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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: 211ed63075 ("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
zero-colors
Jerome Brunet 2017-06-08 14:24:16 +02:00 committed by Thierry Reding
parent d396b20a1e
commit fd7b2be8cb
1 changed files with 10 additions and 6 deletions
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16
drivers/pwm/pwm-meson.c
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@ -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;