[PATCH V2 2/4] pwm: tegra: Increase precision in pwm rate calculation

Laxman Dewangan
2017-04-06T14:41:44Z

The rate of the PWM calculated as follows:
hz = NSEC_PER_SEC / period_ns;
rate = (rate + (hz / 2)) / hz;

This has the precision loss in lower PWM rate.
Changing this to have more precision as:
hz = DIV_ROUND_CLOSE(NSEC_PER_SEC * 100, period_ns);
rate = DIV_ROUND_CLOSE(rate * 100, hz)

Example:
1. period_ns = 16672000, PWM clock rate is 200KHz.
Based on old formula
hz = NSEC_PER_SEC / period_ns
= 1000000000ul/16672000
= 59 (59.98)
rate = (200K + 59/2)/59 = 3390

Based on new method:
hz = 5998
rate = DIV_ROUND_CLOSE(200000*100, 5998) = 3334

If we measure the PWM signal rate, we will get more accurate period
with rate value of 3334 instead of 3390.

2. period_ns = 16803898, PWM clock rate is 200KHz.
Based on old formula:
hz = 60, rate = 3333
Based on new formula:
hz = 5951, rate = 3360

The rate of 3360 is more near to requested period then the 3333.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
---
Changes from V1:
- None

drivers/pwm/pwm-tegra.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
index 0a688da..e9c4de5 100644
--- a/drivers/pwm/pwm-tegra.c
+++ b/drivers/pwm/pwm-tegra.c
@@ -76,6 +76,8 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
struct tegra_pwm_chip *pc = to_tegra_pwm_chip(chip);
unsigned long long c = duty_ns;
unsigned long rate, hz;
+ unsigned long long ns100 = NSEC_PER_SEC;
+ unsigned long precision = 100; /* Consider 2 digit precision */
u32 val = 0;
int err;

@@ -94,9 +96,11 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
* cycles at the PWM clock rate will take period_ns nanoseconds.
*/
rate = clk_get_rate(pc->clk) >> PWM_DUTY_WIDTH;
- hz = NSEC_PER_SEC / period_ns;

- rate = (rate + (hz / 2)) / hz;
+ /* Consider precision in PWM_SCALE_WIDTH rate calculation */
+ ns100 *= precision;
+ hz = DIV_ROUND_CLOSEST_ULL(ns100, period_ns);
+ rate = DIV_ROUND_CLOSEST(rate * precision, hz);

/*
* Since the actual PWM divider is the register's frequency divider
--
2.1.4