coresight: etm-perf: Clarify comment on perf options

In theory, the options should be arbitrary values and are neutral for
any ETM version; so far perf tool uses ETMv3.5/PTM ETMCR config bits
except for register's bit definitions, also uses as options.

This can introduce confusion, especially if we want to add a new option
but the new option is not supported by ETMv3.5/PTM ETMCR.  But on the
other hand, we cannot change options since these options are generic
CoreSight PMU ABI.

For easier maintenance and avoid confusion, this patch refines the
comment to clarify perf options, and gives out the background info for
these bits are coming from ETMv3.5/PTM.  Afterwards, we should take
these options as general knobs, and if there have any confliction with
ETMv3.5/PTM, should consider to define saperate macros for ETMv3.5/PTM
ETMCR config bits.
Suggested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210206150833.42120-2-leo.yan@linaro.org
Link: https://lore.kernel.org/r/20210211172038.2483517-2-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-etm-perf.c

@@ -27,7 +27,10 @@ static bool etm_perf_up;
 static DEFINE_PER_CPU(struct perf_output_handle, ctx_handle);
 static DEFINE_PER_CPU(struct coresight_device *, csdev_src);
 
-/* ETMv3.5/PTM's ETMCR is 'config' */
+/*
+ * The PMU formats were orignally for ETMv3.5/PTM's ETMCR 'config';
+ * now take them as general formats and apply on all ETMs.
+ */
 PMU_FORMAT_ATTR(cycacc,		"config:" __stringify(ETM_OPT_CYCACC));
 PMU_FORMAT_ATTR(contextid,	"config:" __stringify(ETM_OPT_CTXTID));
 PMU_FORMAT_ATTR(timestamp,	"config:" __stringify(ETM_OPT_TS));

include/linux/coresight-pmu.h

@@ -10,7 +10,14 @@
 #define CORESIGHT_ETM_PMU_NAME "cs_etm"
 #define CORESIGHT_ETM_PMU_SEED  0x10
 
-/* ETMv3.5/PTM's ETMCR config bit */
+/*
+ * Below are the definition of bit offsets for perf option, and works as
+ * arbitrary values for all ETM versions.
+ *
+ * Most of them are orignally from ETMv3.5/PTM's ETMCR config, therefore,
+ * ETMv3.5/PTM doesn't define ETMCR config bits with prefix "ETM3_" and
+ * directly use below macros as config bits.
+ */
 #define ETM_OPT_CYCACC		12
 #define ETM_OPT_CTXTID		14
 #define ETM_OPT_TS		28