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

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Message-Id: <793E15AB-D69C-49F5-823E-A57F343BA3F4@collabora.com>
Date: Tue, 26 Aug 2025 09:44:27 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: a.hindborg@...nel.org,
 alex.gaynor@...il.com,
 ojeda@...nel.org,
 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,
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Subject: Re: [PATCH v1 1/2] rust: add udelay() function

Hi Fujita,

> On 21 Aug 2025, at 00:57, FUJITA Tomonori <fujita.tomonori@...il.com> wrote:
> 
> 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);

We should perhaps add a build_assert here to make sure this cast is always valid?

> +
> +    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.
> +    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)
> +    }
> +}
> -- 
> 2.43.0
> 
> 

With the change you suggested for the safety comment in udelay:

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>