Re: [PATCH v3 3/7] rust: cpufreq: always inline functions using build_assert with arguments

Gary Guo
2025-12-15T11:14:41Z

On Mon, 15 Dec 2025 10:36:55 +0530
Viresh Kumar <viresh.kumar@linaro.org> wrote:

> On Mon, 08 Dec 2025 11:47:01 +0900
> Alexandre Courbot <acourbot@nvidia.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@vger.kernel.org
> > Fixes: c6af9a1191d0 ("rust: cpufreq: Extend abstractions for driver registration")
> > Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> > Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
> > Signed-off-by: Alexandre Courbot <acourbot@nvidia.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