linux-kernel - Re: [PATCH V2 2/4] pwm: tegra: Increase precision in pwm rate calculation

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Message-ID: <20170406162417.GB19312@ulmo.ba.sec>
Date:   Thu, 6 Apr 2017 18:24:17 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Laxman Dewangan <ldewangan@...dia.com>
Cc:     robh+dt@...nel.org, jonathanh@...dia.com, mark.rutland@....com,
        linux-pwm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/4] pwm: tegra: Increase precision in pwm rate
 calculation

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@...dia.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
> 

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