linux-kernel - Re: [PATCH] RFC: leds-pwm: don't disable pwm when setting brightness to 0

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Message-ID: <20121025080346.GB9921@S2101-09.ap.freescale.net>
Date:	Thu, 25 Oct 2012 16:03:49 +0800
From:	Shawn Guo <shawn.guo@...aro.org>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Cc:	Thierry Reding <thierry.reding@...onic-design.de>,
	kernel@...gutronix.de, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bryan Wu <bryan.wu@...onical.com>,
	Richard Purdie <rpurdie@...ys.net>, linux-leds@...r.kernel.org
Subject: Re: [PATCH] RFC: leds-pwm: don't disable pwm when setting brightness
 to 0

On Wed, Oct 24, 2012 at 03:52:46PM +0200, Uwe Kleine-König wrote:
> This fixes disabling the LED on i.MX28. The PWM hardware delays using
> the newly set pwm-config until the beginning of a new period.  It's very
> likely that pwm_disable is called before the current period ends. In
> case the LED was on brightness=max before the LED stays on because in
> the disabled PWM block the period never ends.
> 
> It's unclear if the mxs-pwm driver doesn't implement the API as expected
> (i.e. it should block until the newly set config is effective) or if the
> leds-pwm driver makes wrong assumptions. This patch assumes the latter.
> 
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> ---
> Hello,
> 
> I'm not sure this is correct, but this is the workaround I'm using until
> I get some feed back.

I'm fine with it, since it fixes a real problem.  Let's see what
Thierry says.

Shawn

> 
> Best regards
> Uwe
> 
>  drivers/leds/leds-pwm.c |    9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/leds/leds-pwm.c b/drivers/leds/leds-pwm.c
> index f2e44c7..a909f4f 100644
> --- a/drivers/leds/leds-pwm.c
> +++ b/drivers/leds/leds-pwm.c
> @@ -38,13 +38,8 @@ static void led_pwm_set(struct led_classdev *led_cdev,
>  	unsigned int max = led_dat->cdev.max_brightness;
>  	unsigned int period =  led_dat->period;
>  
> -	if (brightness == 0) {
> -		pwm_config(led_dat->pwm, 0, period);
> -		pwm_disable(led_dat->pwm);
> -	} else {
> -		pwm_config(led_dat->pwm, brightness * period / max, period);
> -		pwm_enable(led_dat->pwm);
> -	}
> +	pwm_config(led_dat->pwm, brightness * period / max, period);
> +	pwm_enable(led_dat->pwm);
>  }
>  
>  static int led_pwm_probe(struct platform_device *pdev)
> -- 
> 1.7.10.4
> 
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