Fix preempt-rt on AT91

From: Alexandre Belloni
Date: Sat Jan 16 2016 - 21:24:15 EST

Hi Sebastian, Thomas,

Preemp-rt on at91 has multiple issues:
1/ After apply the preempt-rt patch, the kernel doesn't build anymore,
arm-at91-pit-remove-irq-handler-when-clock-is-unused.patch is not correct
anymore. Can you squash the following patch to solve this build issue?

2/ This approach actually has more issues, in particular, request_irq() must
not be calls from interrupt disabled context. That only works because
might_sleep() is not active during early boot.
For more information, see the following discussion:

However, we can't remove those patches now because else we would suffer from
another issue:
The timer interrupt is shared with other devices, in particular the debug tty,
the rtc dans the pmc. When using preempt-rt, those interrupts become threaded
interrupts. But, the pit interrupt is requested with IRQF_TIMER and so has
IRQF_NO_THREAD. If the pit interrupt is not freed at early boot, all the other
devices will fail requesting their interrupt, for example:

genirq: Flags mismatch irq 16. 00042080 (ttyS0) vs. 00015280 (at91_tick)
atmel_usart ffffee00.serial: atmel_startup - Can't get irq

I'd say that the proper solution would still be to implement the virtual
irqchip because this would still hit people not wanting to use the TCB as
their clock source.

3/ Finally, the kernel will crash when initializing the PMC driver. This is
solved by this series that will hopefully land in the mainline:

Alexandre Belloni (1):
fix arm-at91-pit-remove-irq-handler-when-clock-is-unused.patch

drivers/clocksource/timer-atmel-pit.c | 68 +++++++++++++++++------------------
drivers/clocksource/timer-atmel-st.c | 8 ++---
2 files changed, 38 insertions(+), 38 deletions(-)