On Mon, Jul 16, 2012 at 12:23:46PM +0000, 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. > > This is a client specific data (backlight needs pwm signal inversed) > and not the main device feature (not PWM IP). So we cannot send this in > pwm platform data. This would come as call from client driver (which in > our case is from pwm_bl.c)
Okay, I see.
> > 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.
Okay, that sounds sensible. A couple of comments though.
> 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.
> */
> int pwm_setpolarity(struct pwm_device *pwm, bool inversepol);
This should match the pwm_ops name, i.e.: pwm_set_polarity().
Making the polarity argument a boolean is slightly confusing. For
instance I'd say the logical value if I want normal behaviour would be
to set it to true, which doesn't match your example. So I propose you
define the polarity parameter as an enumeration to make its meaning more
explicit:
enum pwm_polarity {
PWM_POLARITY_NORMAL,
PWM_POLARITY_INVERSE,
};
PWM_POLARITY_{HIGH,LOW} and PWM_POLARITY_{POSITIVE,NEGATIVE} would be
other good name pairs.
> 2. Add "set_polarity" operation support in pwm_ops.
>
> 3. Modification in backlight driver (pwm_bl.c) to support polarity
> configuration.
We also need to think about how this could be represented in the device
tree. The most obvious choice seems to be a third cell for the specifier
and use a custom of_xlate callback for controllers that support polarity
inversion (and later perhaps other flags).
Also would you mind sharing the board setup code that you need this for?
I find it easier to get into the right mindset when looking at code that
actually uses this.
Thierry
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