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

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Message-Id: <20251023.141909.400243634709519102.fujita.tomonori@gmail.com>
Date: Thu, 23 Oct 2025 14:19:09 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: aliceryhl@...gle.com, dakr@...nel.org, miguel.ojeda.sandonis@...il.com
Cc: fujita.tomonori@...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, 22 Oct 2025 14:11:53 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> 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.

Oh, I didn’t know about that. Do you have a pointer or some details I
could look at?


> 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.

You meant that introducing a new type like UdelayDelta, right?

read_poll_timeout() and read_poll_timeout_atomic() use different Delta
types... I'm not sure it's a good idea.

Danilo and Miguel, any ideas for other ways we could do the
compile-time check?