[PATCH 5/5] rust: time: Add Instant::elapsed() for monotonic clocks

[Boqun Feng]

This is a convenient way to do:

	t1 = Clock::now();
	...
	delta =  Clock::now() - t1;

Hence add it.

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 | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 5cd669cbea01..cd1e45169517 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -114,6 +114,31 @@ fn sub(self, other: Self) -> Self::Output {
     }
 }
 
+impl<T: Clock + Monotonic> Instant<T> {
+    /// Returns the time elapsed since this [`Instant`].
+    ///
+    /// This provides a convenient way to calculate time elapsed since a previous [`Clock::now`].
+    /// Note even though the function only exists for monotonic clocks, it could still return
+    /// negative [`Duration`] if the current time is earlier than the time of `&self`, and this
+    /// could happen if `&self` is a timestamp generated by a [`Instant`] + [`Duration`].
+    ///
+    /// But for typical usages, it should always return non-negative [`Duration`]:
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::time::{Clock, clock::KernelTime};
+    ///
+    /// let ts = KernelTime::now();
+    ///
+    /// // `KernelTime` is monotonic.
+    /// assert!(ts.elapsed().to_ns() >= 0);
+    /// ```
+    pub fn elapsed(&self) -> Duration {
+        T::now() - *self
+    }
+}
+
 /// Contains the various clock source types available to the kernel.
 pub mod clock {
     use super::*;
-- 
2.44.0