linux-kernel - Re: [PATCH v3 1/2] rust: add udelay() function

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Message-ID: <aQCTFwswY7dDmfbC@google.com>
Date: Tue, 28 Oct 2025 09:55:35 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: a.hindborg@...nel.org, alex.gaynor@...il.com, dakr@...nel.org, 
	daniel.almeida@...labora.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
Subject: Re: [PATCH v3 1/2] rust: add udelay() function

On Sun, Oct 26, 2025 at 09:54:57PM +0900, FUJITA Tomonori 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>

With below nits addressed:
Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>

> ---
>  rust/helpers/time.c       |  5 +++++
>  rust/kernel/time/delay.rs | 39 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 44 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..0739b75fb9c3 100644
> --- a/rust/kernel/time/delay.rs
> +++ b/rust/kernel/time/delay.rs
> @@ -47,3 +47,42 @@ 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;

We can nest backticks. This should be:

	`[0, MAX_UDELAY_MS]`

> +/// 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.

There's a debug assertion now so I would remove the "maxmimu value"
part.

> +/// 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);
> +
> +    debug_assert!(delta.as_nanos() >= 0);
> +    debug_assert!(delta <= MAX_UDELAY_DELTA);
> +
> +    let delta = if (Delta::ZERO..=MAX_UDELAY_DELTA).contains(&delta) {
> +        delta
> +    } else {
> +        MAX_UDELAY_DELTA
> +    };
> +
> +    // 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)
> +    }
> +}
> -- 
> 2.43.0
>