[RFC PATCH 3/3] printk: Preset tty0 as a pseudo-preferred console

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

The ACPI SPCR (Serial Port Console Redirection) table from a machine's
firmware can specify a serial console which can be used for earlycon.
However, in at least ARM64 systems the same device is also set up as the
preferred console.  Presumably due to the order of acpi_parse_spcr and
VT's register_device calls, setting the specified console as the
preferred one can prevent registering VT as a console.

This might look appropriate for machines which do not have or need
working graphics and whose users most likely have access to a serial
port. However, the use of SPCR tables may not be limited these. For
example, ARM64 QEMU virtual machines include a SPCR table regardless of
the existence of a graphics output or even the nonexistence of a serial
console. Or server hardware which has a SPCR table can be repurposed
into a workstation with the addition of a graphics card. As a result,
boot messages and early userspace prompts can go to an unexpected
console.

This patch presets tty0 as a pseudo-preferred console at compile-time to
ensure that CONFIG_VT_CONSOLE always results in the VT console getting
registered. With this, VT can get registered, these other consoles are
preferred when VT is a dummy, but we can also bump up VTs preference
when working graphics are available.

Without this patch, an ARM64 QEMU virtual machine has roughly the
following order of console events and consoles:

	$ sudo dmesg | grep -i "console\|printk"
	[    0.000000] ACPI: SPCR: console: pl011,mmio,0x9000000,9600
	[    0.000000] printk: bootconsole [pl11] enabled
	[    0.004890] Console: colour dummy device 80x25
	[    0.412252] printk: console [ttyAMA0] enabled
	[    0.416173] printk: bootconsole [pl11] disabled
	[    3.940510] Console: switching to colour frame buffer device 128x48

	$ cat /proc/consoles
	ttyAMA0              -W- (EC   a)  204:64

In addition, boot messages aren't printed to the framebuffer (as tty0 is
not registered). With this patch, boot messages are visible on the
framebuffer and the information above becomes:

	$ sudo dmesg | grep -i "console\|printk"
	[    0.000000] ACPI: SPCR: console: pl011,mmio,0x9000000,9600
	[    0.000000] printk: bootconsole [pl11] enabled
	[    0.002768] Console: colour dummy device 80x25
	[    0.004371] printk: console [tty0] enabled
	[    0.380166] printk: console [ttyAMA0] enabled
	[    0.387337] printk: bootconsole [pl11] disabled
	[    4.695030] Console: switching to colour frame buffer device 128x48
	[    4.709759] printk: switching to console [tty0]

	$ cat /proc/consoles
	tty0                 -WU (EC p  )    4:7
	ttyAMA0              -W- (E    a)  204:64

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
---
 kernel/printk/printk.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 6b16c973587..4b05779ab69 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -277,7 +277,19 @@ static struct console *exclusive_console;
 
 #define MAX_CMDLINECONSOLES 8
 
+/*
+ * The preferred_console and has_preferred_console variables are
+ * intentionally not modified to reflect this so that the first
+ * registered console is still used as the preferred console.
+ */
+#ifdef CONFIG_VT_CONSOLE
+static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES] = {
+	[0].name = "tty",
+	[0].index = 0,
+};
+#elif
 static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
+#endif
 
 static int preferred_console = -1;
 static bool has_preferred_console;
-- 
2.26.0.rc2