pty: Add debug message for ptmx open

Opens of /dev/ptmx don't use tty_open() so debug messages are
not printed for those opens; print a debug message with the
open count (which must always be 1) if TTY_DEBUG_HANGUP is defined.

NB: Each tty core source file undefs support for debug messages.
The relevant source file must be patched/edited to enable these
messages.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/pty.c

@@ -26,6 +26,12 @@
 #include <linux/mutex.h>
 #include <linux/poll.h>
 
+#undef TTY_DEBUG_HANGUP
+#ifdef TTY_DEBUG_HANGUP
+# define tty_debug_hangup(tty, f, args...)	tty_debug(tty, f, ##args)
+#else
+# define tty_debug_hangup(tty, f, args...)	do {} while (0)
+#endif
 
 #ifdef CONFIG_UNIX98_PTYS
 static struct tty_driver *ptm_driver;
@@ -779,6 +785,8 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 	if (retval)
 		goto err_release;
 
+	tty_debug_hangup(tty, "(tty count=%d)\n", tty->count);
+
 	tty_unlock(tty);
 	return 0;
 err_release: