[PATCH] pwm: sun4i: Avoid waiting until the next period

[Thierry Reding]

On Wed, May 12, 2021 at 11:18:24AM +0200, Emil Lenngren wrote:
> Hi Uwe,
> 
> Den ons 12 maj 2021 kl 06:41 skrev Uwe Kleine-König
> <u.kleine-koenig at pengutronix.de>:
> >
> > Hello Emil,
> >
> > On Wed, May 12, 2021 at 02:55:26AM +0200, Emil Lenngren wrote:
> > > Well that's one way of "solving it" ;)
> > >
> > > But on what hardware do you really need to wait until one full pulse
> > > cycle ends, before a disable command takes effect?
> > >
> > > On the hardware I've tested on (GR8 and V3s), it's enough to wait at
> > > most two clock cycles in order for it to take effect before we can
> > > close the gate. And with clock cycle I mean 24 MHz divided by the
> > > prescaler. With prescaler 1, that's 84 nanoseconds. By closing the
> > > gate when the pwm should be disabled, I guess we could save some
> > > nanoampere or microampere (is this important?)
> >
> > If I understood correctly you really have to wait longer to achieve that
> > the output is inactive in the disabled state. Do you talk about the same
> > thing?
> 
> Exactly, i.e. after writing 0 to the EN bit, we don't have to wait
> until the current period ends before we can observe that the output
> signal goes to the inactive state.
> 
> Simple test:
> 
> 1. Set pwm interval to a long time like 2 seconds, and duty to 50%.
> 2. Enable clock gating.
> 3. Enable the pwm by writing 1 to the EN bit.
> 4. Observe the LED blink once per second.
> 5. Now at a random time write 0 to the EN bit in order to disable the
> pwm. Don't turn off the clock gating.
> 6. If you just look with the eye it appears the LED turns off
> immediately, regardless of when in the pulse cycle we disabled it.
> 
> Just tested the above using "devmem" on a V3s.
> 
> By using a large prescaler and testing some different prescalers, I've
> concluded that it takes at least 1 and at most 2 clock cycles before
> we can safely turn off the gate and be certain that the output pin has
> changed to disabled.
> 
> It would be good if people having other hardware could confirm this is
> correct there as well.
> 
> Please take a look at some previous material I wrote:
> https://lkml.org/lkml/2020/3/17/1158
> https://linux-sunxi.org/PWM_Controller_Register_Guide (Observed
> behaviour on GR8 from NextThing)
> https://pastebin.com/GWrhWzPJ

I'm pretty sure Alexandre at the time reported that the instantiation of
the controller that he was using required waiting for the period to
complete before the output went to the disabled state. It's possible
that this was changed in subsequent versions of the IP, so perhaps we
need to distinguish based on compatible string?

Thierry
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