um: add earlyprintk support

User Mode Linux can also benefit from earlyprintk.  UML's earlyprintk
writes kernel messages directly to stdout.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/Kconfig.debug

@@ -37,4 +37,14 @@ config DEBUG_STACK_USAGE
 	  stack seen so far.
 
 	  This option will slow down process creation and destruction somewhat.
+
+config EARLY_PRINTK
+	bool "Early printk"
+	default y
+	---help---
+	  Write kernel log output directly to stdout.
+
+	  This is useful for kernel debugging when your machine crashes very
+	  early before the console code is initialized.
+
 endmenu

arch/um/include/shared/os.h

@@ -244,6 +244,7 @@ extern int raw(int fd);
 extern void setup_machinename(char *machine_out);
 extern void setup_hostinfo(char *buf, int len);
 extern void os_dump_core(void) __attribute__ ((noreturn));
+extern void um_early_printk(const char *s, unsigned int n);
 
 /* time.c */
 extern void idle_sleep(unsigned long long nsecs);

arch/um/kernel/Makefile

@@ -17,6 +17,7 @@ obj-y = config.o exec.o exitcode.o init_task.o irq.o ksyms.o mem.o \
 obj-$(CONFIG_BLK_DEV_INITRD) += initrd.o
 obj-$(CONFIG_GPROF)	+= gprof_syms.o
 obj-$(CONFIG_GCOV)	+= gmon_syms.o
+obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
 
 USER_OBJS := config.o
 

arch/um/kernel/early_printk.c

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2011 Richard Weinberger <richrd@nod.at>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/kernel.h>
+#include <linux/console.h>
+#include <linux/init.h>
+#include "os.h"
+
+static void early_console_write(struct console *con, const char *s, unsigned int n)
+{
+	um_early_printk(s, n);
+}
+
+static struct console early_console = {
+	.name = "earlycon",
+	.write = early_console_write,
+	.flags = CON_BOOT,
+	.index = -1,
+};
+
+static int __init setup_early_printk(char *buf)
+{
+	register_console(&early_console);
+
+	return 0;
+}
+
+early_param("earlyprintk", setup_early_printk);

arch/um/os-Linux/util.c

@@ -139,3 +139,8 @@ void os_dump_core(void)
 
 	uml_abort();
 }
+
+void um_early_printk(const char *s, unsigned int n)
+{
+	printf("%.*s", n, s);
+}