[PATCH v11 5/8] rust: time: Add wrapper for fsleep() function

[FUJITA Tomonori <fujita.tomonori@gmail.com>]

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.

sleep functions including fsleep() belongs to TIMERS, not
TIMEKEEPING. They are maintained separately. rust/kernel/time.rs is an
abstraction for TIMEKEEPING. To make Rust abstractions match the C
side, add rust/kernel/time/delay.rs for this wrapper.

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]
Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/helpers/helpers.c    |  1 +
 rust/helpers/time.c       |  8 +++++++
 rust/kernel/time.rs       |  2 ++
 rust/kernel/time/delay.rs | 49 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 60 insertions(+)
 create mode 100644 rust/helpers/time.c
 create mode 100644 rust/kernel/time/delay.rs

diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 0640b7e115be..9565485a1a54 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -31,6 +31,7 @@
 #include "slab.c"
 #include "spinlock.c"
 #include "task.c"
+#include "time.c"
 #include "uaccess.c"
 #include "vmalloc.c"
 #include "wait.c"
diff --git a/rust/helpers/time.c b/rust/helpers/time.c
new file mode 100644
index 000000000000..7ae64ad8141d
--- /dev/null
+++ b/rust/helpers/time.c
@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/delay.h>
+
+void rust_helper_fsleep(unsigned long usecs)
+{
+	fsleep(usecs);
+}
diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index d64a05a4f4d1..eeb0f6a7e5d4 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -24,6 +24,8 @@
 //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
 //! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
 
+pub mod delay;
+
 /// The number of nanoseconds per microsecond.
 pub const NSEC_PER_USEC: i64 = bindings::NSEC_PER_USEC as i64;
 
diff --git a/rust/kernel/time/delay.rs b/rust/kernel/time/delay.rs
new file mode 100644
index 000000000000..02b8731433c7
--- /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::ffi::c_ulong;
+
+/// 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.as_micros_ceil() as c_ulong)
+    }
+}
-- 
2.43.0