Re: [PATCH V3 2/4] pwm: tegra: Increase precision in pwm rate calculation

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, Apr 07, 2017 at 03:04:00PM +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.
> 
> 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 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 = 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>
> 
> ---
> Changes from v1:
> - None
> 
> Changes from V2:
> - Fix the commit message with exact formula used.
> ---
>  drivers/pwm/pwm-tegra.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
> index 0a688da..21518be 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);

I think hz could overflow for small enough values of period_ns. I've
sent a patch that makes hz unsigned long long. While at it, the patch
also removes the ns100 variable which isn't really necessary here.

Thierry

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