linux-kernel - Re: [PATCH v2 2/7] rust: io: always inline functions using build

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Message-ID: <af76b544-a7ee-43da-878f-cadc1599d7f0@nvidia.com>
Date: Mon, 1 Dec 2025 12:06:55 -0800
From: Edwin Peer <epeer@...dia.com>
To: Alexandre Courbot <acourbot@...dia.com>,
 Danilo Krummrich <dakr@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Daniel Almeida <daniel.almeida@...labora.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>, "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>, Will Deacon <will@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Mark Rutland <mark.rutland@....com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 2/7] rust: io: always inline functions using
 build_assert with arguments


On 11/27/25 18:11, Alexandre Courbot wrote:
> `build_assert` relies on the compiler to optimize out its error path.
> Functions using it with its arguments must thus always be inlined,
> otherwise the error path of `build_assert` might not be optimized out,
> triggering a build error.
>
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
>  rust/kernel/io.rs          | 9 ++++++---
>  rust/kernel/io/resource.rs | 2 ++
>  2 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index ee182b0b5452..ccdd394099cb 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
> @@ -120,7 +120,8 @@ 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 to optimize out error path of `io_addr_assert`.
> +        #[inline(always)]
>          pub fn $name(&self, offset: usize) -> $type_name {
>              let addr = self.io_addr_assert::<$type_name>(offset);
>  
> @@ -149,7 +150,8 @@ 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 to optimize out error path of `io_addr_assert`.
> +        #[inline(always)]
>          pub fn $name(&self, value: $type_name, offset: usize) {
>              let addr = self.io_addr_assert::<$type_name>(offset);
>  
> @@ -217,7 +219,8 @@ fn io_addr<U>(&self, offset: usize) -> Result<usize> {
>          self.addr().checked_add(offset).ok_or(EINVAL)
>      }
>  
> -    #[inline]
> +    // Always inline to optimize out error path of `build_assert`.
> +    #[inline(always)]
>      fn io_addr_assert<U>(&self, offset: usize) -> usize {
>          build_assert!(Self::offset_valid::<U>(offset, SIZE));
>  
> diff --git a/rust/kernel/io/resource.rs b/rust/kernel/io/resource.rs
> index bea3ee0ed87b..d9851923562c 100644
> --- a/rust/kernel/io/resource.rs
> +++ b/rust/kernel/io/resource.rs
> @@ -223,6 +223,8 @@ impl Flags {
>      /// Resource represents a memory region that must be ioremaped using `ioremap_np`.
>      pub const IORESOURCE_MEM_NONPOSTED: Flags = Flags::new(bindings::IORESOURCE_MEM_NONPOSTED);
>  
> +    // Always inline to optimize out error path of `build_assert`.
> +    #[inline(always)]
>      const fn new(value: u32) -> Self {

Does the build_assert problem actually manifest for const functions?

Regards,
Edwin Peer


>          crate::build_assert!(value as u64 <= c_ulong::MAX as u64);
>          Flags(value as c_ulong)
>