[PATCH] vt: Add boot param for setting default vt console

[Adam Saponara]

Presently the default console is hard-coded to vt1.

The param allows for setting a different default. The param defaults to 0
(vt1), preserving the current behavior. It is clamped by the constants
`(MIN|MAX)_NR_CONSOLES`. If set `>= MIN`, `con_init` will initialize that
vt as well (a couple extra kilobytes heap for the `vc_data` and
`vc_screenbuf` structs).

Without this feature, users achieve the same effect with an init
script[0][1][2][3]. This works but requires an extra `chvt(1)` which can
race with user interaction and flicker the screen at login.

[0]: https://bbs.archlinux.org/viewtopic.php?id=232058
[1]: https://unix.stackexchange.com/questions/399986
[2]: https://github.com/systemd/systemd/issues/7247
[3]: https://www.linuxquestions.org/questions/linux-general-1/x-4175722418

Signed-off-by: Adam Saponara <as@xxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  5 +++
 drivers/tty/vt/vt.c                           | 44 +++++++++++++------
 2 files changed, 36 insertions(+), 13 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cb850e5290c2..6694b2edcfd6 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -8429,6 +8429,11 @@ Kernel parameters
 			overridden by individual drivers. 0 will hide
 			cursors, 1 will display them.
 
+	vt.default_console=
+			[VT]
+			Set default console; 0-62.
+			Default: 0 (vt1)
+
 	vt.italic=	[VT] Default color for italic text; 0-15.
 			Default: 2 = green.
 
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index c1f152d8b03b..e566942c380f 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -140,6 +140,7 @@ EXPORT_SYMBOL(vc_cons);
 static const struct consw *con_driver_map[MAX_NR_CONSOLES];
 
 static int con_open(struct tty_struct *, struct file *);
+static void con_init_vc(int console_idx);
 static void vc_init(struct vc_data *vc, int do_clear);
 static void gotoxy(struct vc_data *vc, int new_x, int new_y);
 static void restore_cur(struct vc_data *vc);
@@ -159,10 +160,14 @@ static void unblank_screen(void);
 
 int default_utf8 = true;
 module_param(default_utf8, int, S_IRUGO | S_IWUSR);
+
 int global_cursor_default = -1;
 module_param(global_cursor_default, int, S_IRUGO | S_IWUSR);
 EXPORT_SYMBOL(global_cursor_default);
 
+int default_console;
+module_param(default_console, int, S_IRUGO | S_IWUSR);
+
 static int cur_default = CUR_UNDERLINE;
 module_param(cur_default, int, S_IRUGO | S_IWUSR);
 
@@ -3742,7 +3747,7 @@ static int __init con_init(void)
 {
 	const char *display_desc = NULL;
 	struct vc_data *vc;
-	unsigned int currcons = 0, i;
+	unsigned int i, di;
 
 	console_lock();
 
@@ -3776,18 +3781,18 @@ static int __init con_init(void)
 		mod_timer(&console_timer, jiffies + (blankinterval * HZ));
 	}
 
-	for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++) {
-		vc_cons[currcons].d = vc = kzalloc_obj(struct vc_data,
-						       GFP_NOWAIT);
-		INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
-		tty_port_init(&vc->port);
-		visual_init(vc, currcons, true);
-		/* Assuming vc->vc_{cols,rows,screenbuf_size} are sane here. */
-		vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_NOWAIT);
-		vc_init(vc, currcons || !vc->vc_sw->con_save_screen);
-	}
-	currcons = fg_console = 0;
-	master_display_fg = vc = vc_cons[currcons].d;
+	for (i = 0; i < MIN_NR_CONSOLES; i++)
+		con_init_vc(i);
+
+	/* Init default_console if we didn't already do that above */
+	di = clamp(default_console, 0, MAX_NR_CONSOLES - 1);
+	if (di >= MIN_NR_CONSOLES)
+		con_init_vc(di);
+
+	fg_console = di;
+
+	vc = vc_cons[fg_console].d;
+	master_display_fg = vc;
 	set_origin(vc);
 	save_screen(vc);
 	gotoxy(vc, vc->state.x, vc->state.y);
@@ -3806,6 +3811,19 @@ static int __init con_init(void)
 }
 console_initcall(con_init);
 
+static void con_init_vc(int console_idx)
+{
+	struct vc_data *vc = kzalloc_obj(struct vc_data, GFP_NOWAIT);
+
+	vc_cons[console_idx].d = vc;
+	INIT_WORK(&vc_cons[console_idx].SAK_work, vc_SAK);
+	tty_port_init(&vc->port);
+	visual_init(vc, console_idx, true);
+	/* Assuming vc->vc_{cols,rows,screenbuf_size} are sane here. */
+	vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_NOWAIT);
+	vc_init(vc, console_idx || !vc->vc_sw->con_save_screen);
+}
+
 static const struct tty_operations con_ops = {
 	.install = con_install,
 	.open = con_open,
-- 
2.51.0