rust: device: Create FwNode abstraction for accessing device properties

Accessing device properties is currently done via methods on `Device`
itself, using bindings to device_property_* functions. This is
sufficient for the existing method property_present. However, it's not
sufficient for other device properties we want to access. For example,
iterating over child nodes of a device will yield a fwnode_handle.
That's not a device, so it wouldn't be possible to read the properties
of that child node. Thus, we need an abstraction over fwnode_handle and
methods for reading its properties.

Add a struct FwNode which abstracts over the C struct fwnode_handle.
Implement its reference counting analogous to other Rust abstractions
over reference-counted C structs.

Subsequent patches will add functionality to access FwNode and read
properties with it.

Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Remo Senekowitsch <remo@buenzli.dev>
Link: https://lore.kernel.org/r/20250611102908.212514-2-remo@buenzli.dev
[ Add temporary #[expect(dead_code)] to avoid a warning. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

MAINTAINERS

@@ -7366,6 +7366,7 @@ F:	include/linux/property.h
 F:	include/linux/sysfs.h
 F:	lib/kobj*
 F:	rust/kernel/device.rs
+F:	rust/kernel/device/
 F:	rust/kernel/device_id.rs
 F:	rust/kernel/devres.rs
 F:	rust/kernel/driver.rs

rust/helpers/helpers.c

@@ -30,6 +30,7 @@
 #include "platform.c"
 #include "pci.c"
 #include "pid_namespace.c"
+#include "property.c"
 #include "rbtree.c"
 #include "rcu.c"
 #include "refcount.c"

rust/helpers/property.c

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/property.h>
+
+void rust_helper_fwnode_handle_put(struct fwnode_handle *fwnode)
+{
+	fwnode_handle_put(fwnode);
+}

rust/kernel/device.rs

@@ -14,6 +14,8 @@
 #[cfg(CONFIG_PRINTK)]
 use crate::c_str;
 
+pub mod property;
+
 /// A reference-counted device.
 ///
 /// This structure represents the Rust abstraction for a C `struct device`. This implementation

rust/kernel/device/property.rs

@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Unified device property interface.
+//!
+//! C header: [`include/linux/property.h`](srctree/include/linux/property.h)
+
+use core::ptr;
+
+use crate::{
+    bindings,
+    types::{ARef, Opaque},
+};
+
+/// A reference-counted fwnode_handle.
+///
+/// This structure represents the Rust abstraction for a
+/// C `struct fwnode_handle`. This implementation abstracts the usage of an
+/// already existing C `struct fwnode_handle` within Rust code that we get
+/// passed from the C side.
+///
+/// # Invariants
+///
+/// A `FwNode` instance represents a valid `struct fwnode_handle` created by the
+/// C portion of the kernel.
+///
+/// Instances of this type are always reference-counted, that is, a call to
+/// `fwnode_handle_get` ensures that the allocation remains valid at least until
+/// the matching call to `fwnode_handle_put`.
+#[repr(transparent)]
+pub struct FwNode(Opaque<bindings::fwnode_handle>);
+
+impl FwNode {
+    /// # Safety
+    ///
+    /// Callers must ensure that:
+    /// - The reference count was incremented at least once.
+    /// - They relinquish that increment. That is, if there is only one
+    ///   increment, callers must not use the underlying object anymore -- it is
+    ///   only safe to do so via the newly created `ARef<FwNode>`.
+    #[expect(dead_code)]
+    unsafe fn from_raw(raw: *mut bindings::fwnode_handle) -> ARef<Self> {
+        // SAFETY: As per the safety requirements of this function:
+        // - `NonNull::new_unchecked`:
+        //   - `raw` is not null.
+        // - `ARef::from_raw`:
+        //   - `raw` has an incremented refcount.
+        //   - that increment is relinquished, i.e. it won't be decremented
+        //     elsewhere.
+        // CAST: It is safe to cast from a `*mut fwnode_handle` to
+        // `*mut FwNode`, because `FwNode` is  defined as a
+        // `#[repr(transparent)]` wrapper around `fwnode_handle`.
+        unsafe { ARef::from_raw(ptr::NonNull::new_unchecked(raw.cast())) }
+    }
+
+    /// Obtain the raw `struct fwnode_handle *`.
+    pub(crate) fn as_raw(&self) -> *mut bindings::fwnode_handle {
+        self.0.get()
+    }
+}
+
+// SAFETY: Instances of `FwNode` are always reference-counted.
+unsafe impl crate::types::AlwaysRefCounted for FwNode {
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference guarantees that the
+        // refcount is non-zero.
+        unsafe { bindings::fwnode_handle_get(self.as_raw()) };
+    }
+
+    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
+        // SAFETY: The safety requirements guarantee that the refcount is
+        // non-zero.
+        unsafe { bindings::fwnode_handle_put(obj.cast().as_ptr()) }
+    }
+}