[RFC PATCHv4 3/6] nvme: add generic debugfs support

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

Add generic infrastructure for creating and managing debugfs files in
the NVMe module. This introduces helper APIs that allow NVMe drivers to
register and unregister debugfs entries, along with a reusable attribute
structure for defining new debugfs files.

The implementation uses seq_file interfaces to safely expose per-NS and
per-NS-head statistics, while supporting both simple show callbacks and
full seq_operations.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/Makefile    |   2 +-
 drivers/nvme/host/core.c      |   3 +
 drivers/nvme/host/debugfs.c   | 138 ++++++++++++++++++++++++++++++++++
 drivers/nvme/host/multipath.c |   2 +
 drivers/nvme/host/nvme.h      |  10 +++
 5 files changed, 154 insertions(+), 1 deletion(-)
 create mode 100644 drivers/nvme/host/debugfs.c

diff --git a/drivers/nvme/host/Makefile b/drivers/nvme/host/Makefile
index 6414ec968f99..7962dfc3b2ad 100644
--- a/drivers/nvme/host/Makefile
+++ b/drivers/nvme/host/Makefile
@@ -10,7 +10,7 @@ obj-$(CONFIG_NVME_FC)			+= nvme-fc.o
 obj-$(CONFIG_NVME_TCP)			+= nvme-tcp.o
 obj-$(CONFIG_NVME_APPLE)		+= nvme-apple.o
 
-nvme-core-y				+= core.o ioctl.o sysfs.o pr.o
+nvme-core-y				+= core.o ioctl.o sysfs.o pr.o debugfs.o
 nvme-core-$(CONFIG_NVME_VERBOSE_ERRORS)	+= constants.o
 nvme-core-$(CONFIG_TRACING)		+= trace.o
 nvme-core-$(CONFIG_NVME_MULTIPATH)	+= multipath.o
diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 47f375c63d2d..c15dfcaf3de2 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -4187,6 +4187,8 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
 	if (device_add_disk(ctrl->device, ns->disk, nvme_ns_attr_groups))
 		goto out_cleanup_ns_from_list;
 
+	nvme_debugfs_register(ns->disk);
+
 	if (!nvme_ns_head_multipath(ns->head))
 		nvme_add_ns_cdev(ns);
 
@@ -4276,6 +4278,7 @@ static void nvme_ns_remove(struct nvme_ns *ns)
 
 	nvme_mpath_remove_sysfs_link(ns);
 
+	nvme_debugfs_unregister(ns->disk);
 	del_gendisk(ns->disk);
 
