[PATCH 3/5] rust: time: Introduce clock reading framework

[Boqun Feng]

To make sure Rust code doesn't mix timestamps from different clocks, a
type safe clock reading framework is introduced. It includes:

* A `Clock` trait that represents different clocks, to read a particular
  clock, one needs implement the `Clock::now()` function.

* A `Instant<Clock>` type that represents timestamps of different
  clocks, whose implementation is just a `ktime_t`, so all the
  calculation on `ktime_t` should apply to it as well.

Co-developed-by: Heghedus Razvan <heghedus.razvan@xxxxxxxxxxxxxx>
Signed-off-by: Heghedus Razvan <heghedus.razvan@xxxxxxxxxxxxxx>
Co-developed-by: Asahi Lina <lina@xxxxxxxxxxxxx>
Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 rust/kernel/time.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index b238b3a4e899..0f9f5605ed48 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -6,6 +6,9 @@
 //! 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).
+
+use core::marker::PhantomData;
 
 /// The number of nanoseconds per millisecond.
 pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
@@ -64,3 +67,49 @@ pub fn to_ns(self) -> i64 {
         self.inner
     }
 }
+
+/// Represents a clock, that is, a unique time source and it can be queried for the current time.
+pub trait Clock: Sized {
+    /// Returns the current time for this clock.
+    fn now() -> Instant<Self>;
+}
+
+/// Marker trait for clock sources that are guaranteed to be monotonic.
+pub trait Monotonic {}
+
+/// A timestamp of a given [`Clock`]
+#[repr(transparent)]
+#[derive(Debug)]
+pub struct Instant<T: Clock> {
+    inner: bindings::ktime_t,
+    clock: PhantomData<T>,
+}
+
+impl<T: Clock> Clone for Instant<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<T: Clock> Copy for Instant<T> {}
+
+impl<T: Clock> Instant<T> {
+    fn new(ktime: bindings::ktime_t) -> Self {
+        Self {
+            inner: ktime,
+            clock: PhantomData,
+        }
+    }
+}
+
+impl<T: Clock> core::ops::Sub for Instant<T> {
+    type Output = Duration;
+
+    /// Returns the difference of two timestamps.
+    #[inline]
+    fn sub(self, other: Self) -> Self::Output {
+        // `ktime_t` is an `i64` value of nanoseconds, and kernel defines signed overflow to behave
+        // like 2s-complement, hence `wrapping_sub()` is used here to mirror `ktime_sub()`.
+        Duration::new(self.inner.wrapping_sub(other.inner))
+    }
+}
-- 
2.44.0