[U-Boot] [PATCH v4 3/6] ARM: enable Thumb build

[Aneesh V <aneesh@ti.com>]

Enable Thumb build and ARM-Thumb interworking based on the new
config flag CONFIG_SYS_THUMB_BUILD

Signed-off-by: Aneesh V <aneesh@ti.com>
---
Changes in v4:
- Use ':=' instead of '+=' when computed make variables
  are involved

Changes in v3:
- None

Changes from V1 to V2:
- None

Changes from RFC to V1:
- Fixed review comments from Tom Rini <trini@ti.com>
---
 README             |    8 ++++++++
 arch/arm/config.mk |   22 +++++++++++++++-------
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/README b/README
index 8964672..bdb428e 100644
--- a/README
+++ b/README
@@ -426,6 +426,14 @@ The following options need to be configured:
 		Select high exception vectors of the ARM core, e.g., do not
 		clear the V bit of the c1 register of CP15.
 
+		CONFIG_SYS_THUMB_BUILD
+
+		Use this flag to build U-Boot using the Thumb instruction
+		set for ARM architectures. Thumb instruction set provides
+		better code density. For ARM architectures that support
+		Thumb2 this flag will result in Thumb2 code generated by
+		GCC.
+
 - Linux Kernel Interface:
 		CONFIG_CLOCKS_IN_MHZ
 
diff --git a/arch/arm/config.mk b/arch/arm/config.mk
index 45f9dca..d4fa1f8 100644
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -33,25 +33,33 @@ endif
 
 PLATFORM_CPPFLAGS += -DCONFIG_ARM -D__ARM__
 
-# Explicitly specifiy 32-bit ARM ISA since toolchain default can be -mthumb:
-PF_CPPFLAGS_ARM := $(call cc-option,-marm,)
+# Choose between ARM/Thumb instruction sets
+ifeq ($(CONFIG_SYS_THUMB_BUILD),y)
+PF_CPPFLAGS_ARM := $(call cc-option, -mthumb -mthumb-interwork,\
+			$(call cc-option,-marm,)\
+			$(call cc-option,-mno-thumb-interwork,)\
+		)
+else
+PF_CPPFLAGS_ARM := $(call cc-option,-marm,) \
+		$(call cc-option,-mno-thumb-interwork,)
+endif
 
 # Try if EABI is supported, else fall back to old API,
 # i. e. for example:
 # - with ELDK 4.2 (EABI supported), use:
-#	-mabi=aapcs-linux -mno-thumb-interwork
+#	-mabi=aapcs-linux
 # - with ELDK 4.1 (gcc 4.x, no EABI), use:
-#	-mabi=apcs-gnu -mno-thumb-interwork
+#	-mabi=apcs-gnu
 # - with ELDK 3.1 (gcc 3.x), use:
-#	-mapcs-32 -mno-thumb-interwork
+#	-mapcs-32
 PF_CPPFLAGS_ABI := $(call cc-option,\
-			-mabi=aapcs-linux -mno-thumb-interwork,\
+			-mabi=aapcs-linux,\
 			$(call cc-option,\
 				-mapcs-32,\
 				$(call cc-option,\
 					-mabi=apcs-gnu,\
 				)\
-			) $(call cc-option,-mno-thumb-interwork,)\
+			)\
 		)
 PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_ARM) $(PF_CPPFLAGS_ABI)
 
-- 
1.7.1