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SubjectRe: [PATCHv3 1/3] pwm: make the PWM_POLARITY flag in DTB optional
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On Mon, Apr 7, 2014 at 10:02 PM, Lothar Waßmann <LW@karo-electronics.de> wrote:
> Thierry Reding wrote:

>> No. You cannot emulate polarity inversion in software.
>>
> Why not?
>
> duty_ns = period_ns - duty_ns;

Since I made the same mistake, I will pass along the pointer Thierry gave me.

In include/linux/pwm.h the second difference for an inverted signal is
described.

/**
* enum pwm_polarity - polarity of a PWM signal
* @PWM_POLARITY_NORMAL: a high signal for the duration of the duty-
* cycle, followed by a low signal for the remainder of the pulse
* period
* @PWM_POLARITY_INVERSED: a low signal for the duration of the duty-
* cycle, followed by a high signal for the remainder of the pulse
* period
*/
enum pwm_polarity {
PWM_POLARITY_NORMAL,
PWM_POLARITY_INVERSED,
};

Of course, I suspect not all PWM hardware respects this definition of
inverted output.

Either way, hacking the duty in software certainly would get the
high/low order wrong.

-Tim
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