[PATCH 1/1] arm: rust: Enable Rust support for ARMv7

[Christian Schrefl]

This commit allows building ARMv7 kernels with Rust support.

The rust core library expects some __eabi_... functions
that are not implemented in the kernel.
Those functions are some float operations and __aeabi_uldivmod.

This is based on the code by Sven Van Asbroeck from the original
rust branch and inspired by the AArch version by Jamie Cunliffe.

I have tested the rust samples and a custom simple MMIO module
on on hardware (De1SoC FPGA + Arm A9 CPU).

Signed-off-by: Christian Schrefl <chrisi.schrefl@xxxxxxxxx>
---
 Documentation/rust/arch-support.rst |  1 +
 arch/arm/Kconfig                    |  1 +
 arch/arm/Makefile                   |  1 +
 rust/Makefile                       | 10 +++++++++-
 rust/bindgen_parameters             |  4 ++++
 rust/compiler_builtins.rs           | 24 ++++++++++++++++++++++++
 scripts/generate_rust_target.rs     |  4 +++-
 7 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index b13e19d84744..4bf5205f526d 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -15,6 +15,7 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 =============  ================  ==============================================
 Architecture   Level of support  Constraints
 =============  ================  ==============================================
+``arm``        Maintained        ARMv7 Little Endian only.
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        \-
 ``riscv``      Maintained        ``riscv64`` only.
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ee5115252aac..f07149fe078b 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -126,6 +126,7 @@ 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
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UID16
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 71afdd98ddf2..9cc10e32e8be 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -150,6 +150,7 @@ 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 asm/unified.h -msoft-float
+KBUILD_RUSTFLAGS += --target=arm-unknown-linux-gnueabi
 
 CHECKFLAGS	+= -D__arm__
 
diff --git a/rust/Makefile b/rust/Makefile
index f70d5e244fee..ef177ffb68a8 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -286,7 +286,8 @@ bindgen_skip_c_flags := -mno-fp-ret-in-387 -mpreferred-stack-boundary=% \
 
 # Derived from `scripts/Makefile.clang`.
 BINDGEN_TARGET_x86	:= x86_64-linux-gnu
-BINDGEN_TARGET_arm64	:= aarch64-linux-gnu
+BINDGEN_TARGET_arm64:= aarch64-linux-gnu
+BINDGEN_TARGET_arm	:= arm-linux-gnueabi
 BINDGEN_TARGET		:= $(BINDGEN_TARGET_$(SRCARCH))
 
 # All warnings are inhibited since GCC builds are very experimental,
@@ -413,6 +414,13 @@ redirect-intrinsics = \
 	__muloti4 __multi3 \
 	__udivmodti4 __udivti3 __umodti3
 
+ifdef CONFIG_ARM
+	# Add eabi initrinsics for ARM 32-bit
+	redirect-intrinsics += \
+		__aeabi_fadd __aeabi_fmul __aeabi_fcmpeq __aeabi_fcmple __aeabi_fcmplt __aeabi_fcmpun \
+		__aeabi_dadd __aeabi_dmul __aeabi_dcmple __aeabi_dcmplt __aeabi_dcmpun \
+		__aeabi_uldivmod
+endif
 ifneq ($(or $(CONFIG_ARM64),$(and $(CONFIG_RISCV),$(CONFIG_64BIT))),)
 	# These intrinsics are defined for ARM64 and RISCV64
 	redirect-intrinsics += \
diff --git a/rust/bindgen_parameters b/rust/bindgen_parameters
index a721d466bee4..bf0148b3019e 100644
--- a/rust/bindgen_parameters
+++ b/rust/bindgen_parameters
@@ -24,3 +24,7 @@
 # These functions use the `__preserve_most` calling convention, which neither bindgen
 # nor Rust currently understand, and which Clang currently declares to be unstable.
 --blocklist-function __list_.*_report
+
+# Depending on how the architecute defines ARCH_SLAB_MINALIGN, bindgen might generate a binding.
+# Disable this here as there is a const that will always be generated in bindings_helper.c
+--blocklist-item ARCH_SLAB_MINALIGN
diff --git a/rust/compiler_builtins.rs b/rust/compiler_builtins.rs
index bba2922c6ef7..c37142b16a45 100644
--- a/rust/compiler_builtins.rs
+++ b/rust/compiler_builtins.rs
@@ -70,5 +70,29 @@ pub extern "C" fn $ident() {
     __umodti3,
 });
 
+#[cfg(target_arch = "arm")]
+define_panicking_intrinsics!("`f32` should not be used", {
+    __aeabi_fadd,
+    __aeabi_fmul,
+    __aeabi_fcmpeq,
+    __aeabi_fcmple,
+    __aeabi_fcmplt,
+    __aeabi_fcmpun,
+});
+
+#[cfg(target_arch = "arm")]
+define_panicking_intrinsics!("`f64` should not be used", {
+    __aeabi_dadd,
+    __aeabi_dmul,
+    __aeabi_dcmple,
+    __aeabi_dcmplt,
+    __aeabi_dcmpun,
+});
+
+#[cfg(target_arch = "arm")]
+define_panicking_intrinsics!("`u64` division/modulo should not be used", {
+    __aeabi_uldivmod,
+});
+
 // NOTE: if you are adding a new intrinsic here, you should also add it to
 // `redirect-intrinsics` in `rust/Makefile`.
diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
index 641b713a033a..acfcf2e22e28 100644
--- a/scripts/generate_rust_target.rs
+++ b/scripts/generate_rust_target.rs
@@ -148,7 +148,9 @@ fn main() {
     let mut ts = TargetSpec::new();
 
     // `llvm-target`s are taken from `scripts/Makefile.clang`.
-    if cfg.has("ARM64") {
+    if cfg.has("ARM") {
+        panic!("arm uses the builtin rustc target");
+    } else if cfg.has("ARM64") {
         panic!("arm64 uses the builtin rustc aarch64-unknown-none target");
     } else if cfg.has("RISCV") {
         if cfg.has("64BIT") {
-- 
2.45.2