[PATCH v3 4/6] ARM: rust: Enable Rust support for ARMv5TE

[Karl Mehltretter <kmehltretter@gmail.com>]

The arm-unknown-linux-gnueabi target enables +v6 and emits instructions
that ARMv5 does not support. -Ctarget-cpu=arm926ej-s does not remove
+v6, while -Ctarget-feature=-v6 produces an unstable-feature warning.

Use armv5te-unknown-linux-gnueabi for CPU_32v5. It uses the Linux
soft-float EABI, requires strict alignment and declares 32-bit atomics.
Select it alongside -march so Rust and C follow the same CPU selection.
Kernels containing ARMv4 or ARMv4T CPUs remain excluded. CPU_32v5 also
covers ARM1020, for which C already uses -march=armv5te.

The kernel crate implements atomics through C helpers. The preceding
patch supplies the missing 2-byte xchg() on pre-ARMv6 CPUs.
core::sync::atomic remains unusable because LLVM lowers its operations
to __sync_* libcalls that the kernel does not provide. No in-tree code
uses them.

Tested the Rust samples, KUnit suites and doctests in QEMU versatilepb
with versatile_defconfig and on a Microchip SAM9X75 Curiosity board.

Link: https://lore.kernel.org/rust-for-linux/CACRpkdYF0sVB2-qgy=GzETSR3+2sagVQPGdunDQDJrn8KqJorA@mail.gmail.com/
Link: https://lore.kernel.org/rust-for-linux/b13d37bd-ec68-4713-94e5-e9ed4d6a6354@gmail.com/
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 Documentation/rust/arch-support.rst | 2 +-
 arch/arm/Kconfig                    | 8 +++++++-
 arch/arm/Makefile                   | 7 ++++++-
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index bdc42a24bf30..d6d1bef44c8f 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -15,7 +15,7 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 =============  ================  ==============================================
 Architecture   Level of support  Constraints
 =============  ================  ==============================================
-``arm``        Maintained        ARMv6K and ARMv7, Little Endian only.
+``arm``        Maintained        ARMv5TE, ARMv6K and ARMv7, Little Endian only.
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        \-
 ``riscv``      Maintained        ``riscv64`` and LLVM/Clang only.
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index b5816f28783a..9275a85f95e9 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -138,7 +138,7 @@ config ARM
 	select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RSEQ
-	select HAVE_RUST if CPU_LITTLE_ENDIAN && CPU_32v6K && !CPU_V6 && !KASAN
+	select HAVE_RUST if CPU_LITTLE_ENDIAN && ARM_RUST_TARGET && !KASAN
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UID16
@@ -172,6 +172,12 @@ config ARM
 	  Europe.  There is an ARM Linux project with a web page at
 	  <http://www.arm.linux.org.uk/>.
 
+# The CPU selections for which rustc has a matching target
+config ARM_RUST_TARGET
+	def_bool y
+	depends on (CPU_32v6K && !CPU_V6) || \
+		   (CPU_32v5 && !CPU_32v4T && !CPU_32v4)
+
 config ARM_HAS_GROUP_RELOCS
 	def_bool !COMPILE_TEST
 	help
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 573813ef5e77..168779e9e93d 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -73,6 +73,11 @@ arch-$(CONFIG_CPU_32v4T)	:=-march=armv4t
 arch-$(CONFIG_CPU_32v4)		:=-march=armv4
 arch-$(CONFIG_CPU_32v3)		:=-march=armv3m
 
+# The rustc target fixes the instruction set. -Ctarget-cpu cannot remove
+# the +v6 of the generic target, so older CPUs need their own target.
+rust-target-y				:=arm-unknown-linux-gnueabi
+rust-target-$(CONFIG_CPU_32v5)		:=armv5te-unknown-linux-gnueabi
+
 # Note that GCC does not numerically define an architecture version
 # macro, but instead defines a whole series of macros which makes
 # testing for a specific architecture or later rather impossible.
@@ -150,7 +155,7 @@ endif
 KBUILD_CPPFLAGS	+=$(cpp-y)
 KBUILD_CFLAGS	+=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
 KBUILD_AFLAGS	+=$(CFLAGS_ABI) $(AFLAGS_ISA) -Wa,$(arch-y) $(tune-y) -include $(srctree)/arch/arm/include/asm/unified.h -msoft-float
-KBUILD_RUSTFLAGS += --target=arm-unknown-linux-gnueabi
+KBUILD_RUSTFLAGS += --target=$(rust-target-y)
 
 CHECKFLAGS	+= -D__arm__
 
-- 
2.39.5 (Apple Git-154)