linux-kernel - Re: [PATCH] pwm: lpc32xx

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Date:	Thu, 08 Nov 2012 10:51:35 +0100
From:	Roland Stigge <stigge@...com.de>
To:	Alban Bedel <alban.bedel@...onic-design.de>
CC:	Thierry Reding <thierry.reding@...onic-design.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexandre Pereira da Silva <aletes.xgr@...il.com>
Subject: Re: [PATCH] pwm: lpc32xx - Fix the PWM polarity

On 07/11/12 16:25, Alban Bedel wrote:
> Signed-off-by: Alban Bedel <alban.bedel@...onic-design.de>
> ---
>  drivers/pwm/pwm-lpc32xx.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-lpc32xx.c b/drivers/pwm/pwm-lpc32xx.c
> index adb87f0..0dc278d 100644
> --- a/drivers/pwm/pwm-lpc32xx.c
> +++ b/drivers/pwm/pwm-lpc32xx.c
> @@ -51,7 +51,11 @@ static int lpc32xx_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  
>  	c = 256 * duty_ns;
>  	do_div(c, period_ns);
> -	duty_cycles = c;
> +	if (c == 0)
> +		c = 256;
> +	if (c > 255)
> +		c = 255;
> +	duty_cycles = 256 - c;

Except for the range check (for the original c > 255), this results in:

	duty_cycles = 256 - c

except for (c == 0) where

	duty_cycles = 1

which actually is

	duty_cycles = (256 - c) - 255

(think with the original c)

i.e. nearly a polarity inversion in the case of (c == 0).

Why is the case (c == 0) so special here? Maybe you can document this,
if it is really intended?

>  
>  	writel(PWM_ENABLE | PWM_RELOADV(period_cycles) | PWM_DUTY(duty_cycles),
>  		lpc32xx->base + (pwm->hwpwm << 2));

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