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

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Message-Id: <DET9WD6T0KR4.1MILPHIC2LIWB@nvidia.com>
Date: Tue, 09 Dec 2025 09:52:13 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Gary Guo" <gary@...yguo.net>, "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 Dec 8, 2025 at 10:55 PM JST, Gary Guo wrote:
> 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.

... for now. :)

>
> 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.

It could be useful in the general sense, but for this particular case
the rule "if you do build_assert on a function argument, then always
inline it" also covers us in case `copy_name` gets used outside of const
context, so isn't it the preferable workaround?