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

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Message-ID: <20250821035710.3692455-2-fujita.tomonori@gmail.com>
Date: Thu, 21 Aug 2025 12:57:09 +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,
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	jstultz@...gle.com,
	linux-kernel@...r.kernel.org,
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	rust-for-linux@...r.kernel.org,
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	daniel.almeida@...labora.com
Subject: [PATCH v1 1/2] rust: add udelay() function

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.
+    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