pwm: atmel: Fix calculation of prescale value
The prescale value used for calculating the period was incremented
afterwards, thus the resulting prescale value is by one too high.
This resulted in a PWM frequency only half as high as requested.
This patch moves the 64 bit division out of the prescale loop to
correct the above issue and make the calculation more efficient.
Signed-off-by: Nikolaus Voss <n.voss@weinmann-emt.de>
Tested-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
diff --git a/drivers/pwm/pwm-atmel.c b/drivers/pwm/pwm-atmel.c
index 6e700a5..d3c22de 100644
--- a/drivers/pwm/pwm-atmel.c
+++ b/drivers/pwm/pwm-atmel.c
@@ -102,7 +102,7 @@
int duty_ns, int period_ns)
{
struct atmel_pwm_chip *atmel_pwm = to_atmel_pwm_chip(chip);
- unsigned long clk_rate, prd, dty;
+ unsigned long prd, dty;
unsigned long long div;
unsigned int pres = 0;
u32 val;
@@ -113,20 +113,18 @@
return -EBUSY;
}
- clk_rate = clk_get_rate(atmel_pwm->clk);
- div = clk_rate;
+ /* Calculate the period cycles and prescale value */
+ div = (unsigned long long)clk_get_rate(atmel_pwm->clk) * period_ns;
+ do_div(div, NSEC_PER_SEC);
- /* Calculate the period cycles */
while (div > PWM_MAX_PRD) {
- div = clk_rate / (1 << pres);
- div = div * period_ns;
- /* 1/Hz = 100000000 ns */
- do_div(div, 1000000000);
+ div >>= 1;
+ pres++;
+ }
- if (pres++ > PRD_MAX_PRES) {
- dev_err(chip->dev, "pres exceeds the maximum value\n");
- return -EINVAL;
- }
+ if (pres > PRD_MAX_PRES) {
+ dev_err(chip->dev, "pres exceeds the maximum value\n");
+ return -EINVAL;
}
/* Calculate the duty cycles */
