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

[Lyude Paul <lyude@redhat.com>]

This adds the ability to read the expiry time of the given timer.

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

---
V2:
* Convert from Ktime to Instant
* Use read_volatile instead of read and add a FIXME

V4:
* Correct read_volatile comment in HrTimer::expires()

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/time/hrtimer.rs | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index 0d93866d7b20a..5b508ff81aee2 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -73,7 +73,10 @@
     time::{Delta, Instant},
     types::Opaque,
 };
-use core::{marker::PhantomData, ptr::NonNull};
+use core::{
+    marker::PhantomData,
+    ptr::{addr_of, NonNull},
+};
 use pin_init::PinInit;
 
 /// A timer backed by a C `struct hrtimer`.
@@ -136,7 +139,7 @@ unsafe fn raw_get(this: *const Self) -> *mut bindings::hrtimer {
         // SAFETY: The field projection to `timer` does not go out of bounds,
         // because the caller of this function promises that `this` points to an
         // allocation of at least the size of `Self`.
-        unsafe { Opaque::raw_get(core::ptr::addr_of!((*this).timer)) }
+        unsafe { Opaque::raw_get(addr_of!((*this).timer)) }
     }
 
     /// Cancel an initialized and potentially running timer.
@@ -241,6 +244,26 @@ pub fn forward_now(self: Pin<&mut Self>, interval: Delta) -> u64 {
 
         self.forward(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) -> Instant {
+        // 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(addr_of!((*c_timer_ptr).node.expires)),
+            )
+        }
+    }
 }
 
 /// Implemented by pointer types that point to structs that contain a [`HrTimer`].
-- 
2.48.1