Messages in this thread Patch in this message | | | Subject | [RFC PATCH 3/3] printk: Preset tty0 as a pseudo-preferred console | From | Alper Nebi Yasak <> | Date | Mon, 23 Mar 2020 13:50:49 +0300 |
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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
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