armv7: add a wrapper for romstage's main() for ARM ISA

This adds a wrapper around main() in romstage which is compiled using
-marm. This assumes that the bootblock branches to romstage in ARM
mode.

The long-term idea is to enforce ABI compatibility when handing off to
the next stage by using shims which are which are compiled in a pre-
determiend manner and leave the main portions of each stage up to
whatever the compiler wants. So it will eventually look like this:
1. bootblock_main (ARM/Thumb)
2. bootblock_exit (ARM)
3. romstage_entry (ARM)
4. romstage_main (ARM/Thumb)

(credit to Gabe Black for writing the patch, I'm just uploading it)

Change-Id: I4fdb8d2c6c2c0a7178bcb9154c378ddce0567309
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Signed-off-by: Gabe Black <gabeblack@chromium.org>
Reviewed-on: http://review.coreboot.org/2175
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/armv7/Makefile.inc

@@ -265,21 +265,28 @@ endif
 ################################################################################
 # Build the romstage
 
+romstage_main_c = $(src)/arch/armv7/romstage_main.c
+romstage_main_o = $(obj)/arch/armv7/romstage_main.o
+
+$(romstage_main_o): $(romstage_main_c)
+	@printf "    CC         $(subst $(obj)/,,$(@))\n"
+	$(CC) -nostdlib -nostartfiles -static -c -o $@ $< -marm
+
 # FIXME(dhendrix): added debug printfs
-$(objcbfs)/romstage_null.debug: $$(romstage-objs) $(objgenerated)/romstage_null.ld
+$(objcbfs)/romstage_null.debug: $$(romstage-objs) $(romstage_main_o) $(objgenerated)/romstage_null.ld
 	@printf "    LINK       $(subst $(obj)/,,$(@))\n"
 ifeq ($(CONFIG_COMPILER_LLVM_CLANG),y)
 	$(LD) -nostdlib -nostartfiles -static -o $@ -L$(obj) $(romstage-objs) -T $(objgenerated)/romstage_null.ld
 else
-	$(CC) -nostdlib -nostartfiles -static -o $@ -L$(obj) -T $(objgenerated)/romstage_null.ld -Wl,--start-group $(romstage-objs) $(LIBGCC_FILE_NAME) -Wl,--end-group
+	$(CC) -nostdlib -nostartfiles -static -o $@ -L$(obj) -T $(objgenerated)/romstage_null.ld -Wl,--start-group $(romstage-objs) $(romstage_main_o) $(LIBGCC_FILE_NAME) -Wl,--end-group
 endif
 
 $(objcbfs)/romstage_xip.debug: $$(romstage-objs) $(objgenerated)/romstage_xip.ld
 	@printf "    LINK       $(subst $(obj)/,,$(@))\n"
 ifeq ($(CONFIG_COMPILER_LLVM_CLANG),y)
-	$(LD) -nostdlib -nostartfiles -static -o $@ -L$(obj) $(romstage-objs) -T $(objgenerated)/romstage_xip.ld
+	$(LD) -nostdlib -nostartfiles -static -o $@ -L$(obj) $(romstage-objs) $(romstage_main_o) -T $(objgenerated)/romstage_xip.ld
 else
-	$(CC) -nostdlib -nostartfiles -static -o $@ -L$(obj) -T $(objgenerated)/romstage_xip.ld -Wl,--start-group $(romstage-objs) $(LIBGCC_FILE_NAME) -Wl,--end-group
+	$(CC) -nostdlib -nostartfiles -static -o $@ -L$(obj) -T $(objgenerated)/romstage_xip.ld -Wl,--start-group $(romstage-objs) $(romstage_main_o) $(LIBGCC_FILE_NAME) -Wl,--end-group
 endif
 
 $(objgenerated)/romstage_null.ld: $$(ldscripts) $(obj)/ldoptions

src/arch/armv7/romstage.ld

@@ -44,6 +44,7 @@ SECTIONS
 	.romtext . : {
 		_rom = .;
 		_start = .;
+		*(.text.entry.armv7);
 		*(.text.startup);
 		*(.text);
 	}

src/arch/armv7/romstage_main.c

@@ -0,0 +1,29 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright (C) 2012 The ChromiumOS Authors.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+void main(void);
+
+/* romstage_main is simply a stub to invoke main(), to handle ARM mode switching
+ * (ARM/Thumb) properly. */
+
+void romstage_main(void) __attribute__((section(".text.entry.armv7")));
+void romstage_main(void)
+{
+	main();
+}

src/mainboard/google/snow/romstage.c

@@ -26,6 +26,7 @@ static void mmu_setup(void)
 	dram_bank_mmu_setup(CONFIG_SYS_SDRAM_BASE, CONFIG_DRAM_SIZE_MB * 1024);
 }
 
+void main(void);
 void main(void)
 {
 	mmu_setup();