linux-kernel - [PATCH v8 7/7] rust: hrtimer: Add HrTimer::expires()

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Message-ID: <20250819201334.545001-8-lyude@redhat.com>
Date: Tue, 19 Aug 2025 16:05:58 -0400
From: Lyude Paul <lyude@...hat.com>
To: rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	John Stultz <jstultz@...gle.com>,
	Stephen Boyd <sboyd@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH v8 7/7] rust: hrtimer: Add HrTimer::expires()

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@...hat.com>

---
V8:
* Fix bogus safety comment I noticed after Fujita's comments. In expires()
  we're not guaranteed to get a non-negative ktime_t because of ktime_t
  itself, we're guaranteed to get one because a negative expiration time
  for a timer doesn't make sense.

 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 5c40ebe096357..30f0086c2408f 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_ktime(ktime: bindings::ktime_t) -> Self {
         debug_assert!(ktime >= 0);
diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index e0d78a8859903..856d2d929a008 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:
+        // - Timers cannot have negative ktime_t values as their expiration time.
+        // - There's no actual locking here, a racy read is fine and expected
+        unsafe {
+            Instant::from_ktime(
+                // 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