[PATCH v3 3/6] ARM: rust: Enable Rust support for ARMv6K-only kernels

[Karl Mehltretter <kmehltretter@gmail.com>]

HAVE_RUST currently requires CPU_32v7, excluding ARMv6K-only kernels
even though arm-unknown-linux-gnueabi already builds ARMv6 code.

Select HAVE_RUST for CPU_32v6K. CPU_V7 selects CPU_32v6K, so ARMv7
remains covered.

Keep plain ARMv6 (CPU_V6, ARM1136r0) excluded. It lacks 1- and 2-byte
xchg() and 2-byte cmpxchg(), which the Rust atomic helpers need. The old
condition exposes Rust on combined CPU_V6 and ARMv7 kernels, which do
not link.

Tested the Rust samples, KUnit suites and doctests in QEMU raspi0
(ARM1176) with bcm2835_defconfig without ARCH_MULTI_V7.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Tested the full six-patch series on a Raspberry Pi 1 Model B+
Rev 1.2 (ARM1176JZF-S), with bcm2835_defconfig without ARCH_MULTI_V7,
clang 22.1.8, rustc 1.99.0 and bindgen 0.73.2.

All 368 doctests and the Rust KUnit suites passed. The intentionally
disabled KUnit selftest was skipped. Ten Rust modules loaded and
unloaded. Misc ioctls, configfs, scoped debugfs and rnull read/write
checks passed. No warnings or oopses in the test kernel.

 Documentation/rust/arch-support.rst | 2 +-
 arch/arm/Kconfig                    | 2 +-
 rust/helpers/atomic_ext.c           | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index 4f980815e92a..bdc42a24bf30 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        ARMv7 Little Endian only.
+``arm``        Maintained        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 ffbc7f386131..b5816f28783a 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_32v7 && !KASAN
+	select HAVE_RUST if CPU_LITTLE_ENDIAN && CPU_32v6K && !CPU_V6 && !KASAN
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UID16
diff --git a/rust/helpers/atomic_ext.c b/rust/helpers/atomic_ext.c
index c267d5190529..1d157df29229 100644
--- a/rust/helpers/atomic_ext.c
+++ b/rust/helpers/atomic_ext.c
@@ -42,7 +42,7 @@ GEN_READ_SET_HELPERS(ptr, const void *)
  * xchg helpers depend on ARCH_SUPPORTS_ATOMIC_RMW and on the
  * architecture provding xchg() support for i8 and i16.
  *
- * The architectures that currently support Rust (x86_64, armv7,
+ * The architectures that currently support Rust (x86_64, arm,
  * arm64, riscv, and loongarch) satisfy these requirements.
  */
 #define GEN_XCHG_HELPER(tname, type, suffix)					\
@@ -66,7 +66,7 @@ GEN_XCHG_HELPERS(ptr, const void *)
  * try_cmpxchg helpers depend on ARCH_SUPPORTS_ATOMIC_RMW and on the
  * architecture provding try_cmpxchg() support for i8 and i16.
  *
- * The architectures that currently support Rust (x86_64, armv7,
+ * The architectures that currently support Rust (x86_64, arm,
  * arm64, riscv, and loongarch) satisfy these requirements.
  */
 #define GEN_TRY_CMPXCHG_HELPER(tname, type, suffix)				\
-- 
2.39.5 (Apple Git-154)