[RFC PATCH 2/3] vt: Set as preferred console when a non-dummy backend, is bound

[Alper Nebi Yasak <alpernebiyasak@gmail.com>]

When a machine's device-tree has a "chosen" node with a "stdout-path"
property that specified console is added as the preferred console by
of_console_check via an add_preferred_console call. The property is
quite common in kernel device-tree definitions. As far as I can tell, it
is set to provide a reasonable default value for earlycon, and the
(usually serial) console is set as preferred to avoid output going to
VT's dummy backend instead of a working console.

However, a chosen stdout-path property is included even in device-trees
of systems that are designed to be used with a built-in display, e.g.
several ARM Chromebooks. In these cases where CONFIG_VT_CONSOLE is
enabled and no console argument is given on the kernel commandline, tty0
is still registered (presumably based on the order of of_console_check
and vt's register_console calls) but ends up not being the preferred
console.

As a result, it is possible for early userspace prompts (encryption
passphrase requests, emergency shells) to end up in a console that the
user doesn't expect or even have access to.

This patch tries to set tty0 as the /dev/console whenever a non-dummy
backend tries to register as its default, unless the preferred console
was set from the kernel commandline arguments.

On a Samsung Chromebook Plus (Google Kevin, rk3399-gru-kevin.dts), boot
messages are still visible on the framebuffer without this patch, but it
isn't the preferred console due to the device-tree having a stdout-path
property (from rk3399-gru.dtsi):

	$ sudo dmesg | grep -i "console\|printk"
	[    0.000000] printk: bootconsole [uart0] enabled
	[    0.010232] Console: colour dummy device 80x25
	[    0.015107] printk: console [tty0] enabled
	[    0.019601] printk: bootconsole [uart0] disabled
	[    7.145478] printk: console [ttyS2] enabled
	[    9.316094] Console: switching to colour frame buffer device 300x100

	$ cat /proc/consoles
	ttyS2                -W- (EC p a)    4:66
	tty0                 -WU (E     )    4:7

And on the same machine, with this patch:

	$ sudo dmesg | grep -i "console\|printk"
	[    0.000000] printk: bootconsole [uart0] enabled
	[    0.010257] Console: colour dummy device 80x25
	[    0.015132] printk: console [tty0] enabled
	[    0.019626] printk: bootconsole [uart0] disabled
	[    4.741120] printk: console [ttyS2] enabled
	[    6.779994] Console: switching to colour frame buffer device 300x100
	[    6.836117] printk: switching to console [tty0]

	$ cat /proc/consoles
	tty0                 -WU (EC    )    4:7
	ttyS2                -W- (E  p a)    4:66

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
---
 drivers/tty/vt/vt.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index bbc26d73209..0fc462ae8b2 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3566,6 +3566,13 @@ static int do_bind_con_driver(const struct consw *csw, int first, int last,
 		pr_cont("to %s\n", desc);
 	}
 
+#ifdef CONFIG_VT_CONSOLE
+	if (!console_set_on_cmdline && deflt && conswitchp != &dummy_con) {
+		add_preferred_console("tty", 0, NULL);
+		update_console_to_preferred();
+	}
+#endif
+
 	retval = 0;
 err:
 	module_put(owner);
-- 
2.26.0.rc2