pwm: atmel: Replace loop in prescale calculation by ad-hoc calculation

The calculated values are the same with the modified algorithm. The only
difference is that the calculation is a bit more efficient.

Acked-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

diff --git a/drivers/pwm/pwm-atmel.c b/drivers/pwm/pwm-atmel.c
index 199255b..c9fdf16 100644 (file)
--- a/drivers/pwm/pwm-atmel.c
+++ b/drivers/pwm/pwm-atmel.c
@@ -60,7 +60,7 @@ struct atmel_pwm_registers {
 };
 
 struct atmel_pwm_config {
-       u32 max_period;
+       u32 period_bits;
 };
 
 struct atmel_pwm_data {
@@ -119,17 +119,27 @@ static int atmel_pwm_calculate_cprd_and_pres(struct pwm_chip *chip,
 {
        struct atmel_pwm_chip *atmel_pwm = to_atmel_pwm_chip(chip);
        unsigned long long cycles = state->period;
+       int shift;
 
        /* Calculate the period cycles and prescale value */
        cycles *= clk_get_rate(atmel_pwm->clk);
        do_div(cycles, NSEC_PER_SEC);
 
-       for (*pres = 0; cycles > atmel_pwm->data->cfg.max_period; cycles >>= 1)
-               (*pres)++;
+       /*
+        * The register for the period length is cfg.period_bits bits wide.
+        * So for each bit the number of clock cycles is wider divide the input
+        * clock frequency by two using pres and shift cprd accordingly.
+        */
+       shift = fls(cycles) - atmel_pwm->data->cfg.period_bits;
 
-       if (*pres > PWM_MAX_PRES) {
+       if (shift > PWM_MAX_PRES) {
                dev_err(chip->dev, "pres exceeds the maximum value\n");
                return -EINVAL;
+       } else if (shift > 0) {
+               *pres = shift;
+               cycles >>= *pres;
+       } else {
+               *pres = 0;
        }
 
        *cprd = cycles;
@@ -289,7 +299,7 @@ static const struct atmel_pwm_data atmel_sam9rl_pwm_data = {
        },
        .cfg = {
                /* 16 bits to keep period and duty. */
-               .max_period     = 0xffff,
+               .period_bits    = 16,
        },
 };
 
@@ -302,7 +312,7 @@ static const struct atmel_pwm_data atmel_sama5_pwm_data = {
        },
        .cfg = {
                /* 16 bits to keep period and duty. */
-               .max_period     = 0xffff,
+               .period_bits    = 16,
        },
 };
 
@@ -315,7 +325,7 @@ static const struct atmel_pwm_data mchp_sam9x60_pwm_data = {
        },
        .cfg = {
                /* 32 bits to keep period and duty. */
-               .max_period     = 0xffffffff,
+               .period_bits    = 32,
        },
 };