linux-kernel - Re: [PATCH] rust: io: always inline methods leading to build

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Message-Id: <A1A280D4-836E-4D75-863E-30B1C276C80C@collabora.com>
Date: Thu, 27 Nov 2025 11:53:38 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Danilo Krummrich <dakr@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: io: always inline methods leading to build_assert



> On 27 Nov 2025, at 10:30, Alexandre Courbot <acourbot@...dia.com> wrote:
> 
> `build_assert` relies on the compiler to optimize out its error path,
> lest build fails with the dreaded error:
> 
>    ERROR: modpost: "rust_build_error" [drivers/gpu/nova-core/nova_core.ko] undefined!
> 
> It has been observed that very trivial code performing I/O accesses
> (sometimes even using an immediate value) would seemingly randomly fail
> with this error whenever `CLIPPY=1` was set. Removing the CLIPPY option
> makes the error go away, but that's obviously not a great workaround.
> Clippy appears to influence the way the compiler optimizes things,
> making it on occasion generate a method where we would need it to inline
> in order to satisfy a `build_assert`.
> 
> Fix this by instructing the compiler to always inline the methods
> leading to `build_assert`. This stronger directive is effective even
> when `CLIPPY=1` is specified, which gets rid of this error.
> 
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
> This is the same fix as for another build error triggered by the use of
> `build_assert` [1], which signals that all callers of this macro should
> all be tagged with `#[inline(always)]`, as inlining is a requirement for
> `build_assert` to perform properly anyway.
> 
> [1] https://lore.kernel.org/all/DEEUYUOAEZU3.1J1HM2YQ10EX1@nvidia.com/
> ---
> rust/kernel/io.rs | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index 98e8b84e68d1..f161ec8056ce 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
> @@ -142,7 +142,8 @@ macro_rules! define_read {
>         /// Bound checks are performed on compile time, hence if the offset is not known at compile
>         /// time, the build will fail.
>         $(#[$attr])*
> -        #[inline]
> +        // Always inline so the error path of `io_addr_assert` is optimized out.
> +        #[inline(always)]
>         pub fn $name(&self, offset: usize) -> $type_name {
>             let addr = self.io_addr_assert::<$type_name>(offset);
> 
> @@ -171,7 +172,8 @@ macro_rules! define_write {
>         /// Bound checks are performed on compile time, hence if the offset is not known at compile
>         /// time, the build will fail.
>         $(#[$attr])*
> -        #[inline]
> +        // Always inline so the error path of `io_addr_assert` is optimized out.
> +        #[inline(always)]
>         pub fn $name(&self, value: $type_name, offset: usize) {
>             let addr = self.io_addr_assert::<$type_name>(offset);
> 
> @@ -239,7 +241,8 @@ fn io_addr<U>(&self, offset: usize) -> Result<usize> {
>         self.addr().checked_add(offset).ok_or(EINVAL)
>     }
> 
> -    #[inline]
> +    // Always inline so the error path of `build_assert!` is optimized out.
> +    #[inline(always)]
>     fn io_addr_assert<U>(&self, offset: usize) -> usize {
>         build_assert!(Self::offset_valid::<U>(offset, SIZE));
> 
> 
> ---
> base-commit: ea34511aaf755349999a1067b2984a541bee1492
> change-id: 20251127-io-build-assert-3579a5bfb81c
> 
> Best regards,
> -- 
> Alexandre Courbot <acourbot@...dia.com>
> 
> 

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>

I also faced this with genmask, by the way, i.e.: using genmask with an
in-bounds constant would trigger a build error. Very confusingly, this would be
randomly solved by moving the genmask invocation around in the code.

I wonder if the same fix is needed for it as well?

— Daniel