Re: Adding support for configuring polarity in PWM framework.

From: Lars-Peter Clausen
Date: Mon Jul 16 2012 - 08:42:09 EST

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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