proc tty: add struct tty_operations::proc_fops

Used for gradual switch of TTY drivers from using ->read_proc which helps
with gradual switch from ->read_proc for the whole tree.

As side effect, fix possible race condition when ->data initialized after
PDE is hooked into proc tree.

->proc_fops takes precedence over ->read_proc.
Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/proc/proc_tty.c

@@ -144,16 +144,22 @@ void proc_tty_register_driver(struct tty_driver *driver)
 {
 	struct proc_dir_entry *ent;
 		
-	if (!driver->ops->read_proc || !driver->driver_name ||
-	    driver->proc_entry)
+	if (!driver->driver_name || driver->proc_entry)
 		return;
 
+	if (driver->ops->proc_fops) {
+		ent = proc_create_data(driver->driver_name, 0, proc_tty_driver,
+				       driver->ops->proc_fops, driver);
+		if (!ent)
+			return;
+	} else if (driver->ops->read_proc) {
 		ent = create_proc_entry(driver->driver_name, 0, proc_tty_driver);
 		if (!ent)
 			return;
 		ent->read_proc = driver->ops->read_proc;
 		ent->data = driver;
-
+	} else
+		return;
 	driver->proc_entry = ent;
 }
 

include/linux/tty_driver.h

@@ -264,6 +264,7 @@ struct tty_operations {
 	int (*poll_get_char)(struct tty_driver *driver, int line);
 	void (*poll_put_char)(struct tty_driver *driver, int line, char ch);
 #endif
+	const struct file_operations *proc_fops;
 };
 
 struct tty_driver {