 	mutex_lock(&ns->ctrl->namespaces_lock);
diff --git a/drivers/nvme/host/debugfs.c b/drivers/nvme/host/debugfs.c
new file mode 100644
index 000000000000..6bb57c4b5c3b
--- /dev/null
+++ b/drivers/nvme/host/debugfs.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2025 IBM Corporation
+ *	Nilay Shroff <nilay@linux.ibm.com>
+ */
+
+#include <linux/debugfs.h>
+#include <linux/seq_file.h>
+
+#include "nvme.h"
+
+struct nvme_debugfs_attr {
+	const char *name;
+	umode_t mode;
+	int (*show)(void *data, struct seq_file *m);
+	ssize_t (*write)(void *data, const char __user *buf, size_t count,
+			loff_t *ppos);
+	const struct seq_operations *seq_ops;
+};
+
+struct nvme_debugfs_ctx {
+	void *data;
+	struct nvme_debugfs_attr *attr;
+	int srcu_idx;
+};
+
+static int nvme_debugfs_show(struct seq_file *m, void *v)
+{
+	struct nvme_debugfs_ctx *ctx = m->private;
+	void *data = ctx->data;
+	struct nvme_debugfs_attr *attr = ctx->attr;
+
+	return attr->show(data, m);
+}
+
+static int nvme_debugfs_open(struct inode *inode, struct file *file)
+{
+	void *data = inode->i_private;
+	struct nvme_debugfs_attr *attr = debugfs_get_aux(file);
+	struct nvme_debugfs_ctx *ctx;
+	struct seq_file *m;
+	int ret;
+
+	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
+	if (WARN_ON_ONCE(!ctx))
+		return -ENOMEM;
+
+	ctx->data = data;
+	ctx->attr = attr;
+
+	if (attr->seq_ops) {
+		ret = seq_open(file, attr->seq_ops);
+		if (ret) {
+			kfree(ctx);
+			return ret;
+		}
+		m = file->private_data;
+		m->private = ctx;
+		return ret;
+	}
+
+	if (WARN_ON_ONCE(!attr->show)) {
+		kfree(ctx);
+		return -EPERM;
+	}
+
+	return single_open(file, nvme_debugfs_show, ctx);
+}
+
+static ssize_t nvme_debugfs_write(struct file *file, const char __user *buf,
+			size_t count, loff_t *ppos)
+{
+	struct seq_file *m = file->private_data;
+	struct nvme_debugfs_ctx *ctx = m->private;
+	struct nvme_debugfs_attr *attr = ctx->attr;
+
+	if (!attr->write)
+		return -EPERM;
+
+	return attr->write(ctx->data, buf, count, ppos);
+}
+
+static int nvme_debugfs_release(struct inode *inode, struct file *file)
+{
+	struct seq_file *m = file->private_data;
+	struct nvme_debugfs_ctx *ctx = m->private;
+	struct nvme_debugfs_attr *attr = ctx->attr;
+	int ret;
+
+	if (attr->seq_ops)
+		ret = seq_release(inode, file);
+	else
+		ret = single_release(inode, file);
+
+	kfree(ctx);
+	return ret;
+}
+
+static const struct file_operations nvme_debugfs_fops = {
+	.owner   = THIS_MODULE,
+	.open    = nvme_debugfs_open,
+	.read    = seq_read,
+	.write   = nvme_debugfs_write,
+	.llseek  = seq_lseek,
+	.release = nvme_debugfs_release,
+};
+
+
+static const struct nvme_debugfs_attr nvme_mpath_debugfs_attrs[] = {
+	{},
+};
+
+static const struct nvme_debugfs_attr nvme_ns_debugfs_attrs[] = {
+	{},
+};
+
+static void nvme_debugfs_create_files(struct request_queue *q,
+		const struct nvme_debugfs_attr *attr, void *data)
+{
+	if (WARN_ON_ONCE(!q->debugfs_dir))
+		return;
+
+	for (; attr->name; attr++)
+		debugfs_create_file_aux(attr->name, attr->mode, q->debugfs_dir,
+				data, (void *)attr, &nvme_debugfs_fops);
+}
+
+void nvme_debugfs_register(struct gendisk *disk)
+{
+	const struct nvme_debugfs_attr *attr;
+
+	if (nvme_disk_is_ns_head(disk))
+		attr = nvme_mpath_debugfs_attrs;
+	else
+		attr = nvme_ns_debugfs_attrs;
+
+	nvme_debugfs_create_files(disk->queue, attr, disk->private_data);
+}
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 55dc28375662..047dd9da9cbf 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -1086,6 +1086,7 @@ static void nvme_remove_head(struct nvme_ns_head *head)
 
 		nvme_cdev_del(&head->cdev, &head->cdev_device);
 		synchronize_srcu(&head->srcu);
+		nvme_debugfs_unregister(head->disk);
 		del_gendisk(head->disk);
 	}
 	nvme_put_ns_head(head);
@@ -1192,6 +1193,7 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
 		}
 		nvme_add_ns_head_cdev(head);
 		kblockd_schedule_work(&head->partition_scan_work);
+		nvme_debugfs_register(head->disk);
 	}
 
 	nvme_mpath_add_sysfs_link(ns->head);
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 715c7053054c..1c1ec2a7f9ad 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -1000,6 +1000,16 @@ static inline int nvme_data_dir(const enum req_op op)
 		return NVME_STAT_OTHER;
 }
 
+void nvme_debugfs_register(struct gendisk *disk);
+static inline void nvme_debugfs_unregister(struct gendisk *disk)
+{
+	/*
+	 * Nothing to do for now. When the request queue is unregistered,
+	 * all files under q->debugfs_dir are recursively deleted.
+	 * This is just a placeholder; the compiler will optimize it out.
+	 */
+}
+
 #ifdef CONFIG_NVME_MULTIPATH
 static inline bool nvme_ctrl_use_ana(struct nvme_ctrl *ctrl)
 {
-- 
2.51.0