[PATCH v5 1/9] rust: device: Create FwNode abstraction for accessing device properties

[Remo Senekowitsch <remo@buenzli.dev>]

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.

Signed-off-by: Remo Senekowitsch <remo@buenzli.dev>
---
 MAINTAINERS                    |  1 +
 rust/helpers/helpers.c         |  1 +
 rust/helpers/property.c        |  8 ++++++
 rust/kernel/device.rs          |  2 ++
 rust/kernel/device/property.rs | 47 ++++++++++++++++++++++++++++++++++
 5 files changed, 59 insertions(+)
 create mode 100644 rust/helpers/property.c
 create mode 100644 rust/kernel/device/property.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index 3cbf9ac0d83f6..0b95c4b0f657e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7248,6 +7248,7 @@ 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
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 1e7c84df72521..6aac840417648 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -25,6 +25,7 @@
 #include "platform.c"
 #include "pci.c"
 #include "pid_namespace.c"
+#include "property.c"
 #include "rbtree.c"
 #include "rcu.c"
 #include "refcount.c"
diff --git a/rust/helpers/property.c b/rust/helpers/property.c
new file mode 100644
index 0000000000000..08f68e2dac4a9
--- /dev/null
+++ b/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);
+}
diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index 0353c5552769c..d8619d4485fb4 100644
--- a/rust/kernel/device.rs
+++ b/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
diff --git a/rust/kernel/device/property.rs b/rust/kernel/device/property.rs
new file mode 100644
index 0000000000000..e75d55f5856cf
--- /dev/null
+++ b/rust/kernel/device/property.rs
@@ -0,0 +1,47 @@
+// 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::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 {
+    /// 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()) }
+    }
+}
-- 
2.49.0