Within the hrtimer API there are quite a number of functions that can only
be safely called from one of two contexts:

* When we have exclusive access to the hrtimer and the timer is not active.
* When we're within the hrtimer's callback context as it is being executed.

This commit adds bindings for hrtimer_forward() for the first such context,
along with HrTimer::raw_forward() for later use in implementing the
hrtimer_forward() in the latter context.

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

---
V4:
* Fix the safety contract for raw_forward()
* Require Pin<&mut Self>, not &mut self
* Drop incorrect UniquePin example
* Rewrite documentation a bit (re: Andreas)

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/time/hrtimer.rs | 44 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index ef96c7e2c21ff..76caf9fae8170 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -68,7 +68,11 @@
 //! `start` operation.
 
 use super::ClockId;
-use crate::{prelude::*, time::Instant, types::Opaque};
+use crate::{
+    prelude::*,
+    time::{Delta, Instant},
+    types::Opaque,
+};
 use core::marker::PhantomData;
 use pin_init::PinInit;
 
@@ -164,6 +168,44 @@ pub(crate) unsafe fn raw_cancel(this: *const Self) -> bool {
         // handled on the C side.
         unsafe { bindings::hrtimer_cancel(c_timer_ptr) != 0 }
     }
+
+    /// Forward the timer expiry for a given timer pointer.
+    ///
+    /// # Safety
+    ///
+    /// - `self_ptr` must point to a valid `Self`.
+    /// - The caller must either have exclusive access to the data pointed at by `self_ptr`, or be
+    ///   within the context of the timer callback.
+    #[inline]
+    unsafe fn raw_forward(self_ptr: *mut Self, now: Instant, interval: Delta) -> u64 {
+        // SAFETY:
+        // * The C API requirements for this function are fulfilled by our safety contract.
+        // * `self_ptr` is guaranteed to point to a valid `Self` via our safety contract
+        unsafe {
+            bindings::hrtimer_forward(Self::raw_get(self_ptr), now.as_nanos(), interval.as_nanos())
+        }
+    }
+
+    /// Conditionally forward the timer.
+    ///
+    /// If the timer expires after `now`, this function does nothing and returns 0. If the timer
+    /// expired at or before `now`, this function forwards the timer by `interval` until the timer
+    /// expires after `now` and then returns the number of times the timer was forwarded by
+    /// `interval`.
+    ///
+    /// This function is mainly useful for timer types which can provide exclusive access to the
+    /// timer when the timer is not running. For forwarding the timer from within the timer callback
+    /// context, see [`HrTimerCallbackContext::forward()`].
+    ///
+    /// Returns the number of overruns that occurred as a result of the timer expiry change.
+    pub fn forward(self: Pin<&mut Self>, now: Instant, interval: Delta) -> u64 {
+        // SAFETY:
+        // - `raw_forward` does not move `self`.
+        // - Self is a mutable reference and thus always points to a valid `HrTimer`
+        // - The only way that we could hold a mutable reference to `HrTimer<T>` is if we have
+        //   exclusive access to it - fulfilling the requirements of the C API.
+        unsafe { Self::raw_forward(self.get_unchecked_mut(), now, interval) }
+    }
 }
 
 /// Implemented by pointer types that point to structs that contain a [`HrTimer`].
-- 
2.48.1