Re: [PATCH 4/5] leds: leds-pwm: implement PWM inversion

[Alexandre Belloni <alexandre.belloni@free-electrons.com>]

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(adding Thierry Reding)

Hi,

On 06/04/2014 at 23:20:18 +0100, Russell King wrote :
> Some PWM outputs are wired such that the LED they're controlling is
> connected to supply rather than ground.  These PWMs may not support
> output inversion, or when they do, disabling the PWM may set the
> PWM output low, causing a "brightness" value of zero to turn the LED
> fully on.
> 
> The platform data for this driver already indicates that this was
> thought about, and we have the "active_low" property there already.
> However, the implementation for this is missing.
> 
> Add the trivial implementation for this feature.
> 
> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
> ---
>  drivers/leds/leds-pwm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/leds/leds-pwm.c b/drivers/leds/leds-pwm.c
> index e1b4c23a409a..1d47742c551f 100644
> --- a/drivers/leds/leds-pwm.c
> +++ b/drivers/leds/leds-pwm.c
> @@ -70,6 +70,10 @@ static void led_pwm_set(struct led_classdev *led_cdev,
>  
>  	duty *= brightness;
>  	do_div(duty, max);
> +
> +	if (led_dat->active_low)
> +		duty = led_dat->period - duty;
> +
>  	led_dat->duty = duty;
>  

This will conflict with my patch (which is still lacking proper review)
there:
http://thread.gmane.org/gmane.linux.leds/482

I would say that it is better to hide the polarity inversion in the PWM
driver for your specific PWM. Else we will end up with all the drivers
using PWMs trying to detect whether the PWM supports inversion and if it
is not the case, calculating the inverted duty cycle.

So, I would go for my patch which is adding the missing polarity
inversion setting when using platform data and then implement software
polarity inversion in your underlying PWM driver. That also avoids patch
5/5 and I believe not adding a DT property is always a good idea.

What is your PWM that is not supporting polarity inversion ?

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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