MIPS: Add support for vmlinux.bin appended dtb

Add support for detecting a vmlinux.bin appended dtb and overriding
the boot arguments to match the UHI interface.

Due to the PERCPU section being empty for !SMP, but still modifying
the current address by aligning it to the page size, do not define
it for !SMP builds to allow __appended_dtb to still point to
the actual end of the data.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: John Crispin <blogic@openwrt.org>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: James Hartley <James.Hartley@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/9739/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -2677,6 +2677,33 @@ config USE_OF
 config BUILTIN_DTB
 	bool
 
+choice
+	prompt "Kernel appended dtb support" if OF
+	default MIPS_NO_APPENDED_DTB
+
+	config MIPS_NO_APPENDED_DTB
+		bool "None"
+		help
+		  Do not enable appended dtb support.
+
+	config MIPS_RAW_APPENDED_DTB
+		bool "vmlinux.bin"
+		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.
+endchoice
+
 endmenu
 
 config LOCKDEP_SUPPORT

arch/mips/kernel/head.S

@@ -94,6 +94,22 @@ NESTED(kernel_entry, 16, sp)			# kernel entry point
 	jr	t0
 0:
 
+#ifdef CONFIG_MIPS_RAW_APPENDED_DTB
+	PTR_LA		t0, __appended_dtb
+
+#ifdef CONFIG_CPU_BIG_ENDIAN
+	li		t1, 0xd00dfeed
+#else
+	li		t1, 0xedfe0dd0
+#endif
+	lw		t2, (t0)
+	bne		t1, t2, not_found
+	 nop
+
+	move		a1, t0
+	PTR_LI		a0, -2
+not_found:
+#endif
 	PTR_LA		t0, __bss_start		# clear .bss
 	LONG_S		zero, (t0)
 	PTR_LA		t1, __bss_stop - LONGSIZE

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_RAW_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