console: allow to retain boot console via boot option keep_bootcon

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>

1 files changed, 6 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index e9261e938f6a..c357a31411cd 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -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.