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>

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;
 }