From 318c1fce4aeef298cbb6153416c499c94ad7cda0 Mon Sep 17 00:00:00 2001
From: Jonas Gorski <jogo@openwrt.org>
Date: Tue, 24 Jun 2014 10:53:15 +0200
Subject: [PATCH RFC v3] MIPS: add support for vmlinux.bin appended DTB

Add support for populating initial_boot_params through a dtb
blob appended to raw vmlinux.bin.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
---
Changes RFC v2 -> v3

* fixed !smp kernels (TODO: move it to its own patch

Changes RFC v1 -> v2

* changed all occurences of vmlinux to vmlinux.bin
* clarified this applies to the raw vmlinux.bin without decompressor
* s/initial_device_params/initial_boot_params/

Initial comments by me still valid:

Mostly adapted from how ARM is doing it.

Sent as an RFC PATCH because I am not sure if this is the right way to
it, and whether storing the pointer in initial_boot_params is a good
idea, or a new variable should be introduced.

The reasoning for initial_boot_params is that there is no common
MIPS interface yet, so the next best thing was using that. This also
has the advantage of keeping the original fw_args intact.

This patch works for me on bcm63xx, where the bootloader expects
an lzma compressed kernel, so I didn't want to double compress using
the in-kernel compressed kernel support.

Completely untested on anything except MIPS32 / big endian.

 arch/mips/Kconfig              | 18 ++++++++++++++++++
 arch/mips/kernel/head.S        | 19 +++++++++++++++++++
 arch/mips/kernel/vmlinux.lds.S |  7 +++++++
 3 files changed, 43 insertions(+)

--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -2655,6 +2655,24 @@ config RAPIDIO
 
 source "drivers/rapidio/Kconfig"
 
+config MIPS_APPENDED_DTB
+	bool "Use appended device tree blob to vmlinux.bin (EXPERIMENTAL)"
+	depends on OF
+	help
+	  With this option, the boot code will look for a device tree binary
+	  DTB) appended to raw vmlinux.bin (without decompressor).
+	  (e.g. cat vmlinux.bin <filename>.dtb > vmlinux_w_dtb).
+
+	  This is meant as a backward compatibility convenience for those
+	  systems with a bootloader that can't be upgraded to accommodate
+	  the documented boot protocol using a device tree.
+
+	  Beware that there is very little in terms of protection against
+	  this option being confused by leftover garbage in memory that might
+	  look like a DTB header after a reboot if no actual DTB is appended
+	  to vmlinux.bin.  Do not leave this option active in a production kernel
+	  if you don't intend to always append a DTB.
+
 endmenu
 
 menu "Executable file formats"
--- a/arch/mips/kernel/head.S
+++ b/arch/mips/kernel/head.S
@@ -100,6 +100,22 @@ NESTED(kernel_entry, 16, sp)			# kernel
 	jr	t0
 0:
 
+#ifdef CONFIG_MIPS_APPENDED_DTB
+	PTR_LA		t0, __appended_dtb
+	PTR_LI		t3, 0
+
+#ifdef CONFIG_CPU_BIG_ENDIAN
+	PTR_LI		t1, 0xd00dfeed
+#else
+	PTR_LI		t1, 0xedfe0dd0
+#endif
+	LONG_L		t2, (t0)
+	bne		t1, t2, not_found
+
+	PTR_LA		t3, __appended_dtb
+
+not_found:
+#endif
 	PTR_LA		t0, __bss_start		# clear .bss
 	LONG_S		zero, (t0)
 	PTR_LA		t1, __bss_stop - LONGSIZE
@@ -113,6 +129,10 @@ NESTED(kernel_entry, 16, sp)			# kernel
 	LONG_S		a2, fw_arg2
 	LONG_S		a3, fw_arg3
 
+#ifdef CONFIG_MIPS_APPENDED_DTB
+	LONG_S		t3, initial_boot_params
+#endif
+
 	MTC0		zero, CP0_CONTEXT	# clear context register
 	PTR_LA		$28, init_thread_union
 	/* Set the SP after an empty pt_regs.  */
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -125,8 +125,14 @@ SECTIONS
 	.exit.data : {
 		EXIT_DATA
 	}
-
+#ifdef CONFIG_SMP
 	PERCPU_SECTION(1 << CONFIG_MIPS_L1_CACHE_SHIFT)
+#endif
+#ifdef CONFIG_MIPS_APPENDED_DTB
+	__appended_dtb = .;
+	/* leave space for appended DTB */
+	. = . + 0x100000;
+#endif
 	/*
 	 * Align to 64K in attempt to eliminate holes before the
 	 * .bss..swapper_pg_dir section at the start of .bss.  This
