Using sched_clock for mmio-trace

[Jeff Muizelaar <jeff@infidigm.net>]

I've built a tool with the goal of logging mmio writes and reads by
device drivers. See http://nouveau.freedesktop.org/wiki/MmioTrace.

I'd like to add support for recording a time stamp on each read and
write. Unfortunately, I am not sure which clock api I should use.

I had a look at blktrace and saw that it uses 'sched_clock()' for time
stamps. However, this symbol is not exported to modules, and from what
I've read it sounds like its use is discouraged.

The question is, what api should I be using? I need something that can
be called from inside interrupt handlers, and obviously the more
accurate and the lower the overhead the better.

-Jeff
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