[PATCH v6 7/7] rust: hrtimer: Add HrTimer::expires()

[Lyude Paul <lyude@redhat.com>]

Add a simple callback for retrieving the current expiry time for an
HrTimer. In rvkms, we use the HrTimer expiry value in order to calculate
the approximate vblank timestamp during each emulated vblank interrupt.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/time.rs         |  1 -
 rust/kernel/time/hrtimer.rs | 23 +++++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 75088d080b834..6d6e71710ebdc 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -211,7 +211,6 @@ pub(crate) fn as_nanos(&self) -> i64 {
     /// # Safety
     ///
     /// The caller promises that `nanos` is in the range from 0 to `KTIME_MAX`.
-    #[expect(unused)]
     #[inline]
     pub(crate) unsafe fn from_nanos(nanos: i64) -> Self {
         debug_assert!(nanos >= 0);
diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index fa5a3ae81723b..2d52a1b3b39a0 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -223,6 +223,29 @@ pub fn forward_now(self: Pin<&mut Self>, interval: Delta) -> u64
     {
         self.forward(HrTimerInstant::<T>::now(), interval)
     }
+
+    /// Return the time expiry for this [`HrTimer`].
+    ///
+    /// This value should only be used as a snapshot, as the actual expiry time could change after
+    /// this function is called.
+    pub fn expires(&self) -> HrTimerInstant<T>
+    where
+        T: HasHrTimer<T>,
+    {
+        // SAFETY: `self` is an immutable reference and thus always points to a valid `HrTimer`.
+        let c_timer_ptr = unsafe { HrTimer::raw_get(self) };
+
+        // SAFETY:
+        // - `node.expires` is a ktime_t, so it must be within the range of `0` to `KTIME_MAX`.
+        // - There's no actual locking here, a racy read is fine and expected
+        unsafe {
+            Instant::from_nanos(
+                // This `read_volatile` is intended to correspond to a READ_ONCE call.
+                // FIXME(read_once): Replace with `read_once` when available on the Rust side.
+                core::ptr::read_volatile(&raw const ((*c_timer_ptr).node.expires)),
+            )
+        }
+    }
 }
 
 /// Implemented by pointer types that point to structs that contain a [`HrTimer`].
-- 
2.50.0