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| author | Laxman Dewangan <ldewangan@nvidia.com> | 2017-04-07 11:34:00 +0200 |
|---|---|---|
| committer | Thierry Reding <thierry.reding@gmail.com> | 2017-04-12 16:08:15 +0200 |
| commit | 250b76f43f57d578ebff5e7211eb2c73aa5cd6ca (patch) | |
| tree | e0635c1349f94e77e02ade5103052124b7d75f5e /drivers/pwm | |
| parent | pwm: tegra: Use DIV_ROUND_CLOSEST_ULL() instead of local implementation (diff) | |
| download | linux-250b76f43f57d578ebff5e7211eb2c73aa5cd6ca.tar.xz linux-250b76f43f57d578ebff5e7211eb2c73aa5cd6ca.zip | |
pwm: tegra: Increase precision in PWM rate calculation
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>
Diffstat (limited to 'drivers/pwm')
| -rw-r--r-- | drivers/pwm/pwm-tegra.c | 7 |
1 files changed, 5 insertions, 2 deletions
diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c index 0a688dabd670..21518bea00c6 100644 --- a/drivers/pwm/pwm-tegra.c +++ b/drivers/pwm/pwm-tegra.c @@ -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 |