Re: [PATCH v2 1/2] rust: add udelay() function

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Oct 22, 2025 at 07:32:30PM +0900, FUJITA Tomonori wrote:
> On Tue, 21 Oct 2025 17:20:41 +0200
> "Danilo Krummrich" <dakr@kernel.org> wrote:
> 
> > On Tue Oct 21, 2025 at 5:13 PM CEST, Miguel Ojeda wrote:
> >> i.e. if they aren't sure what the value is, then I would prefer they
> >> clamp it explicitly on the callee side (or we provide an explicitly
> >> clamped version if it is a common case, but it seems to me runtime
> >> values are already the minority).
> > 
> > Absolutely! Especially given the context udelay() is introduced
> > (read_poll_timeout_atomic()), the compile time checked version is what we really
> > want.
> > 
> > Maybe we should even defer a runtime checked / clamped version until it is
> > actually needed.
> 
> Then perhaps something like this?
> 
> #[inline(always)]
> pub fn udelay(delta: Delta) {
>     build_assert!(
>         delta.as_nanos() >= 0 && delta.as_nanos() <= i64::from(bindings::MAX_UDELAY_MS) * 1_000_000
>     );

This is a bad idea. Using build_assert! assert for range checks works
poorly, as we found for register index bounds checks.

If you really want to check it at compile-time, you'll need a wrapper
type around Delta that can only be constructed with delays in the right
range.

Alice