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

1 files changed, 10 insertions, 6 deletions

diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
index defc27d880f3..cb845edfe2b4 100644
--- 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;