Add the `RequestList` type as a safe wrapper around the C `struct
rq_list`. This type provides methods to iterate over and manipulate
lists of block requests, which is needed for implementing the
`queue_rqs` callback.

The abstraction includes methods for popping requests from the list,
checking if the list is empty, and peeking at the head request.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/helpers/blk.c                   |  26 ++++++++
 rust/kernel/block/mq.rs              |   2 +
 rust/kernel/block/mq/request_list.rs | 113 +++++++++++++++++++++++++++++++++++
 3 files changed, 141 insertions(+)

diff --git a/rust/helpers/blk.c b/rust/helpers/blk.c
index 91b72131a04cd..0db567dfc5d53 100644
--- a/rust/helpers/blk.c
+++ b/rust/helpers/blk.c
@@ -27,3 +27,29 @@ bool rust_helper_blk_mq_add_to_batch(struct request *req,
 {
 	return blk_mq_add_to_batch(req, iob, is_error, complete);
 }
+
+__rust_helper struct request *rust_helper_rq_list_pop(struct rq_list *rl)
+{
+	return rq_list_pop(rl);
+}
+
+__rust_helper int rust_helper_rq_list_empty(const struct rq_list *rl)
+{
+	return rq_list_empty(rl);
+}
+
+__rust_helper void rust_helper_rq_list_add_tail(struct rq_list *rl,
+						struct request *rq)
+{
+	rq_list_add_tail(rl, rq);
+}
+
+__rust_helper void rust_helper_rq_list_init(struct rq_list *rl)
+{
+	rq_list_init(rl);
+}
+
+__rust_helper struct request *rust_helper_rq_list_peek(struct rq_list *rl)
+{
+	return rq_list_peek(rl);
+}
diff --git a/rust/kernel/block/mq.rs b/rust/kernel/block/mq.rs
index b36112f7b22b9..786c32f2cb56c 100644
--- a/rust/kernel/block/mq.rs
+++ b/rust/kernel/block/mq.rs
@@ -129,6 +129,7 @@
 pub mod gen_disk;
 mod operations;
 mod request;
+mod request_list;
 mod request_queue;
 pub mod tag_set;
 
@@ -141,6 +142,7 @@
 pub use request::IdleRequest;
 pub use request::Request;
 pub use request::RequestTimerHandle;
+pub use request_list::RequestList;
 pub use request_queue::RequestQueue;
 pub use tag_set::QueueType;
 pub use tag_set::TagSet;
diff --git a/rust/kernel/block/mq/request_list.rs b/rust/kernel/block/mq/request_list.rs
new file mode 100644
index 0000000000000..60284d9ef6c9c
--- /dev/null
+++ b/rust/kernel/block/mq/request_list.rs
@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use core::marker::PhantomData;
+
+use crate::{owned::Owned, types::Opaque};
+
+use super::{IdleRequest, Operations};
+
+/// A list of [`Request`].
+///
+/// # INVARIANTS
+///
+/// - `self.inner` is always a valid list, meaning the `next` and `prev`
+///   pointers point to valid requests, or are both null.
+/// - All requests in the list are valid for use as `IdleRequest<T>`.
+#[repr(transparent)]
+pub struct RequestList<T: Operations> {
+    inner: Opaque<bindings::rq_list>,
+    _p: PhantomData<T>,
+}
+
+impl<T: Operations> RequestList<T> {
+    /// Create a new [`RequestList`].
+    pub fn new() -> Self {
+        let this = Self {
+            inner: Opaque::zeroed(),
+            _p: PhantomData,
+        };
+
+        // NOTE: We are actually good to go, but we call the C initializer for forward
+        // compatibility.
+        // SAFETY: `this.inner` is a valid allocation for use as `bindings::rq_list!.
+        unsafe { bindings::rq_list_init(this.inner.get()) }
+
+        //INVARIANT: `self.inner` was initialized above and is empty.
+        this
+    }
+
+    /// Create a mutable reference to a [`RequestList`] from a raw pointer.
+    ///
+    /// # SAFETY
+    /// - The list pointed to by `ptr` must satisfy the invariants of `Self`.
+    /// - The list pointed to by `ptr` must remain valid for use as a mutable reference for the
+    ///   duration of `'a`.
+    pub unsafe fn from_raw<'a>(ptr: *mut bindings::rq_list) -> &'a mut Self {
+        // SAFETY:
+        // - RequestList is transparent.
+        // - By function safety requirements, `ptr` is valid for us as a mutable reference.
+        unsafe { &mut (*ptr.cast()) }
+    }
+
+    /// Check if the list is empty.
+    pub fn empty(&self) -> bool {
+        // SAFETY: By type invariant, self.inner is valid.
+        let ret = unsafe { bindings::rq_list_empty(self.inner.get()) };
+        ret != 0
+    }
+
+    /// Pop a request from the list.
+    ///
+    /// Returns [`None`] if the list is empty.
+    pub fn pop(&mut self) -> Option<Owned<IdleRequest<T>>> {
+        // SAFETY: By type invariant `self.inner` is a valid list.
+        let ptr = unsafe { bindings::rq_list_pop(self.inner.get()) };
+
+        if !ptr.is_null() {
+            // SAFETY: If `rq_list_pop` returns a non-null pointer, it points to a valid request. By
+            // type invariant all requests in this list are valid for use as `IdleRequest`.
+            Some(unsafe { IdleRequest::from_raw(ptr) })
+        } else {
+            None
+        }
+    }
+
+    /// Push a request on the tail of the list.
+    pub fn push_tail(&mut self, rq: Owned<IdleRequest<T>>) {
+        let ptr = rq.as_raw();
+        core::mem::forget(rq);
+        // INVARIANT: rq is an `IdleRequest<T>`.
+        // SAFETY: By type invariant, `self.inner` is a valid list.
+        unsafe { bindings::rq_list_add_tail(self.inner.get(), ptr) };
+    }
+
+    /// Peek at the head of the list.
+    ///
+    /// Returns a null pointer if the list is empty.
+    pub fn peek_raw(&self) -> *mut bindings::request {
+        // SAFETY: By type invariant, `self.inner` is a valid list.
+        unsafe { bindings::rq_list_peek(self.inner.get()) }
+    }
+}
+
+impl<T: Operations> Default for RequestList<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T: Operations> Drop for RequestList<T> {
+    fn drop(&mut self) {
+        while let Some(rq) = self.pop() {
+            drop(rq)
+        }
+    }
+}
+
+impl<T: Operations> Iterator for &mut RequestList<T> {
+    type Item = Owned<IdleRequest<T>>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.pop()
+    }
+}

-- 
2.51.2