[PATCH v2 3/6] ARM: rust: Enable Rust support for ARMv6K-only kernels

[Karl Mehltretter <kmehltretter@gmail.com>]

HAVE_RUST is selected for CPU_32v7 only. A combined ARMv6K and ARMv7
kernel has CPU_32v7 and can use Rust today. A kernel built for ARMv6K
alone cannot. There is no technical reason for that.

No CPU flags are passed to rustc on ARM. The arm-unknown-linux-gnueabi
target has +v6 in its feature list, so Rust code is built as ARMv6 on
all of these kernels already.

Select HAVE_RUST for CPU_32v6K instead. CPU_V7 selects CPU_32v6K, so
ARMv7 stays covered.

Plain ARMv6 (CPU_V6, ARM1136r0) is excluded. It has no 1- and 2-byte
xchg() and no 2-byte cmpxchg(), which the Rust atomic helpers need. A
kernel that combines CPU_V6 with ARMv7 can enable Rust today and does
not link.

Tested in QEMU raspi0 (ARM1176) with bcm2835_defconfig without
ARCH_MULTI_V7. The Rust samples, all Rust KUnit suites and the
doctests pass with the current and with the minimum tool versions.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 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)