[RFC PATCH 3/4] nvme: add debugfs helpers for NVMe drivers

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

Introduce 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-
namespace or per-path statistics, while supporting both simple show
callbacks and full seq_operations.

This will be used by subsequent patches to expose NVMe-TCP queue
and flow information for tuning NVMe TCP I/O workqueue and network stack
components.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/host/Makefile  |   2 +-
 drivers/nvme/host/debugfs.c | 111 ++++++++++++++++++++++++++++++++++++
 drivers/nvme/host/nvme.h    |  10 ++++
 3 files changed, 122 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/debugfs.c b/drivers/nvme/host/debugfs.c
new file mode 100644
index 000000000000..ee86138487d0
--- /dev/null
+++ b/drivers/nvme/host/debugfs.c
@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 IBM Corporation
+ *	Nilay Shroff <nilay@linux.ibm.com>
+ */
+
+#include <linux/debugfs.h>
+#include <linux/seq_file.h>
+#include <linux/slab.h>
+
+#include "nvme.h"
+
+struct nvme_debugfs_attr {
+	const char *name;
+	umode_t mode;
+	int (*show)(void *data, struct seq_file *m);
+	const struct seq_operations *seq_ops;
+};
+
+struct nvme_debugfs_ctx {
+	void *data;
+	struct nvme_debugfs_attr *attr;
+};
+
+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_obj(*ctx);
+	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 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,
+	.llseek  = seq_lseek,
+	.release = nvme_debugfs_release,
+};
+
+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)
+{
+	nvme_debugfs_create_files(disk->queue, nvme_ns_debugfs_attrs,
+			disk->private_data);
+}
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index ccd5e05dac98..2f3f1d2d19b9 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -468,6 +468,16 @@ struct nvme_ctrl {
 	u16			awupf; /* 0's based value. */
 };
 
+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.
+	 */
+}
+
 static inline enum nvme_ctrl_state nvme_ctrl_state(struct nvme_ctrl *ctrl)
 {
 	return READ_ONCE(ctrl->state);
-- 
2.53.0