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

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 <james.clark@arm.com>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Cc: Al Grant <al.grant@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Branislav Rankov <branislav.rankov@arm.com>
Cc: Denis Nikitin <denik@chromium.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: coresight@lists.linaro.org
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lore.kernel.org/lkml/20210517131741.3027-4-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>