MIPS: Kernel hangs occasionally during boot.

The Kernel hangs occasionally during boot after "Calibrating delay loop..".
This is caused by the c0_compare_int_usable() routine in cevt-r4k.c
returning false which causes the system to disable the timer and hang later.
The false return happens because the routine is using a series of four calls
to irq_disable_hazard() as a delay while it waits for the timer changes to
propagate to the cp0 cause register. On newer MIPS cores, like the 74K, the
series of irq_disable_hazard() calls turn into ehb instructions and can take
as little as a few clock ticks for all 4 instructions. This is not enough of
a delay, so the routine thinks the timer is not working.  This fix uses up
to a max number of cycle counter ticks for the delay and uses
back_to_back_c0_hazard() instead of irq_disable_hazard() to handle the
hazard condition between cp0 writes and cp0 reads.

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2911/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>