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pwm: tegra: Increase precision in PWM rate calculation

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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.

Change this to have more precision as:

	hz = DIV_ROUND_CLOSEST_ULL(NSEC_PER_SEC * 100, period_ns);
	rate = DIV_ROUND_CLOSEST(rate * 100, hz)

Example:

1. period_ns = 16672000, PWM clock rate is 200 KHz.

	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 200 KHz.

	Based on old formula:
		hz = 59, rate = 3390

	Based on new formula:
		hz = 5951, rate = 3360

	The PWM signal rate of 3360 is more near to requested period
	than 3333.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
This commit is contained in:
Laxman Dewangan 2017-04-07 15:04:00 +05:30 committed by Thierry Reding
parent 90241fb9b5
commit 250b76f43f
1 changed files with 5 additions and 2 deletions
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7
drivers/pwm/pwm-tegra.c
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@ -76,6 +76,7 @@ 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;
u32 val = 0;
int err;
@ -94,9 +95,11 @@ 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 *= 100;
hz = DIV_ROUND_CLOSEST_ULL(ns100, period_ns);
rate = DIV_ROUND_CLOSEST(rate * 100, hz);
/*
* Since the actual PWM divider is the register's frequency divider