[RFC] Make SMP secondary CPU up more resilient to failure.

[Russell King - ARM Linux]

On Sat, Dec 18, 2010 at 06:10:49AM -0600, Andrei Warkentin wrote:
> Definitely. This would be exactly the right place to place any holding
> logic...

FYI, I've managed to get some timing figures out of my Versatile
Express platform.  It takes about 100us for a CPU to come online via
hotplug, and a further 222ms to run the calibration before marking the
CPU online.

That leaves a margin of about 750ms before the timeout in the generic code
fires.

CPU requesting hotplug, times in ns:
SMP: Start: 0
SMP: Booting: 750
SMP: Cross call: 3500
SMP: Pen released: 41167
SMP: Unlock: 42417
SMP: Boot returned: 43250

CPU being brought online, referenced to "SMP: Start" above, times in ns.
SMP: Sec: restart: 3834
SMP: Sec: up: 5334
SMP: Sec: enter: 30667
SMP: Sec: pen write: 38917
SMP: Sec: pen done: 41417
SMP: Sec: exit: 42750
SMP: Sec: calibrate: 91834
SMP: Sec: online: 222054375