linux-kernel - Re: [PATCH v3 3/7] rust: cpufreq: always inline functions using build

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Message-ID: <20251208135521.5d1dd7f6.gary@garyguo.net>
Date: Mon, 8 Dec 2025 13:55:21 +0000
From: Gary Guo <gary@...yguo.net>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: 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>, 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>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <linux-pm@...r.kernel.org>,
 <stable@...r.kernel.org>
Subject: Re: [PATCH v3 3/7] rust: cpufreq: always inline functions using
 build_assert with arguments

On Mon, 08 Dec 2025 11:47:01 +0900
Alexandre Courbot <acourbot@...dia.com> 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.
> 
> Cc: stable@...r.kernel.org
> Fixes: c6af9a1191d0 ("rust: cpufreq: Extend abstractions for driver registration")
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
>  rust/kernel/cpufreq.rs | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
> index f968fbd22890..0879a79485f8 100644
> --- a/rust/kernel/cpufreq.rs
> +++ b/rust/kernel/cpufreq.rs
> @@ -1015,6 +1015,8 @@ impl<T: Driver> Registration<T> {
>          ..pin_init::zeroed()
>      };
>  
> +    // Always inline to optimize out error path of `build_assert`.
> +    #[inline(always)]
>      const fn copy_name(name: &'static CStr) -> [c_char; CPUFREQ_NAME_LEN] {
>          let src = name.to_bytes_with_nul();
>          let mut dst = [0; CPUFREQ_NAME_LEN];
> 

This change is not needed as this is a private function only used in
const-eval only.

I wonder if I should add another macro to assert that the function is
only used in const eval instead? Do you think it might be useful to have
something like:

	#[const_only]
	const fn foo() {}

or

	const fn foo() {
	    const_only!();
	}

? If so, I can send a patch that adds this feature. 

Implementation-wise, this will behave similar to build_error, where a
function is going to be added that is never-linked but has a body for
const eval.

Best,
Gary