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

[Uwe Kleine-König]

Hello Michael,

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?)

> >  - emits an irq when configuration changes
> 
> Why would you need the interrupt?

To know that the new setting is active. Currently Thierry's ideal PWM
implementation blocks in pwm_apply_state() until the new setting is
active. So some signaling is nice. 

> > > > If you change only cycle but not mode, does the hardware complete the
> > > > currently running period?
> > > 
> > > No it does not.
> > 
> > Please document this as a Limitation.
> 
> I've discussed this internally, for now its a limitation. In the worst
> case you'd do one 100% duty cycle. Maybe we can fix the hardware. I
> acknowledge that this is a severe limitation, esp. if you use the PWM
> for controlling stuff (for now its only LCD backlight.. so thats ok).

That happens if you reduce the duty cycle from A to B and the counter is
already bigger than B but smaller than A, right? The fix would be to
compare for counter >= match instead of counter = match. (Which then
would result in a period with a duty cycle bigger than B but smaller
than A. Also not ideal, but probably better.)

> > > > 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.

> one 100% duty cycle may happen if you change from a higher to a lower
> duty cycle setting. See above.
> 
> > I never programmed a CPLD to emulate a hardware PWM, but I wonder if
> > these are really edge cases that increase the size of the binary?!
> 
> At the moment there is only one 8bit register which stores the value
> which is used for matching. If you want to change that setting after
> a whole cycle, you'd use another 8bit register to cache the new value.
> So this would at least needs 8 additional flip-flops. This doesn't
> sound much, but we are already near 100% usage of the CPLD. So its
> hard to convince people why this is really necessary.

OK. (Maybe there is enough space to allow implementing 100% for mode 0?)

Best regards
Uwe

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