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

Gary Guo
2025-12-09T12:02:11Z

On Tue, 09 Dec 2025 09:52:13 +0900
"Alexandre Courbot" <acourbot@nvidia.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@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.
>
> ... 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