[patch 07/16] arm: omap: Use clocksource based sched_clock

[tony@atomide.com (Tony Lindgren)]

* Thomas Gleixner <tglx@linutronix.de> [110429 07:16]:
> On Fri, 29 Apr 2011, Tony Lindgren wrote:
> > * Thomas Gleixner <tglx@linutronix.de> [110429 05:25]:
> > > 
> > > The generic sched_clock implementation returns a jiffies based value
> > > as long as there is no clocksource registered. But the above does not
> > > fail in sched_clock() it fails in read_persistent_clock() which does
> > > 
> > >         last_cycles = cycles;
> > >         cycles = clocksource_32k.read(&clocksource_32k);
> > > 
> > > So the following change causes this:
> > > 
> > > -     .read           = omap_32k_read_dummy,
> > 
> > Ah right.
> >  
> > > Which leads to the question why you have a read_persistent_clock() at
> > > all if it always reads 0 via omap_32k_read_dummy ? Or is this meant
> > > just for the resume case? Then the above and the removal of
> > > omap_32k_read_dummy() needs to be undone.
> > 
> > The omap_32k_read_dummy needs to return 0 until the clocks are
> > enabled and the right read function is selected. The .read gets
> > then set in omap_init_clocksource_32k.
> > 
> > It would be nice to handle that in a generic way though,
> > I would assume the same issue exists for other platforms too.
> 
> static inline cycles_t clocksource_read_safe(struct clocksource *c)
> {
> 	return c->read ? c->read(c) : 0;
> }

That adds testing for it for each read.. So the current setup
is better.
 
> For the core timekeeping code that's a non issue as nothing should
> register a clocksource with an invalid read function pointer :)

Looks like omap_device code calls read_persistent_clock before
omap_init_clocksource_32k is run. I'll take a look and see if that
is needed when I refresh other omap init_early patches.

Anyways, currently nothing prevents calling read_persistent_clock
before it's initialized and a platform specific hack is needed.
Maybe the default should be clocksource_read_safe that gets
changed by the platform code when initializing?

Regards,

Tony