vt: introduce and use vt_kmsg_redirect() function

The kernel offers with TIOCL_GETKMSGREDIRECT ioctl() the possibility to
redirect the kernel messages to a specific console.

However, since it's not possible to switch to the kernel message console
after a panic(), it would be nice if the kernel would print the panic
message on the current console.

This patch series adds a new interface to access the global kmsg_redirect
variable by a function to be able to use it in code where
CONFIG_VT_CONSOLE is not set (kernel/panic.c).

This patch:

Instead of using and exporting a global value kmsg_redirect, introduce a
function vt_kmsg_redirect() that both can set and return the console where
messages are printed.

Change all users of kmsg_redirect (the VT code itself and kernel/power.c)
to the new interface.

The main advantage is that vt_kmsg_redirect() can also be used when
CONFIG_VT_CONSOLE is not set.

Signed-off-by: Bernhard Walle <bernhard@bwalle.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 37 insertions, 6 deletions

diff --git a/drivers/char/vt.c b/drivers/char/vt.c
index 1e3d728..e43fbc6 100644
--- a/drivers/char/vt.c
+++ b/drivers/char/vt.c
@@ -184,12 +184,10 @@ static DECLARE_WORK(console_work, console_callback);
  * fg_console is the current virtual console,
  * last_console is the last used one,
  * want_console is the console we want to switch to,
- * kmsg_redirect is the console for kernel messages,
  */
 int fg_console;
 int last_console;
 int want_console = -1;
-int kmsg_redirect;
 
 /*
  * For each existing display, we have a pointer to console currently visible
@@ -2434,6 +2432,37 @@ struct tty_driver *console_driver;
 
 #ifdef CONFIG_VT_CONSOLE
 
+/**
+ * vt_kmsg_redirect() - Sets/gets the kernel message console
+ * @new:	The new virtual terminal number or -1 if the console should stay
+ * 		unchanged
+ *
+ * By default, the kernel messages are always printed on the current virtual
+ * console. However, the user may modify that default with the
+ * TIOCL_SETKMSGREDIRECT ioctl call.
+ *
+ * This function sets the kernel message console to be @new. It returns the old
+ * virtual console number. The virtual terminal number 0 (both as parameter and
+ * return value) means no redirection (i.e. always printed on the currently
+ * active console).
+ *
+ * The parameter -1 means that only the current console is returned, but the
+ * value is not modified. You may use the macro vt_get_kmsg_redirect() in that
+ * case to make the code more understandable.
+ *
+ * When the kernel is compiled without CONFIG_VT_CONSOLE, this function ignores
+ * the parameter and always returns 0.
+ */
+int vt_kmsg_redirect(int new)
+{
+	static int kmsg_con;
+
+	if (new != -1)
+		return xchg(&kmsg_con, new);
+	else
+		return kmsg_con;
+}
+
 /*
  *	Console on virtual terminal
  *
@@ -2448,6 +2477,7 @@ static void vt_console_print(struct console *co, const char *b, unsigned count)
 	const ushort *start;
 	ushort cnt = 0;
 	ushort myx;
+	int kmsg_console;
 
 	/* console busy or not yet initialized */
 	if (!printable)
@@ -2455,8 +2485,9 @@ static void vt_console_print(struct console *co, const char *b, unsigned count)
 	if (!spin_trylock(&printing_lock))
 		return;
 
-	if (kmsg_redirect && vc_cons_allocated(kmsg_redirect - 1))
-		vc = vc_cons[kmsg_redirect - 1].d;
+	kmsg_console = vt_get_kmsg_redirect();
+	if (kmsg_console && vc_cons_allocated(kmsg_console - 1))
+		vc = vc_cons[kmsg_console - 1].d;
 
 	/* read `x' only after setting currcons properly (otherwise
 	   the `x' macro will read the x of the foreground console). */
@@ -2613,7 +2644,7 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
 			ret = set_vesa_blanking(p);
 			break;
 		case TIOCL_GETKMSGREDIRECT:
-			data = kmsg_redirect;
+			data = vt_get_kmsg_redirect();
 			ret = __put_user(data, p);
 			break;
 		case TIOCL_SETKMSGREDIRECT:
@@ -2623,7 +2654,7 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
 				if (get_user(data, p+1))
 					ret = -EFAULT;
 				else
-					kmsg_redirect = data;
+					vt_kmsg_redirect(data);
 			}
 			break;
 		case TIOCL_GETFGCONSOLE: