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>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index e9261e938f6ab8657586f6c068be1766bd725667..c357a31411cdb98978153eb5de0ecb2db025648f 100644 (file)
--- 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.

diff --git a/kernel/printk.c b/kernel/printk.c
index 33284adb21895b7bee8fa74b160f86d7f1458d52..2b591f252e55a93a43964edfd4fdd733c9303f59 100644 (file)
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -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)
                 */