Conflicting use of CCNT by DEV_PMU and HWPMC

[bz]

Summary

In case we compile in device pmu, the kernel will use CCNT when available to expose cycle counts.
In that case we cannot have HWPMC manage that counter (and interrupts for it).
Put safeguards into the HWPMC code to prevent this from happening; in case someone does try to write to it silently accept, and for reading call the generic kernel function.
It is an "artificial" counter on armv7 (and should really be a soft counter), so if worse comes to worse we could just remove it entirely.

[emaste]

sys/dev/hwpmc/hwpmc_armv7.c

68

Do we have a #define for the 31?

235

Maybe continue here instead?

[andrew]

sys/dev/hwpmc/hwpmc_armv7.c

235

Or invert the logic so we have:

 if (pm_armv7_evsel != 0xFF)
   armv7_pmcn_write(ri, v);
#ifndef DEV_PMU
 else
   cp15_pmccntr_set(v);
#endif

[emaste]

Looking through old reviews and came across this one - what's next?

[bz]

I think a dependency was rejected. This was Cambridge Teaching material code. I would assume this is overcome by time and changes a lot since. I have no more access to these trees where this came from to see. I am abandoning it to clean up.