linux-kernel - Re: [PWM PATCH 1/5] API to consolidate PWM devices behind a common user and kernel interface

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Date:	Thu, 11 Feb 2010 22:00:33 +0100
From:	Pavel Machek <pavel@....cz>
To:	Bill Gatliff <bgat@...lgatliff.com>
Cc:	linux-embedded@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PWM PATCH 1/5] API to consolidate PWM devices behind a common
 user and kernel interface


> >> +pwm_free() -- Marks a PWM channel as no longer in use.  The PWM device
> >> +is stopped before it is released by the API.
> >>     
> >
> > free is normally used for something else. Rename to open/close?

> ... or request/release?

Works for me.

> >> +pwm_start(), pwm_stop() -- Turns the PWM signal on and off.  Except
> >> +where stated otherwise by a driver author, signals are stopped at the
> >> +end of the current period, at which time the output is set to its
> >> +inactive state.
> >>     
> >
> > What does it mean to stop a signal? What is the difference between 0%
> > duty cycle and stop() ?
> >   
> 
> Depends on the hardware.  For a true PWM peripheral, a 0% duty cycle
> might still have the base peripheral clock for the device running. 
> Whereas a pwm_stop() signal could be used to turn off the clock to the
> peripheral.

If it is just powersaving... I'd do it automatically when 0% duty is
selected...? Or is that infeasible due to latency...?

> > Is polarity realy required? Can't driver just replace duty with
> > 100%-duty
> 
> Actually, yes in some cases.  Users can always do the 100%-duty math,
> but some hardware asserts a specific output state when you stop the
> peripheral that's potentially different from 0% duty.  Also, some
> hardware begins the PWM cycle with the output high, while others do with
> the output low.  It isn't necessarily the case that the user cares, but
> I was thinking that having the API allow for different polarity might
> prevent some applications having to optionally do the %duty vs.
> 100-%duty conversion themselves.

Ok, ok, but this should go into the docs.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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