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

Message ID 1491488461-24621-3-git-send-email-ldewangan@nvidia.com Superseded show

## Commit Message

Laxman Dewangan April 6, 2017, 2:20 p.m.
```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(-)
```

Thierry Reding April 6, 2017, 4:24 p.m. | #1
```On Thu, Apr 06, 2017 at 07:50:59PM +0530, Laxman Dewangan wrote:
> 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)

DIV_ROUND_CLOSEST(). And I much prefer this to the actual code below. I
don't think it's necessary to have a local variable for the precision.

Thierry

> 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
>
```
Thierry Reding April 6, 2017, 4:28 p.m. | #2
```On Thu, Apr 06, 2017 at 07:50:59PM +0530, Laxman Dewangan wrote:
> 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)

DIV_ROUND_CLOSEST(). Also I very much prefer this notation over the
actual code below. I don't think we need a local variable to hold the
precision.

Thierry

> 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
>
```

## Patch

```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

```