[PATCH 0/2] ARM: rust: Enable Rust support for ARMv5TE

[Karl Mehltretter <kmehltretter@gmail.com>]

Rust is currently limited to ARMv7 on 32-bit ARM. This series extends
it to ARMv5TE.

Linus Walleij asked for older ARM cores during review of the ARMv7
support [1]. Christian Schrefl tried ARMv5 with the armv5te-none-eabi
target at the time [2]. He hit missing core atomics, missing
__aeabi_mem* symbols and a floating-point helper reached from
pr_info!() formatting. None of these show up anymore. The kernel crate
now implements atomics through the C helpers, and the
armv5te-unknown-linux-gnueabi target used here keeps the Linux EABI.

The only remaining problem was a link error. Pre-ARMv6 __arch_xchg()
has no 2-byte case, which the Rust Atomic<i16> helpers need. Patch 1
adds it the way the other pre-ARMv6 atomics work, with interrupts
disabled. Patch 2 enables Rust for ARMv5TE.

Bradley Morgan's two-byte cmpxchg() emulation series [3] does not
overlap with this. Its ARMv6 part was dropped in v2 and it never
covered xchg() or pre-ARMv6.

Tested on v7.3-rc1 with clang 22.1.8, rustc 1.99.0 and bindgen 0.73.2.

- QEMU versatilepb (ARM926EJ-S). The Rust samples, all Rust KUnit
  suites and 355 doctests pass.
- Microchip SAM9X75 Curiosity board (ARM926EJ-S), at91_dt_defconfig
  without the ARMv4T AT91RM9200. The samples, all Rust KUnit suites and
  356 doctests pass. The Rust sample modules (rust_minimal, rust_print,
  rust_misc_device, rust_driver_faux) load and unload cleanly.
- Build test with every Rust driver, abstraction and sample that can be
  selected on ARMv5TE (rnull, binder, the two PHY drivers, cpufreq-dt,
  the DRM panic QR code). PWM_TH1520 is the exception. It uses a native
  u64 division, which does not link on 32-bit ARM.

As a further test I ported the Atmel PWM driver to Rust and ran it next
to the C driver on the SAM9X75 board. The port is not part of this
series. The comparison found a prescaler bug in the C driver for
periods of 2^32 clock cycles or more [4].

I plan to get a Raspberry Pi 1 Model B+ and then add ARMv6K support as
a separate patch.

The ARMv7 enablement went in through Russell's patch system as
"ARM: 9441/1". Unless someone prefers another route I would submit
this series there after review.

[1] https://lore.kernel.org/rust-for-linux/CACRpkdYF0sVB2-qgy=GzETSR3+2sagVQPGdunDQDJrn8KqJorA@mail.gmail.com/
[2] https://lore.kernel.org/rust-for-linux/b13d37bd-ec68-4713-94e5-e9ed4d6a6354@gmail.com/
[3] https://lore.kernel.org/lkml/20260922173354.14404-1-brads@mainlining.org/
[4] https://lore.kernel.org/linux-pwm/20261003083036.21584-1-kmehltretter@gmail.com/

Karl Mehltretter (2):
  ARM: cmpxchg: support 2-byte xchg() on pre-ARMv6 CPUs
  ARM: rust: Enable Rust support for ARMv5TE

 Documentation/rust/arch-support.rst |  2 +-
 arch/arm/Kconfig                    |  2 +-
 arch/arm/Makefile                   |  4 ++++
 arch/arm/include/asm/cmpxchg.h      | 14 +++++++++++---
 4 files changed, 17 insertions(+), 5 deletions(-)


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