the HMT_* macros are telling firmware that "this processor thread
should
run at this priority". Typically used when we're waiting on a
spinlock.
I.e. When we are waiting on a spinlock, we hit the HMT_low macro to
drop
our threads priority, allowing the other thread to use those extra
cycles finish it's stuff quicker, and maybe even release the lock
we're
waiting for. HMT_* is all within the kernel though, no
exposure
to userspace apps.
Actually, those macros translate straight into a single machine insn.
No firmware is involved. See include/asm-powerpc/processor.h. For
example:
#define HMT_very_low() asm volatile("or 31,31,31 # very low
priority")
You can use those same macros from user space, although it is CPU
implementation dependent which priorities you can actually set (you
probably can do low and medium priority).
Segher
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