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    SubjectRE: Adding support for configuring polarity in PWM framework.
    Date
    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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