pwm: bcm2835: Improve precision of PWM

If sending IR with carrier of 455kHz using the pwm-ir-tx driver, the
carrier ends up being 476kHz. The clock is set to bcm2835-pwm with a
rate of 10MHz.

A carrier of 455kHz has a period of 2198ns, but the arithmetic truncates
this to 2100ns rather than 2200ns. So, use DIV_ROUND_CLOSEST() to reduce
rounding errors, and we have a much more accurate carrier of 454.5kHz.

Reported-by: Andreas Christ <andreas@christ-faesch.ch>
Signed-off-by: Sean Young <sean@mess.org>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

diff --git a/drivers/pwm/pwm-bcm2835.c b/drivers/pwm/pwm-bcm2835.c
index 5652f46..f6fe0b9 100644 (file)
--- a/drivers/pwm/pwm-bcm2835.c
+++ b/drivers/pwm/pwm-bcm2835.c
@@ -70,7 +70,7 @@ static int bcm2835_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
                return -EINVAL;
        }
 
-       scaler = NSEC_PER_SEC / rate;
+       scaler = DIV_ROUND_CLOSEST(NSEC_PER_SEC, rate);
 
        if (period_ns <= MIN_PERIOD) {
                dev_err(pc->dev, "period %d not supported, minimum %d\n",
@@ -78,8 +78,10 @@ static int bcm2835_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
                return -EINVAL;
        }
 
-       writel(duty_ns / scaler, pc->base + DUTY(pwm->hwpwm));
-       writel(period_ns / scaler, pc->base + PERIOD(pwm->hwpwm));
+       writel(DIV_ROUND_CLOSEST(duty_ns, scaler),
+              pc->base + DUTY(pwm->hwpwm));
+       writel(DIV_ROUND_CLOSEST(period_ns, scaler),
+              pc->base + PERIOD(pwm->hwpwm));
 
        return 0;
 }