Add thumb support to buildroot for the ARM architecture

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 <[email protected]>

---
 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
===================================================================

Config.in +8 -0

@@ -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
===================================================================

package/Makefile.in +4 -0

@@ -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
===================================================================

toolchain/Config.in.2 +12 -0

@@ -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
===================================================================

toolchain/binutils/binutils.mk +5 -0

@@ -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
===================================================================

toolchain/gcc/Makefile.in +4 -0

@@ -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
===================================================================

toolchain/uClibc/uclibc.mk +0 -0

@@ -322,6 +322,13 @@
 else
 	$(SED) 's,^.*UCLIBC_HAS_WCHAR.*,UCLIBC_HAS_WCHAR=n,g'

$(UCLIBC_DIR)/.oldconfig endif

toolchain/uClibc/uclibc.mk +3 -0

+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 +

toolchain/uClibc/uclibc.mk +2 -0

+	$(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

Thanks for your excellent work! Before committing your patch I have a
question: have you tested it? on which CPU? which buildroot configs?

I'm asking this because for almost half of year I did not managed to build a good ARM EABI toolchain with buildroot. Toolchain seems emit invalid instructions when building busybox. So I managed only to run very simple hello-world when set in cmdline init=hello-world, but not a busybox binary with shell, etc. Boot process just hangs when busybox running. Magic-SysRq key shows that illegal instruction trap was involved. What is surprising that rootfs can be launched ok on qemu with versatilepb machine emulation, but as I told fails to run on real hardware S3C2440 CPU - 920T target.

Hi,

Any comments on that ? Are you interested in merging such a patch ?

Thanks,

Thomas

However, there is quite a big difference between your S3C2440 real hardware and the Qemu versatilepb emulation. S3C2440 is based on 920T as you say, which supports the ARMv4T ISA. But the versatilepb emulation of Qemu emulates an ARM926EJ-S CPU, which supports the ARMv5TEJ ISA. And as far as I understood, there are some differences in the ARM <-> Thumb switch capabilities between ARMv4T and ARMv5T.

Any comments on that ? Are you interested in merging such a patch ?

Le Fri, 14 Mar 2008 14:19:29 +0100,
Thomas Petazzoni <[email protected]> a ?crit :

Quoted 6 lines

However, there is quite a big difference between your S3C2440 real hardware and the Qemu versatilepb emulation. S3C2440 is based on 920T as you say, which supports the ARMv4T ISA. But the versatilepb emulation of Qemu emulates an ARM926EJ-S CPU, which supports the ARMv5TEJ ISA. And as far as I understood, there are some differences in the ARM <-> Thumb switch capabilities between ARMv4T and ARMv5T.

In particular, from the ARM architecture manual:

?
BL (H == 10 and H == 11 forms) is in all T variants. BLX (H == 01 form) is in T variants of ARMv5 and above. ?

So on your S3C2440, the 'blx' instruction is not supported. And this instruction can especially be used to do interworking. So if gcc generated this instruction, it of course will fail with the "illegal instruction" exception.

Sincerly,

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

I compiled for ARM_920T & EABI & SOFTFLOAT.
I checked busybox binary ( arm-linux-objdump ) - there is no blx instruction. Seems all these ARM troubles were related to avr32 patches (see recent posts in mailing list) because yesterday finally I managed to build working rootfs.

However busybox binary seems resulted in OABI format because when in kernel disable OABI_COMPAT - rootfs does not boot.

Quoted 5 lines

I compiled for ARM_920T & EABI & SOFTFLOAT.
I checked busybox binary ( arm-linux-objdump ) - there is no blx instruction. Seems all these ARM troubles were related to avr32 patches (see recent posts in mailing list) because yesterday finally I managed to build working rootfs.

Hum, maybe.

Quoted 2 lines

However busybox binary seems resulted in OABI format because when in kernel disable OABI_COMPAT - rootfs does not boot.

Do you have CONFIG_EABI=y in your buildroot config file ?

But anyway, do you confirm that my patch works ?

Sincerly,

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

Quoted 1 line

Any comments on that ? Are you interested in merging such a patch ?

Still no interest in this patch ? Is the patch wrong ?