Messages in this thread | | | Subject | Re: [PATCH 10/14] pwm: meson: simplify the calculation of the pre-divider and count | From | Neil Armstrong <> | Date | Mon, 27 May 2019 14:37:05 +0200 |
| |
On 26/05/2019 21:41, Uwe Kleine-König wrote: > 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 >
I don't have a strong view on this, Martin showed similar or much greater accuracy in the 2 principal use cases of the driver, so I'm ok with it.
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
| |