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):
# Without earlycon:
$ sudo dmesg | grep -i "console\|printk\|tty[0-9a-z]" | grep -v systemd
[ 0.001447] Console: colour dummy device 80x25
[ 0.001820] printk: console [tty0] enabled
[ 3.012303] ff1a0000.serial: ttyS2 at MMIO 0xff1a0000 (irq = 39, base_baud = 1500000) is a 16550A
[ 4.326549] printk: console [ttyS2] enabled
[ 5.521780] dw-apb-uart ff1a0000.serial: forbid DMA for kernel console
[ 6.359706] Console: switching to colour frame buffer device 300x100
$ cat /proc/consoles
ttyS2 -W- (EC p a) 4:66
tty0 -WU (E p ) 4:7
# With earlycon:
$ sudo dmesg | grep -i "console\|printk\|tty[0-9a-z]" | grep -v systemd
[ 0.000000] printk: bootconsole [uart0] enabled
[ 0.010257] Console: colour dummy device 80x25
[ 0.015131] printk: console [tty0] enabled
[ 0.019625] printk: bootconsole [uart0] disabled
[ 3.034305] ff1a0000.serial: ttyS2 at MMIO 0xff1a0000 (irq = 39, base_baud = 1500000) is a 16550A
[ 4.367601] printk: console [ttyS2] enabled
[ 5.576519] dw-apb-uart ff1a0000.serial: forbid DMA for kernel console
[ 6.435612] 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:
# Without earlycon:
$ sudo dmesg | grep -i "console\|printk\|tty[0-9a-z]" | grep -v systemd
[ 0.001451] Console: colour dummy device 80x25
[ 0.001821] printk: console [tty0] enabled
[ 3.013821] ff1a0000.serial: ttyS2 at MMIO 0xff1a0000 (irq = 39, base_baud = 1500000) is a 16550A
[ 4.328053] printk: console [ttyS2] enabled
[ 5.509658] dw-apb-uart ff1a0000.serial: forbid DMA for kernel console
[ 6.309330] Console: switching to colour frame buffer device 300x100
[ 6.378862] printk: switching to console [tty0]
$ cat /proc/consoles
tty0 -WU (EC p ) 4:7
ttyS2 -W- (E p a) 4:66
# With earlycon:
$ sudo dmesg | grep -i "console\|printk\|tty[0-9a-z]" | grep -v systemd
[ 0.000000] printk: bootconsole [uart0] enabled
[ 0.010259] Console: colour dummy device 80x25
[ 0.015135] printk: console [tty0] enabled
[ 0.019628] printk: bootconsole [uart0] disabled
[ 3.037677] ff1a0000.serial: ttyS2 at MMIO 0xff1a0000 (irq = 39, base_baud = 1500000) is a 16550A
[ 4.370985] printk: console [ttyS2] enabled
[ 5.549226] dw-apb-uart ff1a0000.serial: forbid DMA for kernel console
[ 6.364818] Console: switching to colour frame buffer device 300x100
[ 6.417589] 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@xxxxxxxxx>
---
Changes in v2:
- Refresh dmesg outputs with/without earlycon for next-20200430
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 e5ffed795e4c..218171dca711 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3588,6 +3588,13 @@ 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);
--
2.26.2
