[PATCH v4 3/5] rust: auxiliary: add auxiliary device / driver abstractions

Danilo Krummrich
2025-04-14T13:19:53Z

Implement the basic auxiliary abstractions required to implement a
driver matching an auxiliary device.

The design and implementation is analogous to PCI and platform and is
based on the generic device / driver abstractions.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
MAINTAINERS | 2 +
rust/bindings/bindings_helper.h | 1 +
rust/helpers/auxiliary.c | 23 +++
rust/helpers/helpers.c | 1 +
rust/kernel/auxiliary.rs | 274 ++++++++++++++++++++++++++++++++
rust/kernel/device.rs | 1 -
rust/kernel/lib.rs | 2 +
7 files changed, 303 insertions(+), 1 deletion(-)
create mode 100644 rust/helpers/auxiliary.c
create mode 100644 rust/kernel/auxiliary.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index 96b827049501..a7bc29ca37d4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3872,6 +3872,8 @@ T: git git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git
F: Documentation/driver-api/auxiliary_bus.rst
F: drivers/base/auxiliary.c
F: include/linux/auxiliary_bus.h
+F: rust/kernel/auxiliary.rs
+F: samples/rust/rust_driver_auxiliary.rs

AUXILIARY DISPLAY DRIVERS
M: Andy Shevchenko <andy@kernel.org>
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index ab37e1d35c70..8a2add69e5d6 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -7,6 +7,7 @@
*/

#include <kunit/test.h>
+#include <linux/auxiliary_bus.h>
#include <linux/blk-mq.h>
#include <linux/blk_types.h>
#include <linux/blkdev.h>
diff --git a/rust/helpers/auxiliary.c b/rust/helpers/auxiliary.c
new file mode 100644
index 000000000000..0db3860d774e
--- /dev/null
+++ b/rust/helpers/auxiliary.c
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/auxiliary_bus.h>
+
+void rust_helper_auxiliary_set_drvdata(struct auxiliary_device *adev, void *data)
+{
+ auxiliary_set_drvdata(adev, data);
+}
+
+void *rust_helper_auxiliary_get_drvdata(struct auxiliary_device *adev)
+{
+ return auxiliary_get_drvdata(adev);
+}
+
+void rust_helper_auxiliary_device_uninit(struct auxiliary_device *adev)
+{
+ return auxiliary_device_uninit(adev);
+}
+
+void rust_helper_auxiliary_device_delete(struct auxiliary_device *adev)
+{
+ return auxiliary_device_delete(adev);
+}
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index e1c21eba9b15..6b279279cb12 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -7,6 +7,7 @@
* Sorted alphabetically.
*/

