Re: [PATCH v2 0/3] Inline helpers into Rust without full LTO

[Arnd Bergmann]

On Mon, Mar 30, 2026, at 23:09, Christian Schrefl wrote:
> On 3/26/26 6:30 PM, Miguel Ojeda wrote:
>> On Thu, Mar 26, 2026 at 4:18 PM Russell King (Oracle)
>> <linux@xxxxxxxxxxxxxxx> wrote:
>>>
>>> I'm not sure if this is still true, but I believe it used to be the case
>>> that the -linux-gnueabi target has one behaviour for enums (fixed size)
>>> whereas -none-eabi, the size of the type depends on the range of values
>>> included in the enum.
>>>
>>> Certianly, when Arm Ltd were proposing EABI, EABI had the latter
>>> behaviour, and I think there were cases where Linux used "enum" in
>>> its UAPI.
>> 
>> Short enums? I see `c-enum-min-bits` in the armv7a-none-eabi built-in
>> `rustc` target, and indeed:
>> 
>>     #![no_std]
>> 
>>     #[repr(C)]
>>     enum T {
>>         A,
>>         B,
>>     }
>> 
>>     pub static S: usize = core::mem::size_of::<T>();
>> 
>> is 1 for that one, and 4 for the other.
>
> I guess we could use a custom target spec, but I'm not
> sure if that is worth the hassle of adding another one.

The kernel Makefile forces the Linux enum behavior using

arch/arm/Makefile:CFLAGS_ABI    :=-mabi=aapcs-linux -mfpu=vfp
ifeq ($(CONFIG_CC_IS_CLANG),y)
CFLAGS_ABI      += -meabi gnu
endif
KBUILD_RUSTFLAGS += --target=arm-unknown-linux-gnueabi

regardless of the compiler. I think this should be safe
for any combination of C and rust compilers.

     Arnd