From: Alexey Brodkin <abrodkin@synopsys.com>
Subject: openwrt: arc - add OWRTDTB section

This change allows OpenWRT to patch resulting kernel binary with
external .dtb.

That allows us to re-use exactky the same vmlinux on different boards
given its ARC core configurations match (at least cache line sizes etc).

""patch-dtb" searches for ASCII "OWRTDTB:" strign and copies external
.dtb right after it, keeping the string in place.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
---
 arch/arc/kernel/head.S        | 10 ++++++++++
 arch/arc/kernel/setup.c       |  4 +++-
 arch/arc/kernel/vmlinux.lds.S | 13 +++++++++++++
 3 files changed, 26 insertions(+), 1 deletion(-)

--- a/arch/arc/kernel/head.S
+++ b/arch/arc/kernel/head.S
@@ -59,6 +59,16 @@
 #endif
 .endm
 
+; Here "patch-dtb" will embed external .dtb
+; Note "patch-dtb" searches for ASCII "OWRTDTB:" string
+; and pastes .dtb right after it, hense the string precedes
+; __image_dtb symbol.
+	.section .owrt, "aw",@progbits
+	.ascii	"OWRTDTB:"
+ENTRY(__image_dtb)
+	.fill	0x4000
+END(__image_dtb)
+
 	.section .init.text, "ax",@progbits
 
 ;----------------------------------------------------------------
--- a/arch/arc/kernel/setup.c
+++ b/arch/arc/kernel/setup.c
@@ -404,6 +404,8 @@ static inline bool uboot_arg_invalid(uns
 /* We always pass 0 as magic from U-boot */
 #define UBOOT_MAGIC_VALUE	0
 
+extern struct boot_param_header __image_dtb;
+
 void __init handle_uboot_args(void)
 {
 	bool use_embedded_dtb = true;
@@ -444,7 +446,7 @@ ignore_uboot_args:
 #endif
 
 	if (use_embedded_dtb) {
-		machine_desc = setup_machine_fdt(__dtb_start);
+		machine_desc = setup_machine_fdt(&__image_dtb);
 		if (!machine_desc)
 			panic("Embedded DT invalid\n");
 	}
--- a/arch/arc/kernel/vmlinux.lds.S
+++ b/arch/arc/kernel/vmlinux.lds.S
@@ -30,6 +30,19 @@ SECTIONS
 
 	. = CONFIG_LINUX_LINK_BASE;
 
+	/*
+	 * In OpenWRT we want to patch built binary embedding .dtb of choice.
+	 * This is implemented with "patch-dtb" utility which searches for
+	 * "OWRTDTB:" string in first 16k of image and if it is found
+	 * copies .dtb right after mentioned string.
+	 *
+	 * Note: "OWRTDTB:" won't be overwritten with .dtb, .dtb will follow it.
+	 */
+	.owrt : {
+		*(.owrt)
+		. = ALIGN(PAGE_SIZE);
+	}
+
 	_int_vec_base_lds = .;
 	.vector : {
 		*(.vector)