[PATCH v4 5/7] rust: hrtimer: Add HrTimer::raw_cb_time()

[Lyude Paul <lyude@redhat.com>]

This is a simple private unsafe wrapper for retrieving the current time
according to the hrtimer_clock_base struct for a given timer. This will be
used for implementing functions such as forward_now(), which rely on
retrieving the current time from the hrtimer's clock base.

Signed-off-by: Lyude Paul <lyude@redhat.com>

---
V2:
- Convert safety comment to invariant comment in from_raw()
- Add raw_clock_base() and implement clock_base() on HrTimer<T> as well

V4:
- Drop HrTimerClockBase entirely, reword commit as this is now about adding
  raw_cb_time()

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/time.rs         |  1 -
 rust/kernel/time/hrtimer.rs | 24 ++++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 6dcb65ed954db..ce6a991ce7583 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -89,7 +89,6 @@ 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 {
         // INVARIANT: Our safety contract ensures that `nanos` is in the range from 0 to
diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index b0304b2cf2da9..80270e14daafd 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -186,6 +186,30 @@ unsafe fn raw_forward(self_ptr: *mut Self, now: Instant, interval: Delta) -> u64
         }
     }
 
+    /// Retrieve the current time according to the `struct hrtimer_clock_base` for `self_ptr`.
+    ///
+    /// # Safety
+    ///
+    /// - `self_ptr` must point to a valid `Self`.
+    /// - The caller must ensure that the `hrtimer_clock_base` cannot possibly change in the context
+    ///   this function is being called in. This means either exclusive access to `self_ptr` is
+    ///   required, or we must be from within the timer callback context of `self_ptr`.
+    #[expect(unused)]
+    unsafe fn raw_cb_time(self_ptr: *const Self) -> Instant {
+        // SAFETY: We're guaranteed `self_ptr` points to a valid `Self` by our safety contract.
+        let clock_base = unsafe { (*Self::raw_get(self_ptr)).base };
+
+        // SAFETY: The C API guarantees that `get_time` is initialized to a valid function pointer
+        // for as long as we expose hrtimers to users.
+        let get_time_fn = unsafe { (*clock_base).get_time.unwrap_unchecked() };
+
+        // SAFETY:
+        // - get_time_fn() returns a ktime_t, so we're guaranteed its return value is between `0`
+        //   and `KTIME_MAX`.
+        // - get_time_fn() itself has no special requirements.
+        unsafe { Instant::from_nanos(get_time_fn()) }
+    }
+
     /// Conditionally forward the timer.
     ///
     /// If the timer expires after `now`, this function does nothing and returns 0. If the timer
-- 
2.48.1