Messages in this thread | | | Date | Mon, 16 Jul 2012 14:46:13 +0200 | From | Lars-Peter Clausen <> | Subject | Re: Adding support for configuring polarity in PWM framework. |
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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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