[PATCH v2 1/2] rust: add abstraction for struct device

Danilo Krummrich
2024-06-10T18:03:33Z

Add an (always) reference-counted abstraction for a generic C `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`, such as
`struct pci_dev`.

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 | 97 +++++++++++++++++++++++++++++++++++++++++++
rust/kernel/lib.rs | 1 +
3 files changed, 99 insertions(+)
create mode 100644 rust/kernel/device.rs

diff --git a/rust/helpers.c b/rust/helpers.c
index 2c37a0f5d7a8..0e02b2c64c72 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..6e22b0b97928
--- /dev/null
+++ b/rust/kernel/device.rs
@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Generic devices that are part of the kernel's driver model.
+//!
+//! C header: [`include/linux/device.h`](srctree/include/linux/device.h)
+
+use crate::{
+ bindings,
+ types::{ARef, Opaque},
+};
+use core::ptr;
+
+/// A reference-counted device.
+///
+/// This structure represents the Rust abstraction for a C `struct device`. This implementation
+/// abstracts the usage of an already existing C `struct device` within Rust code that we get
+/// passed from the C side.
+///
+/// An instance of this abstraction can be obtained temporarily or permanent.
+///
+/// A temporary one is bound to the lifetime of the C `struct device` pointer used for creation.
+/// A permanent instance is always reference-counted and hence not restricted by any lifetime
+/// boundaries.
+///
+/// For subsystems it is recommended to create a permanent instance to wrap into a subsystem
+/// specifc device structure (e.g. `pci::Device`). This is useful for passing it to drivers in
+/// `T::probe()`, such that a driver can store the `ARef<Device>` (equivalent to storing a
+/// `struct device` pointer in a C driver) for arbitrary purposes, e.g. allocating DMA coherent
+/// memory.
+///
+/// # 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 the underlying object’s reference count.
+#[repr(transparent)]
+pub struct Device(Opaque<bindings::device>);
+
+impl Device {
+ /// Creates a new reference-counted abstraction instance of an existing `struct device` pointer.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count,
+ /// i.e. it must be ensured that the reference count of the C `struct device` `ptr` points to
+ /// can't drop to zero, for the duration of this function call.
+ 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 C `struct device` pointer to a `&'a Device`.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count,
+ /// i.e. it must be ensured that the reference count of the C `struct device` `ptr` points to
+ /// can't drop to zero, for the duration of this function call and 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 reference-counted.
+unsafe impl crate::types::AlwaysRefCounted for Device {
+ fn inc_ref(&self) {
+ // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
+ unsafe { bindings::get_device(self.as_raw()) };
+ }
+
+ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
+ // SAFETY: The safety requirements guarantee that the refcount is non-zero.
+ unsafe { bindings::put_device(obj.cast().as_ptr()) }
+ }
+}
+
+// SAFETY: `Device` only holds a pointer to a C `struct device`, which is safe to be used from any
+// thread.
+unsafe impl Send for Device {}
+
+// SAFETY: `Device` only holds a pointer to a C `struct 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 fbd91a48ff8b..dd1207f1a873 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