console: allow to retain boot console via boot option keep_bootcon (7bf69395) · Commits · 戴 / test

On some architectures, the boot process involves de-registering the boot console (early boot), initialize drivers and then re-register the console. This mechanism introduces a window in which no printk can happen on the console and messages are buffered and then printed once the new console is available. If a kernel crashes during this window, all it's left on the boot console is "console [foo] enabled, bootconsole disabled" making debug of the crash rather 'interesting'. By adding "keep_bootcon" option, do not unregister the boot console, that will allow to printk everything that is happening up to the crash. The option is clearly meant only for debugging purposes as it introduces lots of duplicated info printed on console, but will make bug report from users easier as it doesn't require a kernel build just to figure out where we crash. Signed-off-by:

Fabio M. Di Nitto <fabbione@fabbione.net> Acked-by:

David S. Miller <davem@davemloft.net> Cc: Alan Cox <alan@lxorguk.ukuu.org.uk> Cc: Greg KH <gregkh@suse.de> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

+6 −0

Original line number	Diff line number	Diff line
		@@ -872,6 +872,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
		If specified, z/VM IUCV HVC accepts connections
		from listed z/VM user IDs only.

		keep_bootcon [KNL]
		Do not unregister boot console at start. This is only
		useful for debugging when something happens in the window
		between unregistering the boot console and initializing
		the real console.

		i2c_bus= [HW] Override the default board specific I2C bus speed
		or register an additional I2C bus that is not
		registered from board initialization code.

kernel/printk.c

+15 −1

Original line number	Diff line number	Diff line
		@@ -1316,6 +1316,18 @@ void console_start(struct console *console)
		}
		EXPORT_SYMBOL(console_start);

		static int __read_mostly keep_bootcon;

		static int __init keep_bootcon_setup(char *str)
		{
		keep_bootcon = 1;
		printk(KERN_INFO "debug: skip boot console de-registration.\n");

		return 0;
		}

		early_param("keep_bootcon", keep_bootcon_setup);

		/*
		* The console driver calls this routine during kernel initialization
		* to register the console printing procedure with printk() and to
		@@ -1463,7 +1475,9 @@ void register_console(struct console *newcon)
		* users know there might be something in the kernel's log buffer that
		* went to the bootconsole (that they do not see on the real console)
		*/
		if (bcon && ((newcon->flags & (CON_CONSDEV \| CON_BOOT)) == CON_CONSDEV)) {
		if (bcon &&
		((newcon->flags & (CON_CONSDEV \| CON_BOOT)) == CON_CONSDEV) &&
		!keep_bootcon) {
		/* we need to iterate through twice, to make sure we print
		* everything out, before we unregister the console(s)
		*/

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