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

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Date:	Mon, 16 Jul 2012 14:46:16 +0000
From:	"Philip, Avinash" <avinashphilip@...com>
To:	Thierry Reding <thierry.reding@...onic-design.de>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: Adding support for configuring polarity in PWM framework.

On Mon, Jul 16, 2012 at 18:16:08, Thierry Reding wrote:
> 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().

Ok.

> 
> 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.

Ok. I will do.

> 
> > 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).
> 

Ok I will try to use modified of_xlate callback and hope this can 
rescue pwm_bl.c modification.

> 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.
>

Here is the TI BSP link to support backlight inverse.

http://arago-project.org/git/projects/?p=linux-am33x.git;a=commitdiff;h=
59e96b24925e64fffd4664d696e41e1090c959b1;hp=
b180dcb341db0ff4ca1adbfac3f5dcd07be9e91d

But here we were Supporting PWM frame work from Bill Gatliff.
Also we were configuring PWM polarity directly using eCAP platform data
not from backlight platform data. 
But I think controlling through device/client is more methodical than 
direct PWM data handling.

Thanks
Avinash

> Thierry
> 

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