+#include "auxiliary.c"
#include "blk.c"
#include "bug.c"
#include "build_assert.c"
diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
new file mode 100644
index 000000000000..75423737032a
--- /dev/null
+++ b/rust/kernel/auxiliary.rs
@@ -0,0 +1,274 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Abstractions for the auxiliary bus.
+//!
+//! C header: [`include/linux/auxiliary_bus.h`](srctree/include/linux/auxiliary_bus.h)
+
+use crate::{
+ bindings, device,
+ device_id::RawDeviceId,
+ driver,
+ error::{to_result, Result},
+ prelude::*,
+ str::CStr,
+ types::{ForeignOwnable, Opaque},
+ ThisModule,
+};
+use core::{
+ marker::PhantomData,
+ ptr::{addr_of_mut, NonNull},
+};
+
+/// An adapter for the registration of auxiliary drivers.
+pub struct Adapter<T: Driver>(T);
+
+// SAFETY: A call to `unregister` for a given instance of `RegType` is guaranteed to be valid if
+// a preceding call to `register` has been successful.
+unsafe impl<T: Driver + 'static> driver::RegistrationOps for Adapter<T> {
+ type RegType = bindings::auxiliary_driver;
+
+ unsafe fn register(
+ adrv: &Opaque<Self::RegType>,
+ name: &'static CStr,
+ module: &'static ThisModule,
+ ) -> Result {
+ // SAFETY: It's safe to set the fields of `struct auxiliary_driver` on initialization.
+ unsafe {
+ (*adrv.get()).name = name.as_char_ptr();
+ (*adrv.get()).probe = Some(Self::probe_callback);
+ (*adrv.get()).remove = Some(Self::remove_callback);
+ (*adrv.get()).id_table = T::ID_TABLE.as_ptr();
+ }
+
+ // SAFETY: `adrv` is guaranteed to be a valid `RegType`.
+ to_result(unsafe {
+ bindings::__auxiliary_driver_register(adrv.get(), module.0, name.as_char_ptr())
+ })
+ }
+
+ unsafe fn unregister(adrv: &Opaque<Self::RegType>) {
+ // SAFETY: `adrv` is guaranteed to be a valid `RegType`.
+ unsafe { bindings::auxiliary_driver_unregister(adrv.get()) }
+ }
+}
+
+impl<T: Driver + 'static> Adapter<T> {
+ extern "C" fn probe_callback(
+ adev: *mut bindings::auxiliary_device,
+ id: *const bindings::auxiliary_device_id,
+ ) -> core::ffi::c_int {
+ // SAFETY: The auxiliary bus only ever calls the probe callback with a valid pointer to a
+ // `struct auxiliary_device`.
+ //
+ // INVARIANT: `adev` is valid for the duration of `probe_callback()`.
+ let adev = unsafe { &*adev.cast::<Device<device::Core>>() };
+
+ // SAFETY: `DeviceId` is a `#[repr(transparent)` wrapper of `struct auxiliary_device_id` and
+ // does not add additional invariants, so it's safe to transmute.
+ let id = unsafe { &*id.cast::<DeviceId>() };
+ let info = T::ID_TABLE.info(id.index());
+
+ match T::probe(adev, info) {
+ Ok(data) => {
+ // Let the `struct auxiliary_device` own a reference of the driver's private data.
+ // SAFETY: By the type invariant `adev.as_raw` returns a valid pointer to a
+ // `struct auxiliary_device`.
+ unsafe { bindings::auxiliary_set_drvdata(adev.as_raw(), data.into_foreign()) };
+ }
+ Err(err) => return Error::to_errno(err),
+ }
+
+ 0
+ }
+
+ extern "C" fn remove_callback(adev: *mut bindings::auxiliary_device) {
+ // SAFETY: The auxiliary bus only ever calls the remove callback with a valid pointer to a
+ // `struct auxiliary_device`.
+ let ptr = unsafe { bindings::auxiliary_get_drvdata(adev) };
+
+ // SAFETY: `remove_callback` is only ever called after a successful call to
+ // `probe_callback`, hence it's guaranteed that `ptr` points to a valid and initialized
+ // `KBox<T>` pointer created through `KBox::into_foreign`.
+ let _ = unsafe { KBox::<T>::from_foreign(ptr) };
+ }
+}
+
+/// Declares a kernel module that exposes a single auxiliary driver.
+#[macro_export]
+macro_rules! module_auxiliary_driver {
+ ($($f:tt)*) => {
+ $crate::module_driver!(<T>, $crate::auxiliary::Adapter<T>, { $($f)* });
+ };
+}
+
+/// Abstraction for `bindings::auxiliary_device_id`.
+#[repr(transparent)]
+#[derive(Clone, Copy)]
+pub struct DeviceId(bindings::auxiliary_device_id);
+
+impl DeviceId {
+ /// Create a new [`DeviceId`] from name.
+ pub const fn new(modname: &'static CStr, name: &'static CStr) -> Self {
+ let name = name.as_bytes_with_nul();
+ let modname = modname.as_bytes_with_nul();
+
+ // TODO: Replace with `bindings::auxiliary_device_id::default()` once stabilized for
+ // `const`.
+ //
+ // SAFETY: FFI type is valid to be zero-initialized.
+ let mut id: bindings::auxiliary_device_id = unsafe { core::mem::zeroed() };
+
+ let mut i = 0;
+ while i < modname.len() {
+ id.name[i] = modname[i];
+ i += 1;
+ }
+
+ // Reuse the space of the NULL terminator.
+ id.name[i - 1] = b'.';
+
+ let mut j = 0;
+ while j < name.len() {
+ id.name[i] = name[j];
+ i += 1;
+ j += 1;
+ }
+
+ Self(id)
+ }
+}
+
+// SAFETY:
+// * `DeviceId` is a `#[repr(transparent)` wrapper of `auxiliary_device_id` and does not add
+// additional invariants, so it's safe to transmute to `RawType`.
+// * `DRIVER_DATA_OFFSET` is the offset to the `driver_data` field.
+unsafe impl RawDeviceId for DeviceId {
