From 325f5b8ed9b8cf162f8a6234de935f89bf3b7418 Mon Sep 17 00:00:00 2001
From: Greg Kroah-Hartman <gregkh@google.com>
Date: Sat, 17 Aug 2024 08:32:02 +0000
Subject: [PATCH] Revert "tty: add the option to have a tty reject a new ldisc"

This reverts commit 3c6332f3bb1578b5b10ac2561247b1d6272ae937 which is
commit 6bd23e0c2bb6c65d4f5754d1456bc9a4427fc59b upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ia8d68d62f97da2536404bfb439f2db596033260f
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
---
 drivers/tty/tty_ldisc.c    |  6 ------
 drivers/tty/vt/vt.c        | 10 ----------
 include/linux/tty_driver.h |  8 --------
 3 files changed, 24 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 7ca7731fa78a..776d8a62f77c 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -546,12 +546,6 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
 		goto out;
 	}
 
-	if (tty->ops->ldisc_ok) {
-		retval = tty->ops->ldisc_ok(tty, disc);
-		if (retval)
-			goto out;
-	}
-
 	old_ldisc = tty->ldisc;
 
 	/* Shutdown the old discipline. */
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index e2f9348725ff..48a9ed7c93c9 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3440,15 +3440,6 @@ static void con_cleanup(struct tty_struct *tty)
 	tty_port_put(&vc->port);
 }
 
-/*
- * We can't deal with anything but the N_TTY ldisc,
- * because we can sleep in our write() routine.
- */
-static int con_ldisc_ok(struct tty_struct *tty, int ldisc)
-{
-	return ldisc == N_TTY ? 0 : -EINVAL;
-}
-
 static int default_color           = 7; /* white */
 static int default_italic_color    = 2; // green (ASCII)
 static int default_underline_color = 3; // cyan (ASCII)
@@ -3575,7 +3566,6 @@ static const struct tty_operations con_ops = {
 	.resize = vt_resize,
 	.shutdown = con_shutdown,
 	.cleanup = con_cleanup,
-	.ldisc_ok = con_ldisc_ok,
 };
 
 static struct cdev vc0_cdev;
diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
index 331b1884ac23..cf6a8bacebc1 100644
--- a/include/linux/tty_driver.h
+++ b/include/linux/tty_driver.h
@@ -156,13 +156,6 @@ struct serial_struct;
  *
  *	Optional. Called under the @tty->termios_rwsem. May sleep.
  *
- * @ldisc_ok: ``int ()(struct tty_struct *tty, int ldisc)``
- *
- *	This routine allows the @tty driver to decide if it can deal
- *	with a particular @ldisc.
- *
- *	Optional. Called under the @tty->ldisc_sem and @tty->termios_rwsem.
- *
  * @set_ldisc: ``void ()(struct tty_struct *tty)``
  *
  *	This routine allows the @tty driver to be notified when the device's
@@ -382,7 +375,6 @@ struct tty_operations {
 	void (*hangup)(struct tty_struct *tty);
 	int (*break_ctl)(struct tty_struct *tty, int state);
 	void (*flush_buffer)(struct tty_struct *tty);
-	int (*ldisc_ok)(struct tty_struct *tty, int ldisc);
 	void (*set_ldisc)(struct tty_struct *tty);
 	void (*wait_until_sent)(struct tty_struct *tty, int timeout);
 	void (*send_xchar)(struct tty_struct *tty, char ch);