[PATCH v3 5/6] ARM: rust: Enable Rust support for ARMv4T

[Karl Mehltretter <kmehltretter@gmail.com>]

The armv5te target emits clz and blx, which ARMv4T lacks. Use rustc's
armv4t-unknown-linux-gnueabi target for CPU_32v4T. A combined ARMv4T and
ARMv5 kernel then uses ARMv4T code for both C and Rust.

Plain ARMv4 remains excluded because rustc has no matching Linux target.
This also depends on the preceding 16-bit xchg() support for pre-ARMv6
CPUs.

Tested the Rust samples, KUnit suites and doctests in QEMU sx1 (ARM925T)
with omap1_defconfig and on a Microchip SAM9X75 Curiosity board with
at91_dt_defconfig.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 Documentation/rust/arch-support.rst | 8 ++++----
 arch/arm/Kconfig                    | 2 +-
 arch/arm/Makefile                   | 1 +
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index d6d1bef44c8f..3f39cf55b6f4 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -12,15 +12,15 @@ which uses ``libclang``.
 Below is a general summary of architectures that currently work. Level of
 support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 
-=============  ================  ==============================================
+=============  ================  ======================================================
 Architecture   Level of support  Constraints
-=============  ================  ==============================================
-``arm``        Maintained        ARMv5TE, ARMv6K and ARMv7, Little Endian only.
+=============  ================  ======================================================
+``arm``        Maintained        ARMv4T, ARMv5TE, ARMv6K and ARMv7, Little Endian only.
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        \-
 ``riscv``      Maintained        ``riscv64`` and LLVM/Clang only.
 ``s390``       Maintained        ``CONFIG_EXPOLINE`` must be disabled.
 ``um``         Maintained        \-
 ``x86``        Maintained        ``x86_64`` only.
-=============  ================  ==============================================
+=============  ================  ======================================================
 
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 9275a85f95e9..cf782fc04872 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -176,7 +176,7 @@ config ARM
 config ARM_RUST_TARGET
 	def_bool y
 	depends on (CPU_32v6K && !CPU_V6) || \
-		   (CPU_32v5 && !CPU_32v4T && !CPU_32v4)
+		   ((CPU_32v5 || CPU_32v4T) && !CPU_32v4)
 
 config ARM_HAS_GROUP_RELOCS
 	def_bool !COMPILE_TEST
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 168779e9e93d..a72843890241 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -77,6 +77,7 @@ arch-$(CONFIG_CPU_32v3)		:=-march=armv3m
 # the +v6 of the generic target, so older CPUs need their own target.
 rust-target-y				:=arm-unknown-linux-gnueabi
 rust-target-$(CONFIG_CPU_32v5)		:=armv5te-unknown-linux-gnueabi
+rust-target-$(CONFIG_CPU_32v4T)	:=armv4t-unknown-linux-gnueabi
 
 # Note that GCC does not numerically define an architecture version
 # macro, but instead defines a whole series of macros which makes
-- 
2.39.5 (Apple Git-154)