[PATCH v6 17/18] rust: block: add remote completion to `Request`

[Andreas Hindborg <a.hindborg@kernel.org>]

Allow users of rust block device driver API to schedule completion of
requests via `blk_mq_complete_request_remote`.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 drivers/block/rnull/rnull.rs       |  9 +++++++++
 rust/kernel/block/mq.rs            |  6 ++++++
 rust/kernel/block/mq/operations.rs | 19 +++++++++++++++----
 rust/kernel/block/mq/request.rs    | 17 +++++++++++++++++
 4 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/drivers/block/rnull/rnull.rs b/drivers/block/rnull/rnull.rs
index 8255236bc550..a19c55717c4f 100644
--- a/drivers/block/rnull/rnull.rs
+++ b/drivers/block/rnull/rnull.rs
@@ -82,4 +82,13 @@ fn queue_rq(_queue_data: (), rq: ARef<mq::Request<Self>>, _is_last: bool) -> Res
     }
 
     fn commit_rqs(_queue_data: ()) {}
+
+    fn complete(rq: ARef<mq::Request<Self>>) {
+        mq::Request::end_ok(rq)
+            .map_err(|_e| kernel::error::code::EIO)
+            // We take no refcounts on the request, so we expect to be able to
+            // end the request. The request reference must be unique at this
+            // point, and so `end_ok` cannot fail.
+            .expect("Fatal error - expected to be able to end request");
+    }
 }
diff --git a/rust/kernel/block/mq.rs b/rust/kernel/block/mq.rs
index 6e546f4f3d1c..c0ec06b84355 100644
--- a/rust/kernel/block/mq.rs
+++ b/rust/kernel/block/mq.rs
@@ -77,6 +77,12 @@
 //!     }
 //!
 //!     fn commit_rqs(_queue_data: ()) {}
+//!
+//!     fn complete(rq: ARef<Request<Self>>) {
+//!         Request::end_ok(rq)
+//!             .map_err(|_e| kernel::error::code::EIO)
+//!             .expect("Fatal error - expected to be able to end request");
+//!     }
 //! }
 //!
 //! let tagset: Arc<TagSet<MyBlkDevice>> =
diff --git a/rust/kernel/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs
index 6fb256f55acc..0fece577de7c 100644
--- a/rust/kernel/block/mq/operations.rs
+++ b/rust/kernel/block/mq/operations.rs
@@ -42,6 +42,9 @@ fn queue_rq(
     /// Called by the kernel to indicate that queued requests should be submitted.
     fn commit_rqs(queue_data: ForeignBorrowed<'_, Self::QueueData>);
 
+    /// Called by the kernel when the request is completed.
+    fn complete(rq: ARef<Request<Self>>);
+
     /// Called by the kernel to poll the device for completed requests. Only
     /// used for poll queues.
     fn poll() -> bool {
@@ -143,13 +146,21 @@ impl<T: Operations> OperationsVTable<T> {
         T::commit_rqs(queue_data)
     }
 
-    /// This function is called by the C kernel. It is not currently
-    /// implemented, and there is no way to exercise this code path.
+    /// 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.
-    unsafe extern "C" fn complete_callback(_rq: *mut bindings::request) {}
+    /// This function may only be called by blk-mq C infrastructure. `rq` must
+    /// point to a valid request that has been marked as completed. The pointee
+    /// of `rq` must be valid for write for the duration of this function.
+    unsafe extern "C" fn complete_callback(rq: *mut bindings::request) {
+        // SAFETY: This function can only be dispatched through
+        // `Request::complete`. We leaked a refcount then which we pick back up
+        // now.
+        let aref = unsafe { Request::aref_from_raw(rq) };
+        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.
diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
index 3848cfe63f77..f7f757f7459f 100644
--- a/rust/kernel/block/mq/request.rs
+++ b/rust/kernel/block/mq/request.rs
@@ -135,6 +135,23 @@ pub fn end_ok(this: ARef<Self>) -> Result<(), ARef<Self>> {
         Ok(())
     }
 
+    /// Complete the request by scheduling `Operations::complete` for
+    /// execution.
+    ///
+    /// The function may be scheduled locally, via SoftIRQ or remotely via IPMI.
+    /// See `blk_mq_complete_request_remote` in [`blk-mq.c`] for details.
+    ///
+    /// [`blk-mq.c`]: srctree/block/blk-mq.c
+    pub fn complete(this: ARef<Self>) {
+        let ptr = ARef::into_raw(this).cast::<bindings::request>().as_ptr();
+        // SAFETY: By type invariant, `self.0` is a valid `struct request`
+        if !unsafe { bindings::blk_mq_complete_request_remote(ptr) } {
+            // SAFETY: We released a refcount above that we can reclaim here.
+            let this = unsafe { Request::aref_from_raw(ptr) };
+            T::complete(this);
+        }
+    }
+
     /// Return a pointer to the [`RequestDataWrapper`] stored in the private area
     /// of the request structure.
     ///

-- 
2.47.2