Re: [PATCH v5 07/13] pwm: add support for sl28cpld PWM controller

[Uwe Kleine-KÃnig]

Hello Michael,

On Wed, Jul 15, 2020 at 10:41:25PM +0200, Michael Walle wrote:
> Am 2020-07-15 20:18, schrieb Uwe Kleine-König:
> > On Wed, Jul 15, 2020 at 07:45:10PM +0200, Michael Walle wrote:
> > > 
> > > Am 2020-07-15 18:36, schrieb Uwe Kleine-König:
> > > > On Tue, Jul 14, 2020 at 11:09:28PM +0200, Michael Walle wrote:
> > > > > > My wishlist (just as it comes to my mind, so no guarantee of
> > > > > > completeness):
> > > > > >
> > > > > >  - can do 0% duty cycle for all supported period lengths
> > > > > >  - can do 100% duty cycle for all supported period lengths
> > > > > >  - supports both polarities
> > > > > >  - supports immediate change of configuration and after completion of
> > > > > >    the currently running period
> > > > > >  - atomic update (i.e. if you go from configuration A to configuration B
> > > > > >    the hardware guarantees to only emit periods of type A and then type
> > > > > >    B. (Depending on the item above, the last A period might be cut off.)
> > > > >
> > > > > We actually discussed this, because the implementation would be
> > > > > easier. But
> > > > > if the change takes place immediately you might end up with a longer
> > > > > duty
> > > > > cycle. Assume the PWM runs at 80% duty cycle and starts with the
> > > > > on-period.
> > > > > If you now change that to 50% you might end up with one successive
> > > > > duty
> > > > > cycle of "130%". Eg. the 80% of the old and right after that you
> > > > > switch to
> > > > > the new 50% and then you'd have a high output which corresponds to a
> > > > > 130%
> > > > > cycle. I don't know if that is acceptable for all applications.
> > > >
> > > > I thought this is a "change takes place immediately" implementation?! So
> > > > these problems are actually real here. (And this not happening is
> > > > exactly
> > > > my wish here. Is there a mis-understanding?)
> > > 
> > > I wasn't talking about the sl28cpld btw. What is the difference
> > > between
> > > your proposed "change take place immediately" and "after the cycle".
> > > I understand how the after the cycle should work. But how would the
> > > immediate change work in your ideal PWM?
> > 
> > If the PWM is running at 1/3 duty cycle and reconfigured for 2/3, then
> > the two scenarios are (the * marks the moment where pwm_apply_state() is
> > called, ^ marks the start of a period):
> > 
> > immediately:
> > 
> >   __       __    _____    _____
> >  /  \_____/  \__/     \__/
> >  ^        ^     ^        ^
> >                 *
> 
> Ok lets assume 2/3 and change it to 1/3:
> 
>    ____     ______      __
>   /    \___/      \____/  \____
>   ^        ^   ^       ^
>                *
> This will then have a longer on period than any of the settings.

I think we agree here. With an immediate change to the new setting both
too long and too short signals can heppen. How bad this is depends on
the use. The consumers currently in the kernel probably don't care too
much.

> > > > > > > > What about disable()?
> > > > > > >
> > > > > > > Mhh well, it would do one 100% cycle.. mhh ;) Lets see if there we can
> > > > > > > fix that (in hardware), not much we can do in the driver here. We are
> > > > > > > _very_ constraint in size, therefore all that little edge cases fall
> > > > > > > off
> > > > > > > the table.
> > > > > >
> > > > > > You're saying that on disable the hardware emits a constant high level
> > > > > > for one cycle? I hope not ...
> > > > >
> > > > > Mh, I was mistaken, disabling the PWM will turn it off immediately,
> > > > > but
> > > >
> > > > And does turn off mean, the output gets inactive?
> > > > If so you might also disable the hardware if a 0% duty cycle is
> > > > configured assuming this saves some energy without modifying the
> > > > resulting wave form.
> > > 
> > > Disabling it has some side effects like switching to another function
> > > for this multi function pin. So I'd rather keep it on ;)
> > 
> > So IMHO you should also keep it on when pwm_apply_state is called with
> > state.enabled = false to ensure a low output.
> 
> That won't work either, because that is how you would turn on that multi
> function. Ie. it is GPIO (default input) as long as the PWM is not enabled,
> otherwise its PWM.

I think you misunderstood what I wrote. The intended behaviour for a
disabled PWM (as in: pwm_apply_state() was called with state.enabled =
false) is that the output is a constant low (assuming a normal
polarity). If disabling your hardware results in something else, don't
disable the hardware. That's another item in the Limitations paragraph.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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