sysfs, kernfs: add kernfs_ops->seq_{start|next|stop}()

kernfs_ops currently only supports single_open() behavior which is
pretty restrictive.  Add optional callbacks ->seq_{start|next|stop}()
which, when implemented, are invoked for seq_file traversal.  This
allows full seq_file functionality for kernfs users.  This currently
doesn't have any user and doesn't change any behavior.

v2: Refreshed on top of the updated "sysfs, kernfs: prepare read path
    for kernfs".
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/sysfs/file.c

@@ -146,6 +146,7 @@ static ssize_t sysfs_kf_bin_read(struct sysfs_open_file *of, char *buf,
 static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
 {
 	struct sysfs_open_file *of = sf->private;
+	const struct kernfs_ops *ops;
 
 	/*
 	 * @of->mutex nests outside active ref and is just to ensure that
@@ -155,26 +156,42 @@ static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
 	if (!sysfs_get_active(of->sd))
 		return ERR_PTR(-ENODEV);
 
-	/*
-	 * The same behavior and code as single_open().  Returns !NULL if
-	 * pos is at the beginning; otherwise, NULL.
-	 */
-	return NULL + !*ppos;
+	ops = kernfs_ops(of->sd);
+	if (ops->seq_start) {
+		return ops->seq_start(sf, ppos);
+	} else {
+		/*
+		 * The same behavior and code as single_open().  Returns
+		 * !NULL if pos is at the beginning; otherwise, NULL.
+		 */
+		return NULL + !*ppos;
+	}
 }
 
 static void *kernfs_seq_next(struct seq_file *sf, void *v, loff_t *ppos)
 {
-	/*
-	 * The same behavior and code as single_open(), always terminate
-	 * after the initial read.
-	 */
-	++*ppos;
-	return NULL;
+	struct sysfs_open_file *of = sf->private;
+	const struct kernfs_ops *ops = kernfs_ops(of->sd);
+
+	if (ops->seq_next) {
+		return ops->seq_next(sf, v, ppos);
+	} else {
+		/*
+		 * The same behavior and code as single_open(), always
+		 * terminate after the initial read.
+		 */
+		++*ppos;
+		return NULL;
+	}
 }
 
 static void kernfs_seq_stop(struct seq_file *sf, void *v)
 {
 	struct sysfs_open_file *of = sf->private;
+	const struct kernfs_ops *ops = kernfs_ops(of->sd);
+
+	if (ops->seq_stop)
+		ops->seq_stop(sf, v);
 
 	sysfs_put_active(of->sd);
 	mutex_unlock(&of->mutex);

include/linux/kernfs.h

@@ -37,8 +37,9 @@ struct kernfs_ops {
 	/*
 	 * Read is handled by either seq_file or raw_read().
 	 *
-	 * If seq_show() is present, seq_file path is active.  The behavior
-	 * is equivalent to single_open().  @sf->private points to the
+	 * If seq_show() is present, seq_file path is active.  Other seq
+	 * operations are optional and if not implemented, the behavior is
+	 * equivalent to single_open().  @sf->private points to the
 	 * associated sysfs_open_file.
 	 *
 	 * read() is bounced through kernel buffer and a read larger than
@@ -46,6 +47,10 @@ struct kernfs_ops {
 	 */
 	int (*seq_show)(struct seq_file *sf, void *v);
 
+	void *(*seq_start)(struct seq_file *sf, loff_t *ppos);
+	void *(*seq_next)(struct seq_file *sf, void *v, loff_t *ppos);
+	void (*seq_stop)(struct seq_file *sf, void *v);
+
 	ssize_t (*read)(struct sysfs_open_file *of, char *buf, size_t bytes,
 			loff_t off);