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]
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/helpers/helpers.c    |  1 +
 rust/helpers/time.c       |  8 ++++++++
 rust/kernel/time.rs       |  4 +++-
 rust/kernel/time/delay.rs | 43 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 55 insertions(+), 1 deletion(-)
 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 30f40149f3a9..c274546bcf78 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -21,6 +21,7 @@
 #include "slab.c"
 #include "spinlock.c"
 #include "task.c"
+#include "time.c"
 #include "uaccess.c"
 #include "wait.c"
 #include "workqueue.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 ae0d3e3ff475..cfaac41fa6e2 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -2,12 +2,14 @@
 
 //! Time related primitives.
 //!
-//! This module contains the kernel APIs related to time and timers that
+//! This module contains the kernel APIs related to time that
 //! have been ported or wrapped for usage by Rust code in the kernel.
 //!
 //! 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..e384a7f7eec4
--- /dev/null
+++ b/rust/kernel/time/delay.rs
@@ -0,0 +1,43 @@
+// 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 core::ffi::c_ulong;
+
+/// Sleeps for a given duration at least.
+///
+/// Equivalent to the kernel's [`fsleep`], flexible sleep function,
+/// which automatically chooses the best sleep method based on a duration.
+///
+/// `delta` must be 0 or greater and no more than `u32::MAX / 2` microseconds.
+/// If a value outside the range is given, the function will sleep
+/// for `u32::MAX / 2` microseconds (= ~2147 seconds or ~36 minutes) at least.
+///
+/// This function can only be used in a nonatomic context.
+pub fn fsleep(delta: Delta) {
+    // The argument of fsleep is an unsigned long, 32-bit on 32-bit architectures.
+    // Considering that fsleep rounds up the duration to the nearest millisecond,
+    // set the maximum value to u32::MAX / 2 microseconds.
+    const MAX_DURATION: Delta = Delta::from_micros(u32::MAX as i64 >> 1);
+
+    let duration = if delta > MAX_DURATION || delta.is_negative() {
+        // TODO: add WARN_ONCE() when it's supported.
+        MAX_DURATION
+    } else {
+        delta
+    };
+
+    // SAFETY: FFI call.
+    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(duration.as_micros_ceil() as c_ulong)
+    }
+}
-- 
2.43.0