Allow `Pin<Box<T>>` to be the target of a timer callback.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/hrtimer.rs      |  3 ++
 rust/kernel/hrtimer/tbox.rs | 95 +++++++++++++++++++++++++++++++++++++
 2 files changed, 98 insertions(+)
 create mode 100644 rust/kernel/hrtimer/tbox.rs

diff --git a/rust/kernel/hrtimer.rs b/rust/kernel/hrtimer.rs
index 38160221f93e..1750016b2b22 100644
--- a/rust/kernel/hrtimer.rs
+++ b/rust/kernel/hrtimer.rs
@@ -493,6 +493,9 @@ unsafe fn raw_get_timer(ptr: *const Self) ->
     }
 }
 
+// `box` is a reserved keyword, so prefix with `t` for timer
+mod tbox;
+
 mod arc;
 mod pin;
 mod pin_mut;
diff --git a/rust/kernel/hrtimer/tbox.rs b/rust/kernel/hrtimer/tbox.rs
new file mode 100644
index 000000000000..089e6ba97801
--- /dev/null
+++ b/rust/kernel/hrtimer/tbox.rs
@@ -0,0 +1,95 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use super::HasTimer;
+use super::RawTimerCallback;
+use super::Timer;
+use super::TimerCallback;
+use super::TimerHandle;
+use super::TimerPointer;
+use crate::prelude::*;
+use crate::time::Ktime;
+use core::mem::ManuallyDrop;
+
+/// A handle for a `Box<HasTimer<U>>` returned by a call to
+/// [`TimerPointer::schedule`].
+pub struct BoxTimerHandle<U>
+where
+    U: HasTimer<U>,
+{
+    pub(crate) inner: *mut U,
+}
+
+// SAFETY: We implement drop below, and we cancel the timer in the drop
+// implementation.
+unsafe impl<U> TimerHandle for BoxTimerHandle<U>
+where
+    U: HasTimer<U>,
+{
+    fn cancel(&mut self) -> bool {
+        // SAFETY: As we obtained `self.inner` from a valid reference when we
+        // created `self`, it must point to a valid `U`.
+        let timer_ptr = unsafe { <U as HasTimer<U>>::raw_get_timer(self.inner) };
+
+        // SAFETY: As `timer_ptr` points into `U` and `U` is valid, `timer_ptr`
+        // must point to a valid `Timer` instance.
+        unsafe { Timer::<U>::raw_cancel(timer_ptr) }
+    }
+}
+
+impl<U> Drop for BoxTimerHandle<U>
+where
+    U: HasTimer<U>,
+{
+    fn drop(&mut self) {
+        self.cancel();
+    }
+}
+
+impl<U> TimerPointer for Pin<Box<U>>
+where
+    U: Send + Sync,
+    U: HasTimer<U>,
+    U: for<'a> TimerCallback<CallbackTarget<'a> = Pin<Box<U>>>,
+    U: for<'a> TimerCallback<CallbackPointer<'a> = &'a U>,
+{
+    type TimerHandle = BoxTimerHandle<U>;
+
+    fn schedule(self, expires: Ktime) -> Self::TimerHandle {
+        use core::ops::Deref;
+        let self_ptr = self.deref() as *const U;
+
+        // SAFETY: Since we generate the pointer passed to `schedule` from a
+        // valid reference, it is a valid pointer.
+        unsafe { U::schedule(self_ptr, expires) };
+
+        // SAFETY: We will not move out of this box during timer callback (we
+        // pass an immutable reference to the callback).
+        let inner = unsafe { Pin::into_inner_unchecked(self) };
+
+        BoxTimerHandle {
+            inner: Box::into_raw(inner),
+        }
+    }
+}
+
+impl<U> RawTimerCallback for Pin<Box<U>>
+where
+    U: HasTimer<U>,
+    U: for<'a> TimerCallback<CallbackTarget<'a> = Pin<Box<U>>>,
+    U: for<'a> TimerCallback<CallbackPointer<'a> = &'a U>,
+{
+    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
+        // `Timer` is `repr(transparent)`
+        let timer_ptr = ptr.cast::<kernel::hrtimer::Timer<U>>();
+
+        // SAFETY: By C API contract `ptr` is the pointer we passed when
+        // queuing the timer, so it is a `Timer<T>` embedded in a `T`.
+        let data_ptr = unsafe { U::timer_container_of(timer_ptr) };
+
+        // SAFETY: We called `Box::into_raw` when we queued the timer.
+        let tbox = ManuallyDrop::new(unsafe { Box::from_raw(data_ptr) });
+
+        use core::ops::Deref;
+        U::run(tbox.deref()).into()
+    }
+}
-- 
2.46.0