Re: [PATCH v12 4/9] OMAP2+: dmtimer: convert to platform devices

[Tony Lindgren <tony@atomide.com>]

* Santosh Shilimkar <santosh.shilimkar@ti.com> [110318 21:31]:
> > * Kevin Hilman <khilman@ti.com> [110317 14:58]:
> > > Tony Lindgren <tony@atomide.com> writes:
> > > >
> > > > In the long run, think "local timer" for runtime, and then
> > > > "wakeup timer" that gets only programmed when we enter idle.
> > > >  The "local timer" will continue operating normally after we
> > > > wake-up and the "wakeuptimer" will be just one shot event.
> > > > Of course in the omap[23] case the "local timer" is really a
> > > > there's are real local timers.
> 
> This is already the case for OMAP4 with PM series. But it doesn't
> work the way it is stated above. Wakeup timer is always programmed
> and kept handy by the timer framework because switching of
> clock event happened dynamically and it's too late to reprogram
> the next timer expiry etc. What framework does, is just switch
> the clock-event to wakeup capable clock-event.

Hmm, the next_timer_interrupt gets called before we enter idle, is
that what you mean maybe?

Assuming so, to set up the wake-up timer we can read the programmed
value from hardware to program the wake-up timer when entering
suspend-idle or off-idle. That part can be done just fine with
dmtimer framework.

But in general, we don't really want to start changing the physical
clock for WFI idle because of the time it takes to lock it. For
retention-idle and off-idle, we have much more time. But in these
case there's really no need to change the sources, all we care
about is the physical timer wake-up event and can then restore the
"local timer".
 
> Why do you want to waste one timer hardware for this purpose
> alone especially when generic framework has a support?

In this setup the additional wake-up timer is "only" needed in the
retention-idle and off-idle cases.

For keeping the wake-up timer separate, there are at least three
reasons that I can think of:

1. We don't need to touch the clockevent and clocksource code
   after init except to restore them when waking from retention-idle
   or off-idle.

2. We don't need to initialize anything early for omaps except
   clock framework to set up the clockevent and clocksource.

3. We can avoid switching the physical clock for the timer which
   cuts down latencies at least for the basic WFI case.

> > > In addition, we already have a usecase for switching the
> > > clocksource as well.  We currently setup a single clocksource
> > > (not using the timer_32ka dmtimer.)  However, we already have
> > > use-cases where we would like to switch to a higher resolution
> > > clocksource (e.g. trace infrastructure for PM instrumentation.)
> >
> I agree with Kevin. Infact I tried prototyping this one. Clock-event
> switching works seamlessly. Clock-source switching though isn't
> supported yet by generic timer framework.

Sure. We need to be able to change between the local timer and
the dmtimer also, and I don't see how the current dmtimer series
would make that any easier.
 
> > Again that can be done with a separate physical timer, no need to
> > switch and reprogram the clocksource.
> >
> If this can done via existing timer framework, I don't see point
> wasting another physical timer for this.

In this setup we don't really need to do anything via existing timer
framework, except to restore the clockevent and clocksource when
waking up from retention-idle or off-idle.
 
> I agree your basic point of making clock-source and clock-event
> not depend on any frameworks. This is probably essential
> considering any generic kernel changes can impact these. Recent
> early_init() was a good example. May be I hold on my comments
> since you plan to do some patches for system timer handling.

Yes, we need to cut down the early_init dependencies to minimum.

The reason for that is that then we can initialize everything else
that's omap specific in normal way much later than we do currently.

For the timer, clock framework is essential to init early, but
hwmod is not.

Regards,

Tony