[PATCH V2] [ARM] Add ARCH_PROVIDES_UDELAY config option

[Russell King - ARM Linux]

On Fri, Apr 30, 2010 at 03:11:20PM -0700, Kevin Hilman wrote:
> Colin Cross <ccross at android.com> writes:
> 
> > An alternative to this patch would be to add a config option to use
> > sched_clock() to provide the counter instead of the cycle loop.  The
> > same loops_per_jiffy calibration could be done to  determine the
> > sched_clock frequency.  Any machine with an available constant tick
> > rate counter, which is likely to be used for sched_clock() already,
> > can enable CONFIG_UDELAY_USES_SCHED_CLOCK.
> 
> Or even better, why not have an option to use the clocksource which is
> most likely using the constant tick timer as well.

We may be running into the same problem which we did with the printk
clock - that is using a machine provided sched_clock() or clocksource
requires MMIO accesses, which can only be done after the IO mappings
have been initialized.

Let's hope no one ever uses udelay() before the necessary IO mappings
are present.