perf pmus: Describe semantics of 'core_pmus' and 'other_pmus'

Notion of 'core_pmus' and 'other_pmus' are independent of hw core and
uncore pmus. For example, AMD IBS PMUs are present in each SMT-thread
but they belongs to 'other_pmus'. Add a comment describing what these
list contains and how they are treated.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ali Saidi <alisaidi@amazon.com>
Cc: Ananth Narayan <ananth.narayan@amd.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sandipan Das <sandipan.das@amd.com>
Cc: Santosh Shukla <santosh.shukla@amd.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20230615051700.1833-2-ravi.bangoria@amd.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/pmus.c

@@ -12,6 +12,21 @@
 #include "pmu.h"
 #include "print-events.h"
 
+/*
+ * core_pmus:  A PMU belongs to core_pmus if it's name is "cpu" or it's sysfs
+ *             directory contains "cpus" file. All PMUs belonging to core_pmus
+ *             must have pmu->is_core=1. If there are more than one PMU in
+ *             this list, perf interprets it as a heterogeneous platform.
+ *             (FWIW, certain ARM platforms having heterogeneous cores uses
+ *             homogeneous PMU, and thus they are treated as homogeneous
+ *             platform by perf because core_pmus will have only one entry)
+ * other_pmus: All other PMUs which are not part of core_pmus list. It doesn't
+ *             matter whether PMU is present per SMT-thread or outside of the
+ *             core in the hw. For e.g., an instance of AMD ibs_fetch// and
+ *             ibs_op// PMUs is present in each hw SMT thread, however they
+ *             are captured under other_pmus. PMUs belonging to other_pmus
+ *             must have pmu->is_core=0 but pmu->is_uncore could be 0 or 1.
+ */
 static LIST_HEAD(core_pmus);
 static LIST_HEAD(other_pmus);
 static bool read_sysfs_core_pmus;