rust/kernel/auxiliary.rs - linux - Git at Google

 // 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, container_of, device,
     device_id::{RawDeviceId, RawDeviceIdIndex},
     driver,
     error::{from_result, to_result, Result},
     prelude::*,
     types::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,
     ) -> kernel::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::CoreInternal>>() };

         // 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());

         from_result(|| {
             let data = T::probe(adev, info)?;

             adev.as_ref().set_drvdata(data);
             Ok(0)
         })
     }

     extern "C" fn remove_callback(adev: *mut bindings::auxiliary_device) {
         // 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::CoreInternal>>() };

         // SAFETY: `remove_callback` is only ever called after a successful call to
         // `probe_callback`, hence it's guaranteed that `Device::set_drvdata()` has been called
         // and stored a `Pin<KBox<T>>`.
         drop(unsafe { adev.as_ref().drvdata_obtain::<Pin<KBox<T>>>() });
     }
 }

 /// 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`.
 unsafe impl RawDeviceId for DeviceId {
     type RawType = bindings::auxiliary_device_id;
 }

 // SAFETY: `DRIVER_DATA_OFFSET` is the offset to the `driver_data` field.
 unsafe impl RawDeviceIdIndex for DeviceId {
     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()
     }
 }

 impl Device {
     extern "C" fn release(dev: *mut bindings::device) {
         // SAFETY: By the type invariant `self.0.as_raw` is a pointer to the `struct device`
         // embedded in `struct auxiliary_device`.
         let adev = unsafe { container_of!(dev, bindings::auxiliary_device, dev) };

         // SAFETY: `adev` points to the memory that has been allocated in `Registration::new`, via
         // `KBox::new(Opaque::<bindings::auxiliary_device>::zeroed(), GFP_KERNEL)`.
         let _ = unsafe { KBox::<Opaque<bindings::auxiliary_device>>::from_raw(adev.cast()) };
     }
 }

 // 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::from_raw(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 {}

 /// The registration of an auxiliary device.
 ///
 /// This type represents the registration of a [`struct auxiliary_device`]. When an instance of this
 /// type is dropped, its respective auxiliary device will be unregistered from the system.
 ///
 /// # Invariants
 ///
 /// `self.0` always holds a valid pointer to an initialized and registered
 /// [`struct auxiliary_device`].
 pub struct Registration(NonNull<bindings::auxiliary_device>);

 impl Registration {
     /// Create and register a new auxiliary device.
     pub fn new(parent: &device::Device, name: &CStr, id: u32, modname: &CStr) -> Result<Self> {
         let boxed = KBox::new(Opaque::<bindings::auxiliary_device>::zeroed(), GFP_KERNEL)?;
         let adev = boxed.get();

         // SAFETY: It's safe to set the fields of `struct auxiliary_device` on initialization.
         unsafe {
             (*adev).dev.parent = parent.as_raw();
             (*adev).dev.release = Some(Device::release);
             (*adev).name = name.as_char_ptr();
             (*adev).id = id;
         }

         // SAFETY: `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`,
         // which has not been initialized yet.
         unsafe { bindings::auxiliary_device_init(adev) };

         // Now that `adev` is initialized, leak the `Box`; the corresponding memory will be freed
         // by `Device::release` when the last reference to the `struct auxiliary_device` is dropped.
         let _ = KBox::into_raw(boxed);

         // SAFETY:
         // - `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`, which has
         //   been initialialized,
         // - `modname.as_char_ptr()` is a NULL terminated string.
         let ret = unsafe { bindings::__auxiliary_device_add(adev, modname.as_char_ptr()) };
         if ret != 0 {
             // SAFETY: `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`,
             // which has been initialialized.
             unsafe { bindings::auxiliary_device_uninit(adev) };

             return Err(Error::from_errno(ret));
         }

         // SAFETY: `adev` is guaranteed to be non-null, since the `KBox` was allocated successfully.
         //
         // INVARIANT: The device will remain registered until `auxiliary_device_delete()` is called,
         // which happens in `Self::drop()`.
         Ok(Self(unsafe { NonNull::new_unchecked(adev) }))
     }
 }

 impl Drop for Registration {
     fn drop(&mut self) {
         // SAFETY: By the type invariant of `Self`, `self.0.as_ptr()` is a valid registered
         // `struct auxiliary_device`.
         unsafe { bindings::auxiliary_device_delete(self.0.as_ptr()) };

