwm_channel *channel = pwm_get_chip_data(pwm); > unsigned int duty, period, pre_div, cnt, duty_cnt; > unsigned long fin_freq = -1; > - u64 fin_ps; > > duty = state->duty_cycle; > period = state->period; > @@ -164,24 +164,19 @@ static int meson_pwm_calc(struct meson_pwm *meson, struct pwm_device *pwm, > } > > dev_dbg(meson->chip.dev, "fin_freq: %lu Hz\n", fin_freq); > - fin_ps = (u64)NSEC_PER_SEC * 1000; > - do_div(fin_ps, fin_freq); > - > - /* Calc pre_div with the period */ > - for (pre_div = 0; pre_div <= MISC_CLK_DIV_MASK; pre_div++) { > - cnt = DIV_ROUND_CLOSEST_ULL((u64)period * 1000, > - fin_ps * (pre_div + 1)); > - dev_dbg(meson->chip.dev, "fin_ps=%llu pre_div=%u cnt=%u\n", > - fin_ps, pre_div, cnt); > - if (cnt <= 0xffff) > - break; > - } > > + pre_div = div64_u64(fin_freq * (u64)period, NSEC_PER_SEC * 0xffffLL); > if (pre_div > MISC_CLK_DIV_MASK) { > dev_err(meson->chip.dev, "unable to get period pre_div\n"); > return -EINVAL; > } > > + cnt = div64_u64(fin_freq * (u64)period, NSEC_PER_SEC * (pre_div + 1)); > + if (cnt > 0xffff) { > + dev_err(meson->chip.dev, "unable to get period cnt\n"); > + return -EINVAL; > + } > +
There is a slight modification in the calculation of pre_div that isn't catched by the examples above.
Before this patch we had:
pick smallest pre_div such that round_closest(period * 1000 / (round_down(1e12 / fin_freq) * (pre_div + 1)) <= 0xffff
New approach is:
pre_div = round_down(fin_freq * period / (1e9 * 0xffff))
An advantage of the new approach is better as it rounds only once and is easier.
Consider fin_freq = 99990001 and period = 655355, then the old algorithm picks pre_div = 1 while the new picks pre_div = 0.
I didn't continue here to check which are the resulting waveforms, I assume they are different though.
As there is currently no definition what is a "better" approximation for a given requested pair (duty_cycle, period) I cannot say if these changes are good or not.
And that's a pity, so I still think there should be a documented definition that lays down how a lowlevel driver should round. Without that a consumer that cares about fine differences can not rely an the abstraction provided by the PWM framework because each low-level driver might behave differently.
@Thierry: So can you please continue the discussion about this topic. The longer this is delayed the more patches are created and submitted that eventually might be wrong which is a waste of developer and reviewer time.
Assuming the people who care about meson don't object after reading this I wouldn't want to stop this patch going in though. So:
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Best regards Uwe
-- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |
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