[Buildroot] [PATCH] Add thumb support to buildroot for the ARM architecture

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Hi,

You'll find enclosed in this mail a patch that adds Thumb support for
buildroot. It allows to generate an interworking-enabled toolchain,
with an uClibc compiled in ARM mode, and then to compile userspace
applications in Thumb mode.

More details are available in the patch description.

Of course, your comments, reviews and suggestions are welcome!

Thanks for developing such a great tool,

Sincerly,

Thomas

---
Add thumb support to buildroot for the ARM architecture

The Thumb instruction set is encoded on 16 bits opcode instead of 32
bits opcode for the regular ARM instruction set. It generally allows
for greater code density, leading to reduced code size, which is nice
on memory-constrained embedded systems.

Currently, uClibc cannot be fully compiled using the Thumb instruction
set. Therefore, this patch still compile uClibc in regular ARM mode,
but allows to enable interworking support for the
toolchain. Interworking allows Thumb code to call ARM code and
vice-versa, allowing a mixed Thumb/ARM userspace.

Enabling interworking at the toolchain level is done using the
BR2_INTERWORKING_SUPPORT option, added by the patch. When enabled, it
adds --enable-interwork to the binutils and gcc configure options,
adds -mthumb-interwork to the CFLAGS during uClibc compilation, and
enables the USE_BX config option of uClibc (bx is an ARM instruction
that allows to switch between ARM and Thumb mode).

Once interworking is enabled, it is then possible to compile the
userspace applications in Thumb mode. This is what the new
BR2_THUMB_BINARIES option allows, by adding -mthumb to the CFLAGS used
during the compilation of userspace applications.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

---
 Config.in                      |    8 ++++++++
 package/Makefile.in            |    4 ++++
 toolchain/Config.in.2          |   12 ++++++++++++
 toolchain/binutils/binutils.mk |    5 +++++
 toolchain/gcc/Makefile.in      |    4 ++++
 toolchain/uClibc/uclibc.mk     |    7 +++++++
 6 files changed, 40 insertions(+)

Index: buildroot.new/Config.in
===================================================================
--- buildroot.new.orig/Config.in
+++ buildroot.new/Config.in
@@ -223,6 +223,14 @@
 
 	  WARNING: This is highly experimental at the moment.
 
+config BR2_THUMB_BINARIES
+	bool "generate Thumb binaries"
+	default n
+	depends on BR2_INTERWORKING_SUPPORT
+	help
+	  Compile binaries in Thumb mode. Only valid for ARM
+	  architecture.
+
 config BR2_HAVE_MANPAGES
 	bool "manpages on the target"
 	default n
Index: buildroot.new/package/Makefile.in
===================================================================
--- buildroot.new.orig/package/Makefile.in
+++ buildroot.new/package/Makefile.in
@@ -46,6 +46,10 @@
 TARGET_CFLAGS+=-msoft-float
 endif
 
+ifeq ($(BR2_THUMB_BINARIES),y)
+TARGET_CFLAGS+=-mthumb
+endif
+
 # enable VIS for v9a and v9b
 ifeq ($(findstring y,$(BR2_sparc_v9a)$(BR2_sparc64_v9a)$(BR2_sparc_v9b)$(BR2_sparc64_v9b)),y)
 TARGET_CFLAGS+=-mvis
Index: buildroot.new/toolchain/Config.in.2
===================================================================
--- buildroot.new.orig/toolchain/Config.in.2
+++ buildroot.new/toolchain/Config.in.2
@@ -61,6 +61,18 @@
 #	bool "Use softfp"
 #	default n
 
+config BR2_INTERWORKING_SUPPORT
+	bool "Enable interworking support"
+	default n
+	depends on BR2_arm || BR2_armeb
+	depends on !BR2_generic_arm && !BR2_arm610 && !BR2_arm710
+	help
+	  Interworking allows Thumb userspace binaries to be linked
+	  against the C standard library compiled in ARM mode.
+
+	  This is only possible on Thumb-aware architectures, i.e not
+	  Generic ARM, ARM610 or ARM710.
+
 config BR2_TARGET_OPTIMIZATION
 	string "Target Optimizations"
 	default "-Os -pipe"
Index: buildroot.new/toolchain/binutils/binutils.mk
===================================================================
--- buildroot.new.orig/toolchain/binutils/binutils.mk
+++ buildroot.new/toolchain/binutils/binutils.mk
@@ -65,6 +65,10 @@
 BINUTILS_TARGET_CONFIG_OPTIONS+=--with-mpfr="$(MPFR_TARGET_DIR)"
 endif
 
+ifeq ($(BR2_INTERWORKING_SUPPORT),y)
+INTERWORKING_CONFIG_OPTION=--enable-interwork
+endif
+
 BINUTILS_OFFICIAL_VERSION:=$(BINUTILS_VERSION)$(VENDOR_SUFFIX)$(VENDOR_BINUTILS_RELEASE)
 ifeq ($(BR2_TOOLCHAIN_BUILDROOT),y)
 BINUTILS_PATCH_DIR:=toolchain/binutils/$(BINUTILS_VERSION)
@@ -112,6 +116,7 @@
 		--disable-werror \
 		$(SOFT_FLOAT_CONFIG_OPTION) \
 		$(EXTRA_BINUTILS_CONFIG_OPTIONS) \
+		$(INTERWORKING_CONFIG_OPTION) \
 	)
 	touch $@
 
Index: buildroot.new/toolchain/gcc/Makefile.in
===================================================================
--- buildroot.new.orig/toolchain/gcc/Makefile.in
+++ buildroot.new/toolchain/gcc/Makefile.in
@@ -65,6 +65,10 @@
 endif
 endif
 
+ifeq ($(BR2_INTERWORKING_SUPPORT),y)
+EXTRA_GCC_CONFIG_OPTIONS+=--enable-interwork
+endif
+
 # AVR32 GCC configuration
 ifeq ($(strip $(BR2_avr32)),y)
 EXTRA_GCC_CONFIG_OPTIONS+=--disable-libmudflap
Index: buildroot.new/toolchain/uClibc/uclibc.mk
===================================================================
--- buildroot.new.orig/toolchain/uClibc/uclibc.mk
+++ buildroot.new/toolchain/uClibc/uclibc.mk
@@ -322,6 +322,13 @@
 else
 	$(SED) 's,^.*UCLIBC_HAS_WCHAR.*,UCLIBC_HAS_WCHAR=n,g' $(UCLIBC_DIR)/.oldconfig
 endif
+ifeq ($(BR2_INTERWORKING_SUPPORT),y)
+	$(SED) 's,^.*USE_BX.*,USE_BX=y,g' $(UCLIBC_DIR)/.oldconfig
+	$(SED) 's,^.*UCLIBC_EXTRA_CFLAGS.*,UCLIBC_EXTRA_CFLAGS="-mthumb-interwork",g' $(UCLIBC_DIR)/.oldconfig
+else
+
+	$(SED) 's,^.*USE_BX.*,USE_BX=n,g' $(UCLIBC_DIR)/.oldconfig
+endif
 ifeq ($(BR2_UCLIBC_PROGRAM_INVOCATION),y)
 	$(SED) 's,^.*UCLIBC_HAS_PROGRAM_INVOCATION_NAME.*,UCLIBC_HAS_PROGRAM_INVOCATION_NAME=y,g' $(UCLIBC_DIR)/.oldconfig
 else


-- 
Thomas Petazzoni, Free Electrons
Free Embedded Linux Training Materials
on http://free-electrons.com/training
(More than 1500 pages!)
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