pwm: meson: Improve PWM calculation precision

When using input clocks with high rates, such as clk81 (166MHz), the
fin_ns = NSEC_PER_SEC / fin_freq can introduce a significant error.

Ex: fin_freq = 166666667, NSEC_PER_SEC = 1000000000
    fin_ns = 5,9999999

which is, of course, rounded down to 5. This introduces an error of ~20%
on the period requested from the PWM.

This patch uses ps instead of ns (and 64 bit integers) to perform the
calculation. This should give a good enough precision.

Fixes: 211ed630753d ("pwm: Add support for Meson PWM Controller")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
squash! pwm: meson: Improve pwm calculation precision

diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
index defc27d..cb845ed 100644 (file)
--- a/drivers/pwm/pwm-meson.c
+++ b/drivers/pwm/pwm-meson.c
@@ -163,7 +163,8 @@ static int meson_pwm_calc(struct meson_pwm *meson,
                          unsigned int duty, unsigned int period)
 {
        unsigned int pre_div, cnt, duty_cnt;
-       unsigned long fin_freq = -1, fin_ns;
+       unsigned long fin_freq = -1;
+       u64 fin_ps;
 
        if (~(meson->inverter_mask >> id) & 0x1)
                duty = period - duty;
@@ -179,13 +180,15 @@ static int meson_pwm_calc(struct meson_pwm *meson,
        }
 
        dev_dbg(meson->chip.dev, "fin_freq: %lu Hz\n", fin_freq);
-       fin_ns = NSEC_PER_SEC / 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(period, fin_ns * (pre_div + 1));
-               dev_dbg(meson->chip.dev, "fin_ns=%lu pre_div=%u cnt=%u\n",
-                       fin_ns, pre_div, cnt);
+               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;
        }
@@ -208,7 +211,8 @@ static int meson_pwm_calc(struct meson_pwm *meson,
                channel->lo = cnt;
        } else {
                /* Then check is we can have the duty with the same pre_div */
-               duty_cnt = DIV_ROUND_CLOSEST(duty, fin_ns * (pre_div + 1));
+               duty_cnt = DIV_ROUND_CLOSEST_ULL((u64)duty * 1000,
+                                                fin_ps * (pre_div + 1));
                if (duty_cnt > 0xffff) {
                        dev_err(meson->chip.dev, "unable to get duty cycle\n");
                        return -EINVAL;