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

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Message-ID: <20250429214752.1637859-8-lyude@redhat.com>
Date: Tue, 29 Apr 2025 17:44:43 -0400
From: Lyude Paul <lyude@...hat.com>
To: rust-for-linux@...r.kernel.org,
	Andreas Hindborg <a.hindborg@...nel.org>,
	linux-kernel@...r.kernel.org
Cc: Boqun Feng <boqun.feng@...il.com>,
	FUJITA Tomonori <fujita.tomonori@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 <benno.lossin@...ton.me>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH v4 7/7] rust: hrtimer: Add HrTimer::expires()

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

Signed-off-by: Lyude Paul <lyude@...hat.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@...hat.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