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SubjectRe: [PATCH v2 3/3] pwm: meson: improve pwm calculation precision.
On Thu, Jun 08, 2017 at 02:24:16PM +0200, Jerome Brunet wrote:
> 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 introduce an error of ~20%
> on the period requested from the pwm.
>
> This patch use ps instead of ns (and 64bits integer) 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>
> ---
> drivers/pwm/pwm-meson.c | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
> index b911a944744a..4cdc66f7f718 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 +180,14 @@ 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) / fin_freq;

This failed to build, so I had to change this division to a do_div().

Thierry
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