[PATCH 2/5] rust: time: Introduce Duration type

[Boqun Feng <boqun.feng@gmail.com>]

From: Alice Ryhl <aliceryhl@google.com>

Introduce a type representing time duration. Define our own type instead
of using `core::time::Duration` because in kernel C code, an i64
(ktime_t) is used for representing time durations, an i64 backed
duration type is more efficient when interacting with time APIs in C.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
[boqun: Rename `Ktime` to `Duration`, and make it a type of durations]
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/kernel/time.rs | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index bbb666e64dd7..b238b3a4e899 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -7,6 +7,9 @@
 //!
 //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
 
+/// The number of nanoseconds per millisecond.
+pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
+
 /// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
 pub type Jiffies = core::ffi::c_ulong;
 
@@ -20,3 +23,44 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
     // matter what the argument is.
     unsafe { bindings::__msecs_to_jiffies(msecs) }
 }
+
+/// A time duration.
+///
+/// # Examples
+///
+/// ```
+/// let one_second = kernel::time::Duration::new(1000_000_000);
+///
+/// // 1 second is 1000 milliseconds.
+/// assert_eq!(one_second.to_ms(), 1000);
+/// ```
+#[repr(transparent)]
+#[derive(Copy, Clone, Debug)]
+pub struct Duration {
+    inner: i64,
+}
+
+impl Duration {
+    /// Creates a new duration of `ns` nanoseconds.
+    pub const fn new(ns: i64) -> Self {
+        Self { inner: ns }
+    }
+
+    /// Divides the number of nanoseconds by a compile-time constant.
+    #[inline]
+    fn divns_constant<const DIV: i64>(self) -> i64 {
+        self.to_ns() / DIV
+    }
+
+    /// Returns the number of milliseconds.
+    #[inline]
+    pub fn to_ms(self) -> i64 {
+        self.divns_constant::<NSEC_PER_MSEC>()
+    }
+
+    /// Returns the number of nanoseconds.
+    #[inline]
+    pub fn to_ns(self) -> i64 {
+        self.inner
+    }
+}
-- 
2.44.0