linux-kernel - Re: [PATCH v1 1/2] rust: add udelay() function

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Message-ID: <87bjo2witj.fsf@t14s.mail-host-address-is-not-set>
Date: Tue, 26 Aug 2025 11:09:12 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: FUJITA Tomonori <fujita.tomonori@...il.com>, alex.gaynor@...il.com,
 ojeda@...nel.org
Cc: aliceryhl@...gle.com, anna-maria@...utronix.de,
 bjorn3_gh@...tonmail.com, boqun.feng@...il.com, dakr@...nel.org,
 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, acourbot@...dia.com, daniel.almeida@...labora.com
Subject: Re: [PATCH v1 1/2] rust: add udelay() function

"FUJITA Tomonori" <fujita.tomonori@...il.com> writes:

> Add udelay() function, inserts a delay based on microseconds with busy
> waiting, in preparation for supporting read_poll_timeout_atomic().
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
> ---
>  rust/helpers/time.c       |  5 +++++
>  rust/kernel/time/delay.rs | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 39 insertions(+)
>
> diff --git a/rust/helpers/time.c b/rust/helpers/time.c
> index a318e9fa4408..67a36ccc3ec4 100644
> --- a/rust/helpers/time.c
> +++ b/rust/helpers/time.c
> @@ -33,3 +33,8 @@ s64 rust_helper_ktime_to_ms(const ktime_t kt)
>  {
>  	return ktime_to_ms(kt);
>  }
> +
> +void rust_helper_udelay(unsigned long usec)
> +{
> +	udelay(usec);
> +}
> diff --git a/rust/kernel/time/delay.rs b/rust/kernel/time/delay.rs
> index eb8838da62bc..baae3238d419 100644
> --- a/rust/kernel/time/delay.rs
> +++ b/rust/kernel/time/delay.rs
> @@ -47,3 +47,37 @@ pub fn fsleep(delta: Delta) {
>          bindings::fsleep(delta.as_micros_ceil() as c_ulong)
>      }
>  }
> +
> +/// Inserts a delay based on microseconds with busy waiting.
> +///
> +/// Equivalent to the C side [`udelay()`], which delays in microseconds.
> +///
> +/// `delta` must be within `[0, `MAX_UDELAY_MS`]` in milliseconds;
> +/// otherwise, it is erroneous behavior. That is, it is considered a bug to
> +/// call this function with an out-of-range value, in which case the function
> +/// will insert a delay for at least the maximum value in the range and
> +/// may warn in the future.
> +///
> +/// The behavior above differs from the C side [`udelay()`] for which out-of-range
> +/// values could lead to an overflow and unexpected behavior.
> +///
> +/// [`udelay()`]: https://docs.kernel.org/timers/delay_sleep_functions.html#c.udelay
> +pub fn udelay(delta: Delta) {
> +    const MAX_UDELAY_DELTA: Delta = Delta::from_millis(bindings::MAX_UDELAY_MS as i64);
> +
> +    let delta = if (Delta::ZERO..=MAX_UDELAY_DELTA).contains(&delta) {
> +        delta
> +    } else {
> +        // TODO: Add WARN_ONCE() when it's supported.
> +        MAX_UDELAY_DELTA
> +    };
> +
> +    // SAFETY: It is always safe to call `udelay()` with any duration.

Function documentation says it is overflow and unexpected behavior to
call `udelay` with out of range value, but above comment says any
duration is safe. Which is it?


Best regards,
Andreas Hindborg