pwm: meson: Simplify the calculation of the pre-divider and count

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>

1 files changed, 10 insertions, 15 deletions

diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
index 8a5658f2b947..99815b26f6b2 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;
+        }
+
         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;