linux-kernel - [PATCH v1 2/2] rust: time: Add wrapper for fsleep() function

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Message-ID: <20250617144155.3903431-3-fujita.tomonori@gmail.com>
Date: Tue, 17 Jun 2025 23:41:55 +0900
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: a.hindborg@...nel.org,
	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,
	Fiona Behrens <me@...enk.dev>,
	Daniel Almeida <daniel.almeida@...labora.com>
Subject: [PATCH v1 2/2] rust: time: Add wrapper for fsleep() function

Add a wrapper for fsleep(), flexible sleep functions in
include/linux/delay.h which typically deals with hardware delays.

The kernel supports several sleep functions to handle various lengths
of delay. This adds fsleep(), automatically chooses the best sleep
method based on a duration.

fsleep() can only be used in a nonatomic context. This requirement is
not checked by these abstractions, but it is intended that klint [1]
or a similar tool will be used to check it in the future.

Link: https://rust-for-linux.com/klint [1]
Reviewed-by: Gary Guo <gary@...yguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
Reviewed-by: Fiona Behrens <me@...enk.dev>
Tested-by: Daniel Almeida <daniel.almeida@...labora.com>
Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
---
 rust/helpers/time.c       |  6 +++++
 rust/kernel/time.rs       |  1 +
 rust/kernel/time/delay.rs | 49 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 rust/kernel/time/delay.rs

diff --git a/rust/helpers/time.c b/rust/helpers/time.c
index 08755db43fc2..a318e9fa4408 100644
--- a/rust/helpers/time.c
+++ b/rust/helpers/time.c
@@ -1,8 +1,14 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#include <linux/delay.h>
 #include <linux/ktime.h>
 #include <linux/timekeeping.h>
 
+void rust_helper_fsleep(unsigned long usecs)
+{
+	fsleep(usecs);
+}
+
 ktime_t rust_helper_ktime_get_real(void)
 {
 	return ktime_get_real();
diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 445877039395..cbe8949ce074 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -26,6 +26,7 @@
 
 use core::marker::PhantomData;
 
+pub mod delay;
 pub mod hrtimer;
 
 /// The number of nanoseconds per microsecond.
diff --git a/rust/kernel/time/delay.rs b/rust/kernel/time/delay.rs
new file mode 100644
index 000000000000..a2fcc15ebfd4
--- /dev/null
+++ b/rust/kernel/time/delay.rs
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Delay and sleep primitives.
+//!
+//! This module contains the kernel APIs related to delay and sleep that
+//! have been ported or wrapped for usage by Rust code in the kernel.
+//!
+//! C header: [`include/linux/delay.h`](srctree/include/linux/delay.h).
+
+use super::Delta;
+use crate::prelude::*;
+
+/// Sleeps for a given duration at least.
+///
+/// Equivalent to the C side [`fsleep()`], flexible sleep function,
+/// which automatically chooses the best sleep method based on a duration.
+///
+/// `delta` must be within `[0, i32::MAX]` microseconds;
+/// 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 sleep for at least the maximum value in the range and may warn
+/// in the future.
+///
+/// The behavior above differs from the C side [`fsleep()`] for which out-of-range
+/// values mean "infinite timeout" instead.
+///
+/// This function can only be used in a nonatomic context.
+///
+/// [`fsleep()`]: https://docs.kernel.org/timers/delay_sleep_functions.html#c.fsleep
+pub fn fsleep(delta: Delta) {
+    // The maximum value is set to `i32::MAX` microseconds to prevent integer
+    // overflow inside fsleep, which could lead to unintentional infinite sleep.
+    const MAX_DELTA: Delta = Delta::from_micros(i32::MAX as i64);
+
+    let delta = if (Delta::ZERO..=MAX_DELTA).contains(&delta) {
+        delta
+    } else {
+        // TODO: Add WARN_ONCE() when it's supported.
+        MAX_DELTA
+    };
+
+    // SAFETY: It is always safe to call `fsleep()` with any duration.
+    unsafe {
+        // Convert the duration to microseconds and round up to preserve
+        // the guarantee; `fsleep()` sleeps for at least the provided duration,
+        // but that it may sleep for longer under some circumstances.
+        bindings::fsleep(delta.into_micros_ceil() as c_ulong)
+    }
+}
-- 
2.43.0