+ type RawType = bindings::auxiliary_device_id;
+
+ const DRIVER_DATA_OFFSET: usize =
+ core::mem::offset_of!(bindings::auxiliary_device_id, driver_data);
+
+ fn index(&self) -> usize {
+ self.0.driver_data
+ }
+}
+
+/// IdTable type for auxiliary drivers.
+pub type IdTable<T> = &'static dyn kernel::device_id::IdTable<DeviceId, T>;
+
+/// Create a auxiliary `IdTable` with its alias for modpost.
+#[macro_export]
+macro_rules! auxiliary_device_table {
+ ($table_name:ident, $module_table_name:ident, $id_info_type: ty, $table_data: expr) => {
+ const $table_name: $crate::device_id::IdArray<
+ $crate::auxiliary::DeviceId,
+ $id_info_type,
+ { $table_data.len() },
+ > = $crate::device_id::IdArray::new($table_data);
+
+ $crate::module_device_table!("auxiliary", $module_table_name, $table_name);
+ };
+}
+
+/// The auxiliary driver trait.
+///
+/// Drivers must implement this trait in order to get an auxiliary driver registered.
+pub trait Driver {
+ /// The type holding information about each device id supported by the driver.
+ ///
+ /// TODO: Use associated_type_defaults once stabilized:
+ ///
+ /// type IdInfo: 'static = ();
+ type IdInfo: 'static;
+
+ /// The table of device ids supported by the driver.
+ const ID_TABLE: IdTable<Self::IdInfo>;
+
+ /// Auxiliary driver probe.
+ ///
+ /// Called when an auxiliary device is matches a corresponding driver.
+ fn probe(dev: &Device<device::Core>, id_info: &Self::IdInfo) -> Result<Pin<KBox<Self>>>;
+}
+
+/// The auxiliary device representation.
+///
+/// This structure represents the Rust abstraction for a C `struct auxiliary_device`. The
+/// implementation abstracts the usage of an already existing C `struct auxiliary_device` within
+/// Rust code that we get passed from the C side.
+///
+/// # Invariants
+///
+/// A [`Device`] instance represents a valid `struct auxiliary_device` created by the C portion of
+/// the kernel.
+#[repr(transparent)]
+pub struct Device<Ctx: device::DeviceContext = device::Normal>(
+ Opaque<bindings::auxiliary_device>,
+ PhantomData<Ctx>,
+);
+
+impl<Ctx: device::DeviceContext> Device<Ctx> {
+ fn as_raw(&self) -> *mut bindings::auxiliary_device {
+ self.0.get()
+ }
+
+ /// Returns the auxiliary device' id.
+ pub fn id(&self) -> u32 {
+ // SAFETY: By the type invariant `self.as_raw()` is a valid pointer to a
+ // `struct auxiliary_device`.
+ unsafe { (*self.as_raw()).id }
+ }
+
+ /// Returns a reference to the parent [`device::Device`], if any.
+ pub fn parent(&self) -> Option<&device::Device> {
+ let ptr: *const Self = self;
+ // CAST: `Device<Ctx: DeviceContext>` types are transparent to each other.
+ let ptr: *const Device = ptr.cast();
+ // SAFETY: `ptr` was derived from `&self`.
+ let this = unsafe { &*ptr };
+
+ this.as_ref().parent()
+ }
+}
+
+// SAFETY: `Device` is a transparent wrapper of a type that doesn't depend on `Device`'s generic
+// argument.
+kernel::impl_device_context_deref!(unsafe { Device });
+kernel::impl_device_context_into_aref!(Device);
+
+// 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_ref().as_raw()) };
+ }
+
+ unsafe fn dec_ref(obj: NonNull<Self>) {
+ // CAST: `Self` a transparent wrapper of `bindings::auxiliary_device`.
+ let adev: *mut bindings::auxiliary_device = obj.cast().as_ptr();
+
+ // SAFETY: By the type invariant of `Self`, `adev` is a pointer to a valid
+ // `struct auxiliary_device`.
+ let dev = unsafe { addr_of_mut!((*adev).dev) };
+
+ // SAFETY: The safety requirements guarantee that the refcount is non-zero.
+ unsafe { bindings::put_device(dev) }
+ }
+}
+
+impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
+ fn as_ref(&self) -> &device::Device<Ctx> {
+ // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
+ // `struct auxiliary_device`.
+ let dev = unsafe { addr_of_mut!((*self.as_raw()).dev) };
+
+ // SAFETY: `dev` points to a valid `struct device`.
+ unsafe { device::Device::as_ref(dev) }
+ }
+}
+
+// SAFETY: A `Device` is always reference-counted and can be released from any thread.
+unsafe impl Send for Device {}
+
+// SAFETY: `Device` can be shared among threads because all methods of `Device`
+// (i.e. `Device<Normal>) are thread safe.
+unsafe impl Sync for Device {}
diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index 911045e32c34..bbd308b8c694 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -68,7 +68,6 @@ pub(crate) fn as_raw(&self) -> *mut bindings::device {
}

/// Returns a reference to the parent device, if any.
- #[expect(unused)]
pub(crate) fn parent(&self) -> Option<&Self> {
// SAFETY:
// - By the type invariant `self.as_raw()` is always valid.
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index de07aadd1ff5..55a8dfeece0b 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -38,6 +38,8 @@
pub use ffi;

pub mod alloc;
+#[cfg(CONFIG_AUXILIARY_BUS)]
+pub mod auxiliary;
#[cfg(CONFIG_BLOCK)]
pub mod block;
#[doc(hidden)]
--
2.49.0