Hi!

The Rust firmware abstractions FTBFS on arm64 and current stable (6.13.4) when compiled with rustc 1.85.0:

``` RUSTC L rust/kernel.o error[E0308]: mismatched types --> rust/kernel/firmware.rs:20:14 | 20 | Self(bindings::request_firmware) | ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found fn item | | | arguments to this function are incorrect | = note: expected fn pointer `unsafe extern "C" fn(_, *const i8, _) -> _` found fn item `unsafe extern "C" fn(_, *const u8, _) -> _ {request_firmware}` note: tuple struct defined here --> rust/kernel/firmware.rs:14:8 | 14 | struct FwFunc( | ^^^^^^

error[E0308]: mismatched types --> rust/kernel/firmware.rs:24:14 | 24 | Self(bindings::firmware_request_nowarn) | ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found fn item | | | arguments to this function are incorrect | = note: expected fn pointer `unsafe extern "C" fn(_, *const i8, _) -> _` found fn item `unsafe extern "C" fn(_, *const u8, _) -> _ {firmware_request_nowarn}` note: tuple struct defined here --> rust/kernel/firmware.rs:14:8 | 14 | struct FwFunc( | ^^^^^^

error[E0308]: mismatched types --> rust/kernel/firmware.rs:64:45 | 64 | let ret = unsafe { func.0(pfw as _, name.as_char_ptr(), dev.as_raw()) }; | ------ ^^^^^^^^^^^^^^^^^^ expected `*const i8`, found `*const u8` | | | arguments to this function are incorrect | = note: expected raw pointer `*const i8` found raw pointer `*const u8`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0308`. ```

This is because rustc 1.85 (now stable) switched core::ffi::c_char from i8 to u8 on arm64 and other platforms [1], and because current stable still uses rustc's core's instead of the custom ffi integer types like 6.14 will. Looking for other i8's in *.rs files tells me only the QR code panic screen should be affected in addition to the firmware abstractions, and that was already reported in [2].

A simple fix would be to switch i8 to u8 in `struct FwFunc`, but that breaks rustc <= 1.84 so I guess a more robust solution is needed.

Cheers!

[1] https://github.com/rust-lang/rust/pull/132975 [2] https://lore.kernel.org/all/20250120124531.2581448-1-linkmauve@linkmauve.fr/