perf/arm-cmn: Add sysfs identifier

Expose a sysfs identifier encapsulating the CMN part number and revision
so that jevents can narrow down a fundamental set of possible events for
calculating metrics. Configuration-dependent aspects - such as whether a
given node type is present, and/or a given node ID is valid - are still
not covered, and in general it's hard to see how userspace could handle
them, so we won't be removing any data or validation logic from the
driver any time soon, but at least it's a step in a useful direction.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-and-tested-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Tested-by: Jing Zhang <renyu.zj@linux.alibaba.com>
Link: https://lore.kernel.org/r/b8a14c14fcdf028939ebf57849863e8ae01743de.1686588640.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

drivers/perf/arm-cmn.c

@@ -1199,19 +1199,31 @@ static ssize_t arm_cmn_cpumask_show(struct device *dev,
 static struct device_attribute arm_cmn_cpumask_attr =
 		__ATTR(cpumask, 0444, arm_cmn_cpumask_show, NULL);
 
-static struct attribute *arm_cmn_cpumask_attrs[] = {
+static ssize_t arm_cmn_identifier_show(struct device *dev,
+				       struct device_attribute *attr, char *buf)
+{
+	struct arm_cmn *cmn = to_cmn(dev_get_drvdata(dev));
+
+	return sysfs_emit(buf, "%03x%02x\n", cmn->part, cmn->rev);
+}
+
+static struct device_attribute arm_cmn_identifier_attr =
+		__ATTR(identifier, 0444, arm_cmn_identifier_show, NULL);
+
+static struct attribute *arm_cmn_other_attrs[] = {
 	&arm_cmn_cpumask_attr.attr,
+	&arm_cmn_identifier_attr.attr,
 	NULL,
 };
 
-static const struct attribute_group arm_cmn_cpumask_attr_group = {
-	.attrs = arm_cmn_cpumask_attrs,
+static const struct attribute_group arm_cmn_other_attrs_group = {
+	.attrs = arm_cmn_other_attrs,
 };
 
 static const struct attribute_group *arm_cmn_attr_groups[] = {
 	&arm_cmn_event_attrs_group,
 	&arm_cmn_format_attrs_group,
-	&arm_cmn_cpumask_attr_group,
+	&arm_cmn_other_attrs_group,
 	NULL
 };