genirq/exynos timers: Fix the CPU hotplug wreckage sanely

The current implementation of irq_set_affinity() refuses rightfully to route an interrupt to an offline cpu.

But there is a special case, where this is actually desired. Some of the ARM SoCs have per cpu timers which require setting the affinity during cpu startup where the cpu is not yet in the online mask.

If we can't do that, then the local timer interrupt for the about to become online cpu is routed to some random online cpu.

The developers of the affected machines tried to work around that issue, but that results in a massive mess in that timer code.

It's saner to provide a facility to force the affinity and make the affected machines use that.

The following series implements that logic w/o impact on any existing users.

The change to the genirq core code is not that bad:

 arch/mips/cavium-octeon/octeon-irq.c |    2 +-
 include/linux/interrupt.h            |   35 ++++++++++++++++++++++++++++++++++-
 include/linux/irq.h                  |    3 ++-
 kernel/irq/manage.c                  |   17 ++++++-----------
 4 files changed, 43 insertions(+), 14 deletions(-)

The resulting fixup for gic/exynos_mct is:

 clocksource/exynos_mct.c |   12 +++---------
 irqchip/irq-gic.c        |    8 ++++++--
 2 files changed, 9 insertions(+), 11 deletions(-)

Krzysztofs proposed workaround was slightly smaller than that, but I prefer having a clean solution for backporting to stable rather than a messy hack around which works.

@Krzysztof: Can you please retest the series? I've changed the core implementation versus the first attempt to make it less intrusive.

Thanks,

tglx