[PATCH 3/6] rust: sync: introduce FfiMutex

[Daniel Almeida <daniel.almeida@collabora.com>]

Introduce an owned mutex that can be passed as bindings::mutex when
initializing C data structures. The kernel will be locking it without
user intervention so there is no data to protect and no guard type.

Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
---
 rust/kernel/sync.rs           |  1 +
 rust/kernel/sync/ffi_mutex.rs | 70 +++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)
 create mode 100644 rust/kernel/sync/ffi_mutex.rs

diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index b07db83972ac..1415062be34d 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -26,6 +26,7 @@ use core::{cell::UnsafeCell, mem::MaybeUninit, pin::Pin};
 
 mod arc;
 mod condvar;
+pub mod ffi_mutex;
 mod guard;
 mod locked_by;
 mod mutex;
diff --git a/rust/kernel/sync/ffi_mutex.rs b/rust/kernel/sync/ffi_mutex.rs
new file mode 100644
index 000000000000..e615ec4059f6
--- /dev/null
+++ b/rust/kernel/sync/ffi_mutex.rs
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! An owned mutex that can be passed as bindings::mutex when initializing C
+//! data structures. The kernel will be locking it without user intervention so
+//! there is no data to protect and no guard type.
+
+#![allow(dead_code)]
+
+use crate::prelude::*;
+use crate::sync::{LockClassKey, LockIniter};
+use core::marker::PhantomPinned;
+use core::pin::Pin;
+
+use crate::Opaque;
+
+/// An owned mutex that can be passed as bindings::mutex when initializing C
+/// data structures. The kernel will be locking it without user intervention so
+/// there is no data to protect and no guard type.
+pub struct FfiMutex {
+    mutex: Opaque<bindings::mutex>,
+    _pin: PhantomPinned,
+}
+
+impl FfiMutex {
+    /// Constructs a new Mutex for FFI purposes.
+    ///
+    /// # Safety
+    ///
+    /// The caller must call [`FfiMutex::init_lock`] before using the raw Mutex.
+    pub const unsafe fn new() -> Self {
+        Self {
+            mutex: Opaque::uninit(),
+            _pin: PhantomPinned,
+        }
+    }
+
+    /// Returns the inner bindings::mutex
+    ///
+    /// # Safety
+    ///
+    /// The caller must call [`FfiMutex::init_lock`] before using the raw Mutex.
+    pub(crate) unsafe fn raw(self: &mut Pin<&mut Self>) -> *mut bindings::mutex {
+        let this = self.as_mut();
+        // SAFETY: mutex is pinned when Lock is. The argument to the function is not moved.
+        let this = unsafe { this.map_unchecked_mut(|x| &mut x.mutex) };
+        // This does not move from the field.
+        this.get()
+    }
+}
+
+impl LockIniter for FfiMutex {
+    fn init_lock(self: Pin<&mut Self>, name: &'static CStr, key: &'static LockClassKey) {
+        unsafe { bindings::__mutex_init(self.mutex.get(), name.as_char_ptr(), key.get()) };
+    }
+}
+
+// SAFETY: the underlying bindings::mutex can be used from any thread.
+unsafe impl Send for FfiMutex {}
+// SAFETY: two threads can try locking the underlying bindings::mutex at the
+// same thread without issues.
+unsafe impl Sync for FfiMutex {}
+
+/// Safely initialises a [`FfiMutex`] with the given name, generating a new lock
+/// class.
+#[macro_export]
+macro_rules! ffi_mutex_init {
+    ($mutex:expr, $name:literal) => {
+        $crate::init_with_lockdep!($mutex, $name);
+    };
+}
-- 
2.40.0