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

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Message-Id: <20251022.193230.585171330619599845.fujita.tomonori@gmail.com>
Date: Wed, 22 Oct 2025 19:32:30 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: dakr@...nel.org
Cc: miguel.ojeda.sandonis@...il.com, fujita.tomonori@...il.com,
 aliceryhl@...gle.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 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
    );

    // SAFETY: It is always safe to call `udelay()` with any duration.
    // Note that the kernel is compiled with `-fno-strict-overflow`
    // so any out-of-range value could lead to unexpected behavior
    // but won't lead to undefined behavior.
    unsafe {
        // Convert the duration to microseconds and round up to preserve
        // the guarantee; `udelay()` inserts a delay for at least
        // the provided duration, but that it may delay for longer
        // under some circumstances.
        bindings::udelay(delta.as_micros_ceil() as c_ulong)
    }
}