[PATCH 1/2] rust: time: Implement Add<Delta>/Sub<Delta> for Instant

[Lyude Paul <lyude@redhat.com>]

In order to maintain the invariants of Instant, we use saturating
addition/subtraction that is clamped to the valid value range for a
non-negative Ktime.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/time.rs | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 64c8dcf548d63..ac5cab62070c6 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -25,6 +25,7 @@
 //! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
 
 use core::marker::PhantomData;
+use core::ops;
 
 pub mod delay;
 pub mod hrtimer;
@@ -202,7 +203,7 @@ pub(crate) fn as_nanos(&self) -> i64 {
     }
 }
 
-impl<C: ClockSource> core::ops::Sub for Instant<C> {
+impl<C: ClockSource> ops::Sub for Instant<C> {
     type Output = Delta;
 
     // By the type invariant, it never overflows.
@@ -214,6 +215,32 @@ fn sub(self, other: Instant<C>) -> Delta {
     }
 }
 
+impl<T: ClockSource> ops::Add<Delta> for Instant<T> {
+    type Output = Self;
+
+    #[inline]
+    fn add(self, rhs: Delta) -> Self::Output {
+        // INVARIANT: We clamp the resulting value to be between `0` and `KTIME_MAX`.
+        Self {
+            inner: self.inner.saturating_add(rhs.nanos).clamp(0, i64::MAX),
+            _c: PhantomData,
+        }
+    }
+}
+
+impl<T: ClockSource> ops::Sub<Delta> for Instant<T> {
+    type Output = Self;
+
+    #[inline]
+    fn sub(self, rhs: Delta) -> Self::Output {
+        // INVARIANT: We clamp the resulting value to be between `0` and `KTIME_MAX`.
+        Self {
+            inner: self.inner.saturating_sub(rhs.nanos).clamp(0, i64::MAX),
+            _c: PhantomData,
+        }
+    }
+}
+
 /// A span of time.
 ///
 /// This struct represents a span of time, with its value stored as nanoseconds.
-- 
2.50.0