perf cs-etm: Remove callback cs_etm_find_snapshot()

perf cs-etm: Remove callback cs_etm_find_snapshot()

The callback cs_etm_find_snapshot() is invoked for snapshot mode, its
main purpose is to find the correct AUX trace data and returns "head"
and "old" (we can call "old" as "old head") to the caller, the caller
__auxtrace_mmap__read() uses these two pointers to decide the AUX trace
data size.

This patch removes cs_etm_find_snapshot() with below reasons:

- The first thing in cs_etm_find_snapshot() is to check if the head has
  wrapped around, if it is not, directly bails out.  The checking is
  pointless, this is because the "head" and "old" pointers both are
  monotonical increasing so they never wrap around.

- cs_etm_find_snapshot() adjusts the "head" and "old" pointers and
  assumes the AUX ring buffer is fully filled with the hardware trace
  data, so it always subtracts the difference "mm->len" from "head" to
  get "old".  Let's imagine the snapshot is taken in very short
  interval, the tracers only fill a small chunk of the trace data into
  the AUX ring buffer, in this case, it's wrongly to copy the whole the
  AUX ring buffer to perf file.

- As the "head" and "old" pointers are monotonically increased, the
  function __auxtrace_mmap__read() handles these two pointers properly.
  It calculates the reminders for these two pointers, and the size is
  clamped to be never more than "snapshot_size".  We can simply reply on
  the function __auxtrace_mmap__read() to calculate the correct result
  for data copying, it's not necessary to add Arm CoreSight specific
  callback.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: James Clark <james.clark@arm.com>
Tested-by: James Clark <james.clark@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Daniel Kiss <daniel.kiss@arm.com>
Cc: Denis Nikitin <denik@google.com>
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: Peter Zijlstra <peterz@infradead.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: coresight@lists.linaro.org
Link: http://lore.kernel.org/lkml/20210701093537.90759-3-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>