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

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Message-ID: <20251215111430.756f8872.gary@garyguo.net>
Date: Mon, 15 Dec 2025 11:14:30 +0000
From: Gary Guo <gary@...yguo.net>
To: Viresh Kumar <viresh.kumar@...aro.org>
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, 15 Dec 2025 10:36:55 +0530
Viresh Kumar <viresh.kumar@...aro.org> 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.
> > 
> > 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 ?
> 

Thinking again about this I think `#[inline(always)]` is fine to keep as
it can also be used to indicate "this function shall never be codegenned".

However I do still think the comment is confusing per-se as there is no
"optimization" for this function at all.

RE: the patch I am fine either without this patch picked or having this
patch in and fix the comment later.

Best,
Gary