[PATCH v3 2/4] pwm: jz4740: Enhance precision in calculation of duty cycle

From: Paul Cercueil
Date: Wed May 27 2020 - 07:52:59 EST

Calculating the hardware value for the duty from the hardware value of
the period resulted in a precision loss versus calculating it from the
clock rate directly.

(Also remove a cast that doesn't really need to be here)

Fixes: f6b8a5700057 ("pwm: Add Ingenic JZ4740 support")
Cc: <stable@xxxxxxxxxxxxxxx>
Suggested-by: Uwe Kleine-KÃnig <u.kleine-koenig@xxxxxxxxxxxxxx>
Reviewed-by: Uwe Kleine-KÃnig <u.kleine-koenig@xxxxxxxxxxxxxx>
Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>

v2: New patch. I don't consider this a fix but an enhancement, since the old
behaviour was in place since the driver was born in ~2010, so no Fixes tag.
v3: Add Fixes tag and Uwe's Reviewed-by

drivers/pwm/pwm-jz4740.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-jz4740.c b/drivers/pwm/pwm-jz4740.c
index 3cd5c054ad9a..4fe9d99ac9a9 100644
--- a/drivers/pwm/pwm-jz4740.c
+++ b/drivers/pwm/pwm-jz4740.c
@@ -158,11 +158,11 @@ static int jz4740_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
/* Calculate period value */
tmp = (unsigned long long)rate * state->period;
do_div(tmp, NSEC_PER_SEC);
- period = (unsigned long)tmp;
+ period = tmp;

/* Calculate duty value */
- tmp = (unsigned long long)period * state->duty_cycle;
- do_div(tmp, state->period);
+ tmp = (unsigned long long)rate * state->duty_cycle;
+ do_div(tmp, NSEC_PER_SEC);
duty = period - tmp;

if (duty >= period)