[PATCHv3 2/4] tree: add queue-depth attribute for nvme path object

[Nilay Shroff <nilay@linux.ibm.com>]

Introduce a new sysfs attribute, queue_depth, under the NVMe path object.
This attribute is used by the queue-depth I/O policy introduced in kernel
v6.11, but the sysfs interface for this attribute was only added later in
kernel v6.15.

The queue_depth value is useful for observing which paths are selected
for I/O forwarding based on their current queue depths. To make this
information available to user space tools such as nvme-cli, the attribute
is now exported in libnvme.map.

As queue_depth value could change frequently, nvme_path_get_queue_depth()
is implemented to always fetch the latest queue_depth value, rather than
relying solely on a cached version. If fetching the latest value fails,
the function gracefully falls back to the cached value.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 src/libnvme.map    |  1 +
 src/nvme/private.h |  1 +
 src/nvme/tree.c    | 19 ++++++++++++++++++-
 src/nvme/tree.h    |  8 ++++++++
 4 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/src/libnvme.map b/src/libnvme.map
index 4314705f..e53fad6b 100644
--- a/src/libnvme.map
+++ b/src/libnvme.map
@@ -317,6 +317,7 @@ LIBNVME_1_0 {
 		nvme_path_get_ctrl;
 		nvme_path_get_name;
 		nvme_path_get_ns;
+		nvme_path_get_queue_depth;
 		nvme_path_get_sysfs_dir;
 		nvme_paths_filter;
 		nvme_read_config;
diff --git a/src/nvme/private.h b/src/nvme/private.h
index f45c5823..f94276e2 100644
--- a/src/nvme/private.h
+++ b/src/nvme/private.h
@@ -34,6 +34,7 @@ struct nvme_path {
 	char *sysfs_dir;
 	char *ana_state;
 	int grpid;
+	int queue_depth;
 };
 
 struct nvme_ns_head {
diff --git a/src/nvme/tree.c b/src/nvme/tree.c
index 2f82fd77..20d4c2e2 100644
--- a/src/nvme/tree.c
+++ b/src/nvme/tree.c
@@ -903,6 +903,18 @@ const char *nvme_path_get_name(nvme_path_t p)
 	return p->name;
 }
 
+int nvme_path_get_queue_depth(nvme_path_t p)
+{
+	_cleanup_free_ char *queue_depth = NULL;
+
+	queue_depth = nvme_get_path_attr(p, "queue_depth");
+	if (queue_depth) {
+		sscanf(queue_depth, "%d", &p->queue_depth);
+	}
+
+	return p->queue_depth;
+}
+
 const char *nvme_path_get_ana_state(nvme_path_t p)
 {
 	return p->ana_state;
@@ -921,7 +933,7 @@ void nvme_free_path(struct nvme_path *p)
 static int nvme_ctrl_scan_path(nvme_root_t r, struct nvme_ctrl *c, char *name)
 {
 	struct nvme_path *p;
-	_cleanup_free_ char *path = NULL, *grpid = NULL;
+	_cleanup_free_ char *path = NULL, *grpid = NULL, *queue_depth = NULL;
 	int ret;
 
 	nvme_msg(r, LOG_DEBUG, "scan controller %s path %s\n",
@@ -955,6 +967,11 @@ static int nvme_ctrl_scan_path(nvme_root_t r, struct nvme_ctrl *c, char *name)
 		sscanf(grpid, "%d", &p->grpid);
 	}
 
+	queue_depth = nvme_get_path_attr(p, "queue_depth");
+	if (queue_depth) {
+		sscanf(queue_depth, "%d", &p->queue_depth);
+	}
+
 	list_node_init(&p->nentry);
 	list_node_init(&p->entry);
 	list_add_tail(&c->paths, &p->entry);
diff --git a/src/nvme/tree.h b/src/nvme/tree.h
index 9f382e9c..a9082f8e 100644
--- a/src/nvme/tree.h
+++ b/src/nvme/tree.h
@@ -867,6 +867,14 @@ const char *nvme_path_get_sysfs_dir(nvme_path_t p);
  */
 const char *nvme_path_get_ana_state(nvme_path_t p);
 
+/**
+ * nvme_path_get_queue_depth() - Queue depth of an nvme_path_t object
+ * @p: &nvme_path_t object
+ *
+ * Return: Queue depth of @p
+ */
+int nvme_path_get_queue_depth(nvme_path_t p);
+
 /**
  * nvme_path_get_ctrl() - Parent controller of an nvme_path_t object
  * @p:	&nvme_path_t object
-- 
2.49.0