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

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Message-ID: <20251209120206.1b95a7c0.gary@garyguo.net>
Date: Tue, 9 Dec 2025 12:02:06 +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 Tue, 09 Dec 2025 09:52:13 +0900
"Alexandre Courbot" <acourbot@...dia.com> wrote:

> 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?

In this particular case the `copy_name` shouldn't be used at all
outside const eval. It's specificially for building a table during
const eval. It's a bug if it's outside, hence I think
`#[inline(always)]` adds confusion to the reader of this code.

I get that you want to have a general rule of "if you're using
something with `build_assert!`, then use `#[inline(always)]`", but I
think applying that rule here is detrimental.

Hence I suggested adding a marker to indicate const-eval only function,
so we can either say const-eval-only functions are fine without inline
markers, or perhaps just use normal panicking-assertion inside these
functions (as `build_assert!` behave identical to just `assert!` in
const-eval).

Best,
Gary