MIPS: Allow shared IRQ for timer & perf counter

MIPS: Allow shared IRQ for timer & perf counter

Before release 2 of the architecture there weren't separate interrupt
pending bits for the local CPU interrupts (timer & perf counter
overflow), so when they were connected to the same interrupt line the
timer handler had to call the performance counter handler before knowing
whether a timer interrupt was actually pending.

Now another CPU local interrupt, for the Fast Debug Channel (FDC), can
also be routed to an arbitrary interrupt line. It isn't scalable to keep
adding cross-calls between handlers for these cases of shared interrupt
lines, especially since the FDC could in theory share its interrupt line
with the performance counter, timer, or both.

Fortunately since release 2 of the architecture separate interrupt
pending bits do exist in the Cause register. This allows local
interrupts which share an interrupt line to have separate handlers using
IRQF_SHARED. Unfortunately they can't easily have their own irqchip as
there is no generic way to individually mask them.

Enable this sharing to happen by removing the special case for when the
perf count shares an IRQ with the timer. cp0_perfcount_irq and
cp0_compare_irq can then be set to the same value with shared interrupt
handlers registered for both of them.

Pre-R2 code should be unaffected. cp0_perfcount_irq will always be -1
and the timer handler will contnue to call into the perf counter
handler.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9131/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>