linux-kernel - Re: Adding support for configuring polarity in PWM framework.

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Date:	Mon, 16 Jul 2012 14:46:13 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	"Philip, Avinash" <avinashphilip@...com>
CC:	Thierry Reding <thierry.reding@...onic-design.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Adding support for configuring polarity in PWM framework.

On 07/16/2012 02:23 PM, Philip, Avinash wrote:
> On Mon, Jul 16, 2012 at 17:09:21, Thierry Reding wrote:
>> On Mon, Jul 16, 2012 at 11:15:50AM +0000, Philip, Avinash wrote:
>>> Hi Thierry,
>>>
>>> On one of the custom boards we are using, uses PWM to drive the backlight. However, for
>>> this device, PWM signal needs to be inversed.
>>> So, we need to a platform data to indicate this parameter.
>>> Current PWM framework doesn't provide .support for setting polarity (or inverse polarity).
>>>
>>> Have you come across any such requirements? If so, do you have any plans to implement it?
>>
>> I don't have any plans to implement such a feature.
> 
> Ok. Thanks for the quick response.
>>
>>> I am planning to add support for the same but want to avoid duplication of work.
>>>
>>> If you have no plans, then I will send a patch to support the same.
>>
>> I wonder how you want to implement this. You'll need special hardware
>> support for it 
> 
> Yes. Our custom hardware (backlight booster) requires the pwm signal to be 
> inverted.
> 
>> you may be able to implement it in the driver itself 
>> instead of putting it into the framework. 
> 

I think this is a common feature amongst PWM chips that they are able to
invert the PWM signal. And some applications of PWM require that you are
able to specify the polarity, so I think it makes sense to put this into the
common framework.

>> Anyway I'm interested in seeing your patch.
> 
> I am planning to modify PWM framework as below.
> 1. Configure PWM polarity from client driver (using platform data provided 
> to pwm backlight driver).
> 2. PWM device needs to be disabled before calling the set-polarity API.
> 
> This involves
> 
> 1. PWM framework API addition.
> 	PWM frame work API support.
> 	/**
> 	 * pwm_setpolarity() - change a PWM device Polarity	
> 	 * @pwm: PWM device
> 	 * @polarity: Configure polarity of PWM
>  	*
> 	 * polarity     - false -> "on" time defined by duty ns
> 	*              - true  -> "off' time defined by duty ns.
>  	*/

Isn't this more about whether we start with a low or a high signal? If it is
just about the duty time you can easily achieve the same effect by setting
it to (period_ns - duty_ns).

- Lars

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