[PATCH v2] nvme-multipath: add iopolicies sysfs attribute

[loberman@redhat.com]

From: Laurence Oberman <loberman@redhat.com>

The subsystem iopolicy attribute only shows the active policy, so
there is no way to see which policies are available.

Changing the output of iopolicy would break existing userspace,
since nvme-cli compares the value against a bare policy name. Leave
iopolicy unchanged and add a read-only iopolicies attribute that
lists the available policies:

  # cat iopolicies
  numa round-robin queue-depth

The current policy is still read from iopolicy.

Signed-off-by: Laurence Oberman <loberman@redhat.com>
Assisted-by: Claude:claude-sonnet-5
---
v2:
  - Drop the [ ] notation and list plain policy names (Christoph)

 drivers/nvme/host/multipath.c | 15 +++++++++++++++
 drivers/nvme/host/nvme.h      |  1 +
 drivers/nvme/host/sysfs.c     |  1 +
 3 files changed, 17 insertions(+)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 75dbb58286a3..ea7d0c59587b 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -1085,6 +1085,21 @@ static ssize_t nvme_subsys_iopolicy_store(struct device *dev,
 SUBSYS_ATTR_RW(iopolicy, S_IRUGO | S_IWUSR,
 		      nvme_subsys_iopolicy_show, nvme_subsys_iopolicy_store);
 
+static ssize_t iopolicies_show(struct device *dev,
+			       struct device_attribute *attr, char *buf)
+{
+	int i, len = 0;
+
+	for (i = 0; i < ARRAY_SIZE(nvme_iopolicy_names); i++)
+		len += sysfs_emit_at(buf, len, "%s%s", i ? " " : "",
+				     nvme_iopolicy_names[i]);
+
+	len += sysfs_emit_at(buf, len, "\n");
+	return len;
+}
+
+struct device_attribute subsys_attr_iopolicies = __ATTR_RO(iopolicies);
+
 static ssize_t ana_grpid_show(struct device *dev, struct device_attribute *attr,
 		char *buf)
 {
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 75e5d5a8a77c..b419c6fc6081 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -1098,6 +1098,7 @@ extern struct device_attribute dev_attr_multipath_failover_count;
 extern struct device_attribute dev_attr_io_requeue_no_usable_path_count;
 extern struct device_attribute dev_attr_io_fail_no_available_path_count;
 extern struct device_attribute subsys_attr_iopolicy;
+extern struct device_attribute subsys_attr_iopolicies;
 
 static inline bool nvme_disk_is_ns_head(struct gendisk *disk)
 {
diff --git a/drivers/nvme/host/sysfs.c b/drivers/nvme/host/sysfs.c
index abf8edaae371..4c4518b6b7d5 100644
--- a/drivers/nvme/host/sysfs.c
+++ b/drivers/nvme/host/sysfs.c
@@ -1298,6 +1298,7 @@ static struct attribute *nvme_subsys_attrs[] = {
 	&subsys_attr_subsystype.attr,
 #ifdef CONFIG_NVME_MULTIPATH
 	&subsys_attr_iopolicy.attr,
+	&subsys_attr_iopolicies.attr,
 #endif
 	NULL,
 };
-- 
2.55.0