perf cs-etm: Prevent and warn on underflows during timestamp calculation.

When a zero timestamp is encountered, warn once. This is to make
hardware or configuration issues visible. Also suggest that the issue
can be worked around with the --itrace=Z option.

When an underflow with a non-zero timestamp occurs, warn every time.
This is an unexpected scenario, and with increasing timestamps, it's
unlikely that it would occur more than once, therefore it should be
ok to warn every time.

Only try to calculate the timestamp by subtracting the instruction
count if neither of the above cases are true. This makes attempting
to decode files with zero timestamps in non-timeless mode
more consistent. Currently it can half work if the timestamp wraps
around and becomes non-zero, although the behavior is undefined and
unpredictable.

Signed-off-by: James Clark <[email protected]>
Reviewed-by: Leo Yan <[email protected]>
Cc: Al Grant <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Branislav Rankov <[email protected]>
Cc: Denis Nikitin <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Garry <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Poirier <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Suzuki Poulouse <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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