[RFC PATCH 1/2] rust: block: mq: safely abstract the timeout callback

[Wenzhao Liao]

Add a typed TimeoutReturn enum for blk-mq timeout handlers and
extend the Operations trait with an optional timeout callback.

The new callback borrows Request instead of taking an ARef because
timeout is a synchronous notification from blk-mq and does not
transfer request ownership to the driver.

Wire the callback into OperationsVTable and keep drivers that do not
implement timeout on the existing timeout: None path.

Signed-off-by: Wenzhao Liao <wenzhaoliao@xxxxxxxxxx>
---
 rust/kernel/block/mq.rs            |  2 +-
 rust/kernel/block/mq/operations.rs | 40 +++++++++++++++++++++++++++++-
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/block/mq.rs b/rust/kernel/block/mq.rs
index 1fd0d54dd549..cfcfcd99addc 100644
--- a/rust/kernel/block/mq.rs
+++ b/rust/kernel/block/mq.rs
@@ -98,6 +98,6 @@
 mod request;
 mod tag_set;
 
-pub use operations::Operations;
+pub use operations::{Operations, TimeoutReturn};
 pub use request::Request;
 pub use tag_set::TagSet;
diff --git a/rust/kernel/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs
index 8ad46129a52c..a8ff5eb8dd31 100644
--- a/rust/kernel/block/mq/operations.rs
+++ b/rust/kernel/block/mq/operations.rs
@@ -16,6 +16,16 @@
 
 type ForeignBorrowed<'a, T> = <T as ForeignOwnable>::Borrowed<'a>;
 
+/// Return value for blk-mq timeout handlers.
+#[repr(u32)]
+pub enum TimeoutReturn {
+    /// The driver completed the request or will complete it later.
+    Done = bindings::blk_eh_timer_return_BLK_EH_DONE,
+
+    /// Reset the request timer and keep waiting for completion.
+    ResetTimer = bindings::blk_eh_timer_return_BLK_EH_RESET_TIMER,
+}
+
 /// Implement this trait to interface blk-mq as block devices.
 ///
 /// To implement a block device driver, implement this trait as described in the
@@ -46,6 +56,11 @@ fn queue_rq(
     /// Called by the kernel when the request is completed.
     fn complete(rq: ARef<Request<Self>>);
 
+    /// Called by the kernel when a request times out.
+    fn timeout(_rq: &Request<Self>) -> TimeoutReturn {
+        build_error!(crate::error::VTABLE_DEFAULT_ERROR)
+    }
+
     /// Called by the kernel to poll the device for completed requests. Only
     /// used for poll queues.
     fn poll() -> bool {
@@ -163,6 +178,25 @@ impl<T: Operations> OperationsVTable<T> {
         T::complete(aref);
     }
 
+    /// This function is called by the C kernel. A pointer to this function is
+    /// installed in the `blk_mq_ops` vtable for the driver.
+    ///
+    /// # Safety
+    ///
+    /// This function may only be called by blk-mq C infrastructure. `rq` must
+    /// point to a valid request that is still live for the duration of this
+    /// callback.
+    unsafe extern "C" fn timeout_callback(
+        rq: *mut bindings::request,
+    ) -> bindings::blk_eh_timer_return {
+        // SAFETY: `rq` is valid as required by the safety requirements for
+        // this function, and the private data is initialized while the request
+        // is live.
+        let rq = unsafe { &*rq.cast::<Request<T>>() };
+
+        T::timeout(rq) as bindings::blk_eh_timer_return
+    }
+
     /// This function is called by the C kernel. A pointer to this function is
     /// installed in the `blk_mq_ops` vtable for the driver.
     ///
@@ -262,7 +296,11 @@ impl<T: Operations> OperationsVTable<T> {
         put_budget: None,
         set_rq_budget_token: None,
         get_rq_budget_token: None,
-        timeout: None,
+        timeout: if T::HAS_TIMEOUT {
+            Some(Self::timeout_callback)
+        } else {
+            None
+        },
         poll: if T::HAS_POLL {
             Some(Self::poll_callback)
         } else {
-- 
2.34.1