of/fdt: add FDT serial scanning for earlycon

This adds FDT parsing of {linux,}stdout-path to setup an early serial
console. Enabling of the early console is triggered with "earlycon"
(with no options) on the kernel command line.

Platforms must either have fixmap permanent mapping support,
have a functioning ioremap when early params are parsed, or explicitly
call early_init_dt_scan_chosen_serial from architecture code.
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Grant Likely <grant.likely@linaro.org>

drivers/of/fdt.c

@@ -21,6 +21,7 @@
 #include <linux/slab.h>
 #include <linux/libfdt.h>
 #include <linux/debugfs.h>
+#include <linux/serial_core.h>
 
 #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
 #include <asm/page.h>
@@ -696,6 +697,61 @@ static inline void early_init_dt_check_for_initrd(unsigned long node)
 }
 #endif /* CONFIG_BLK_DEV_INITRD */
 
+#ifdef CONFIG_SERIAL_EARLYCON
+extern struct of_device_id __earlycon_of_table[];
+
+int __init early_init_dt_scan_chosen_serial(void)
+{
+	int offset;
+	const char *p;
+	int l;
+	const struct of_device_id *match = __earlycon_of_table;
+	const void *fdt = initial_boot_params;
+
+	offset = fdt_path_offset(fdt, "/chosen");
+	if (offset < 0)
+		offset = fdt_path_offset(fdt, "/chosen@0");
+	if (offset < 0)
+		return -ENOENT;
+
+	p = fdt_getprop(fdt, offset, "stdout-path", &l);
+	if (!p)
+		p = fdt_getprop(fdt, offset, "linux,stdout-path", &l);
+	if (!p || !l)
+		return -ENOENT;
+
+	/* Get the node specified by stdout-path */
+	offset = fdt_path_offset(fdt, p);
+	if (offset < 0)
+		return -ENODEV;
+
+	while (match->compatible) {
+		unsigned long addr;
+		if (fdt_node_check_compatible(fdt, offset, match->compatible)) {
+			match++;
+			continue;
+		}
+
+		addr = fdt_translate_address(fdt, offset);
+		if (!addr)
+			return -ENXIO;
+
+		of_setup_earlycon(addr, match->data);
+		return 0;
+	}
+	return -ENODEV;
+}
+
+static int __init setup_of_earlycon(char *buf)
+{
+	if (buf)
+		return 0;
+
+	return early_init_dt_scan_chosen_serial();
+}
+early_param("earlycon", setup_of_earlycon);
+#endif
+
 /**
  * early_init_dt_scan_root - fetch the top level address and size cells
  */