um: stdio_console: Make preferred console

The addition of the "ttynull" console driver did break the ordering of the
UML stdio console driver.
The UML stdio console driver is added in late_initcall (7), whereby the
ttynull driver is added in device_initcall (6), which always does make the
ttynull driver the default console.

Fix it by explicitly adding the UML stdio console as the preferred console,
in case no 'console=' command line option was specified.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

Author: thomasmey

arch/um/kernel/um_arch.c

@@ -26,7 +26,8 @@
 #include <mem_user.h>
 #include <os.h>
 
-#define DEFAULT_COMMAND_LINE "root=98:0"
+#define DEFAULT_COMMAND_LINE_ROOT "root=98:0"
+#define DEFAULT_COMMAND_LINE_CONSOLE "console=tty"
 
 /* Changed in add_arg and setup_arch, which run before SMP is started */
 static char __initdata command_line[COMMAND_LINE_SIZE] = { 0 };
@@ -109,7 +110,8 @@ unsigned long end_vm;
 int ncpus = 1;
 
 /* Set in early boot */
-static int have_root __initdata = 0;
+static int have_root __initdata;
+static int have_console __initdata;
 
 /* Set in uml_mem_setup and modified in linux_main */
 long long physmem_size = 32 * 1024 * 1024;
@@ -161,6 +163,17 @@ __uml_setup("debug", no_skas_debug_setup,
 "    this flag is not needed to run gdb on UML in skas mode\n\n"
 );
 
+static int __init uml_console_setup(char *line, int *add)
+{
+	have_console = 1;
+	return 0;
+}
+
+__uml_setup("console=", uml_console_setup,
+"console=<preferred console>\n"
+"    Specify the preferred console output driver\n\n"
+);
+
 static int __init Usage(char *line, int *add)
 {
 	const char **p;
@@ -264,7 +277,10 @@ int __init linux_main(int argc, char **argv)
 			add_arg(argv[i]);
 	}
 	if (have_root == 0)
-		add_arg(DEFAULT_COMMAND_LINE);
+		add_arg(DEFAULT_COMMAND_LINE_ROOT);
+
+	if (have_console == 0)
+		add_arg(DEFAULT_COMMAND_LINE_CONSOLE);
 
 	host_task_size = os_get_top_address();
 	/*