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

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Message-ID: <fmdoyqoyksspygcjg3wbqxtqqntunk2wfny6vvt3iq6wddwuzr@a4kfi2hcc5x2>
Date: Mon, 15 Dec 2025 10:36:55 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Gary Guo <gary@...yguo.net>
Cc: 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>, 
	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>, 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.

I already applied this from V2, should I drop this change ?

-- 
viresh