pwm: meson: Simplify the calculation of the pre-divider and count (fb2081e8) · Commits · 戴 / test

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. Three 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%) 3) Theoretical case where pre_div is different clock input: 24MHz (XTAL) period: 2730624ns duty cycle: 1365312ns old algorithm: pre_div=1, cnt=32768 new algorithm: pre_div=0, cnt=65534 Using a scope (24MHz sampling rate) shows the actual difference: - old: 2731000ns - new: 2731000ns (my scope is not precise enough to measure the difference if there's any) Suggested-by:

Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Acked-by:

Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Reviewed-by:

Neil Armstrong <narmstrong@baylibre.com> Signed-off-by:

Martin Blumenstingl <martin.blumenstingl@googlemail.com> Signed-off-by:

Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-meson.c

+10 −15

Original line number	Diff line number	Diff line
		@@ -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;
		}

		dev_dbg(meson->chip.dev, "period=%u pre_div=%u cnt=%u\n", period,
		pre_div, cnt);

		@@ -195,8 +190,8 @@ static int meson_pwm_calc(struct meson_pwm meson, struct pwm_device pwm,
		channel->lo = cnt;
		} else {
		/* Then check is we can have the duty with the same pre_div */
		duty_cnt = DIV_ROUND_CLOSEST_ULL((u64)duty * 1000,
		fin_ps * (pre_div + 1));
		duty_cnt = div64_u64(fin_freq * (u64)duty,
		NSEC_PER_SEC * (pre_div + 1));
		if (duty_cnt > 0xffff) {
		dev_err(meson->chip.dev, "unable to get duty cycle\n");
		return -EINVAL;

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