Re: [PATCH] rust: fix building firmware abstraction on 32bit arm

[Danilo Krummrich <dakr@kernel.org>]

On Fri, Apr 11, 2025 at 09:14:48AM +0200, Christian Schrefl wrote:
> When trying to build the rust firmware abstractions on 32 bit arm the
> following build error occures:
> 
> ```
> 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}`

This looks like you have local changes in your tree, running in this error. I
get the exact same errors when I apply the following diff:

diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
index f04b058b09b2..a67047e3aa6b 100644
--- a/rust/kernel/firmware.rs
+++ b/rust/kernel/firmware.rs
@@ -12,7 +12,7 @@
 /// One of the following: `bindings::request_firmware`, `bindings::firmware_request_nowarn`,
 /// `bindings::firmware_request_platform`, `bindings::request_firmware_direct`.
 struct FwFunc(
-    unsafe extern "C" fn(*mut *const bindings::firmware, *const u8, *mut bindings::device) -> i32,
+    unsafe extern "C" fn(*mut *const bindings::firmware, *const i8, *mut bindings::device) -> i32,
 );

> 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
> ```

I did a test build with multi_v7_defconfig and I can't reproduce this issue.

I think the kernel does always use -funsigned-char, as also documented in commit
1bae8729e50a ("rust: map `long` to `isize` and `char` to `u8`")?

> 
> To fix this error the char pointer type in `FwFunc` is converted to
> `ffi::c_char`.
> 
> Fixes: de6582833db0 ("rust: add firmware abstractions")
> Cc: stable@vger.kernel.org # Backport only to 6.15 needed
> 
> Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
> ---
>  rust/kernel/firmware.rs | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
> index f04b058b09b2d2397e26344d0e055b3aa5061432..1d6284316f2a4652ef3f76272670e5e29b0ff924 100644
> --- a/rust/kernel/firmware.rs
> +++ b/rust/kernel/firmware.rs
> @@ -5,14 +5,18 @@
>  //! C header: [`include/linux/firmware.h`](srctree/include/linux/firmware.h)
>  
>  use crate::{bindings, device::Device, error::Error, error::Result, str::CStr};
> -use core::ptr::NonNull;
> +use core::{ffi, ptr::NonNull};

The change itself seems to be fine anyways, but I think we should use crate::ffi
instead.