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

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Message-ID: <aPjmKSrETqrchW_e@google.com>
Date: Wed, 22 Oct 2025 14:11:53 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: dakr@...nel.org, miguel.ojeda.sandonis@...il.com, 
	daniel.almeida@...labora.com, a.hindborg@...nel.org, alex.gaynor@...il.com, 
	ojeda@...nel.org, anna-maria@...utronix.de, bjorn3_gh@...tonmail.com, 
	boqun.feng@...il.com, frederic@...nel.org, gary@...yguo.net, 
	jstultz@...gle.com, linux-kernel@...r.kernel.org, lossin@...nel.org, 
	lyude@...hat.com, rust-for-linux@...r.kernel.org, sboyd@...nel.org, 
	tglx@...utronix.de, tmgross@...ch.edu, gregkh@...uxfoundation.org
Subject: Re: [PATCH v2 1/2] rust: add udelay() function

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@...nel.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