serial: earlycon: Enable earlycon without command line param

Earlycon matching can only be triggered if 'earlycon=...' has been
specified on the kernel command line. To workaround this limitation
requires tight coupling between arches and specific serial drivers
in order to start an earlycon. Devicetree avoids this limitation
with a link table that contains the required data to match earlycons.

Mirror this approach for earlycon match by name. Re-purpose
EARLYCON_DECLARE to generate a table entry which associates name with
setup() function. Re-purpose setup_earlycon() to scan this table for
an earlycon match, which is registered if found.

Declare one "earlycon" early_param, which calls setup_earlycon().

This design allows setup_earlycon() to be called directly with a
param string (as if 'earlycon=...' had been set on the command line).
Re-registration (either directly or by early_param) is prevented.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 11 insertions, 8 deletions

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 980170e5a982..8aeec4913a9c 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -337,18 +337,21 @@ struct earlycon_device {
 	char options[16];		/* e.g., 115200n8 */
 	unsigned int baud;
 };
-int setup_earlycon(char *buf, const char *match,
-		   int (*setup)(struct earlycon_device *, const char *));
 
+struct earlycon_id {
+	char	name[16];
+	int	(*setup)(struct earlycon_device *, const char *options);
+};
+
+extern int setup_earlycon(char *buf);
 extern int of_setup_earlycon(unsigned long addr,
 			     int (*setup)(struct earlycon_device *, const char *));
 
-#define EARLYCON_DECLARE(name, func) \
-static int __init name ## _setup_earlycon(char *buf) \
-{ \
-	return setup_earlycon(buf, __stringify(name), func); \
-} \
-early_param("earlycon", name ## _setup_earlycon);
+#define EARLYCON_DECLARE(_name, func)					\
+	static const struct earlycon_id __earlycon_##_name		\
+		__used __section(__earlycon_table)			\
+		 = { .name  = __stringify(_name),			\
+		     .setup = func  }
 
 #define OF_EARLYCON_DECLARE(name, compat, fn)				\
 	_OF_DECLARE(earlycon, name, compat, fn, void *)