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-koe...@pengutronix.de>
>> Signed-off-by: Martin Blumenstingl <martin.blumensti...@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-koe...@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 <narmstr...@baylibre.com>
