[RFC PATCH 01/11] rust: add abstraction for struct device

[Danilo Krummrich <dakr@redhat.com>]

Add an (always) reference counted abstraction for a generic struct
device. This abstraction encapsulates existing struct device instances
and manages its reference count.

Subsystems may use this abstraction as a base to abstract subsystem
specific device instances based on a generic struct device.

Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
---
 rust/helpers.c        |  1 +
 rust/kernel/device.rs | 76 +++++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs    |  1 +
 3 files changed, 78 insertions(+)
 create mode 100644 rust/kernel/device.rs

diff --git a/rust/helpers.c b/rust/helpers.c
index 4c8b7b92a4f4..f9d2db1d1f33 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -23,6 +23,7 @@
 #include <kunit/test-bug.h>
 #include <linux/bug.h>
 #include <linux/build_bug.h>
+#include <linux/device.h>
 #include <linux/err.h>
 #include <linux/errname.h>
 #include <linux/mutex.h>
diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
new file mode 100644
index 000000000000..fafec70effb6
--- /dev/null
+++ b/rust/kernel/device.rs
@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Generic devices that are part of the kernel's driver model.
+//!
+//! C header: [`include/linux/device.h`](../../../../include/linux/device.h)
+
+use crate::{
+    bindings,
+    types::{ARef, Opaque},
+};
+use core::ptr;
+
+/// A ref-counted device.
+///
+/// # Invariants
+///
+/// The pointer stored in `Self` is non-null and valid for the lifetime of the ARef instance. In
+/// particular, the ARef instance owns an increment on underlying object’s reference count.
+#[repr(transparent)]
+pub struct Device(Opaque<bindings::device>);
+
+impl Device {
+    /// Creates a new ref-counted instance of an existing device pointer.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count.
+    pub unsafe fn from_raw(ptr: *mut bindings::device) -> ARef<Self> {
+        // SAFETY: By the safety requirements, ptr is valid.
+        // Initially increase the reference count by one to compensate for the final decrement once
+        // this newly created `ARef<Device>` instance is dropped.
+        unsafe { bindings::get_device(ptr) };
+
+        // CAST: `Self` is a `repr(transparent)` wrapper around `bindings::device`.
+        let ptr = ptr.cast::<Self>();
+
+        // SAFETY: By the safety requirements, ptr is valid.
+        unsafe { ARef::from_raw(ptr::NonNull::new_unchecked(ptr)) }
+    }
+
+    /// Obtain the raw `struct device *`.
+    pub(crate) fn as_raw(&self) -> *mut bindings::device {
+        self.0.get()
+    }
+
+    /// Convert a raw `struct device` pointer to a `&Device`.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count for
+    /// the entire duration when the returned reference exists.
+    pub unsafe fn as_ref<'a>(ptr: *mut bindings::device) -> &'a Self {
+        // SAFETY: Guaranteed by the safety requirements of the function.
+        unsafe { &*ptr.cast() }
+    }
+}
+
+// SAFETY: Instances of `Device` are always ref-counted.
+unsafe impl crate::types::AlwaysRefCounted for Device {
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference guarantees that the refcount is nonzero.
+        unsafe { bindings::get_device(self.as_raw()) };
+    }
+
+    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
+        // SAFETY: The safety requirements guarantee that the refcount is nonzero.
+        unsafe { bindings::put_device(obj.cast().as_ptr()) }
+    }
+}
+
+// SAFETY: `Device` only holds a pointer to a C device, which is safe to be used from any thread.
+unsafe impl Send for Device {}
+
+// SAFETY: `Device` only holds a pointer to a C device, references to which are safe to be used
+// from any thread.
+unsafe impl Sync for Device {}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 9a943d99c71a..4ba3d4a49e9c 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -28,6 +28,7 @@
 
 pub mod alloc;
 mod build_assert;
+pub mod device;
 pub mod error;
 pub mod init;
 pub mod ioctl;
-- 
2.45.1