         // This drops the reference we acquired through `auxiliary_device_init()`.
         //
         // SAFETY: By the type invariant of `Self`, `self.0.as_ptr()` is a valid registered
         // `struct auxiliary_device`.
         unsafe { bindings::auxiliary_device_uninit(self.0.as_ptr()) };
     }
 }

 // SAFETY: A `Registration` of a `struct auxiliary_device` can be released from any thread.
 unsafe impl Send for Registration {}

 // SAFETY: `Registration` does not expose any methods or fields that need synchronization.
 unsafe impl Sync for Registration {}
	// 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, container_of, device,
	device_id::{RawDeviceId, RawDeviceIdIndex},
	driver,
	error::{from_result, to_result, Result},
	prelude::*,
	types::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,
	) -> kernel::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::CoreInternal>>() };

	// 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());

	from_result(\|\| {
	let data = T::probe(adev, info)?;

	adev.as_ref().set_drvdata(data);
	Ok(0)
	})
	}

	extern "C" fn remove_callback(adev: *mut bindings::auxiliary_device) {
	// 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::CoreInternal>>() };

	// SAFETY: `remove_callback` is only ever called after a successful call to
	// `probe_callback`, hence it's guaranteed that `Device::set_drvdata()` has been called
	// and stored a `Pin<KBox<T>>`.
	drop(unsafe { adev.as_ref().drvdata_obtain::<Pin<KBox<T>>>() });
	}
	}

	/// 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`.
	unsafe impl RawDeviceId for DeviceId {
	type RawType = bindings::auxiliary_device_id;
	}

	// SAFETY: `DRIVER_DATA_OFFSET` is the offset to the `driver_data` field.
	unsafe impl RawDeviceIdIndex for DeviceId {
	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()
	}
	}

	impl Device {
	extern "C" fn release(dev: *mut bindings::device) {
	// SAFETY: By the type invariant `self.0.as_raw` is a pointer to the `struct device`
	// embedded in `struct auxiliary_device`.
	let adev = unsafe { container_of!(dev, bindings::auxiliary_device, dev) };

	// SAFETY: `adev` points to the memory that has been allocated in `Registration::new`, via
	// `KBox::new(Opaque::<bindings::auxiliary_device>::zeroed(), GFP_KERNEL)`.
	let _ = unsafe { KBox::<Opaque<bindings::auxiliary_device>>::from_raw(adev.cast()) };
	}
	}

	// 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::from_raw(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 {}

	/// The registration of an auxiliary device.
	///
	/// This type represents the registration of a [`struct auxiliary_device`]. When an instance of this
	/// type is dropped, its respective auxiliary device will be unregistered from the system.
	///
	/// # Invariants
	///
	/// `self.0` always holds a valid pointer to an initialized and registered
	/// [`struct auxiliary_device`].
	pub struct Registration(NonNull<bindings::auxiliary_device>);

	impl Registration {
	/// Create and register a new auxiliary device.
	pub fn new(parent: &device::Device, name: &CStr, id: u32, modname: &CStr) -> Result<Self> {
	let boxed = KBox::new(Opaque::<bindings::auxiliary_device>::zeroed(), GFP_KERNEL)?;
	let adev = boxed.get();

	// SAFETY: It's safe to set the fields of `struct auxiliary_device` on initialization.
	unsafe {
	(*adev).dev.parent = parent.as_raw();
	(*adev).dev.release = Some(Device::release);
	(*adev).name = name.as_char_ptr();
	(*adev).id = id;
	}

	// SAFETY: `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`,
	// which has not been initialized yet.
	unsafe { bindings::auxiliary_device_init(adev) };

	// Now that `adev` is initialized, leak the `Box`; the corresponding memory will be freed
	// by `Device::release` when the last reference to the `struct auxiliary_device` is dropped.
	let _ = KBox::into_raw(boxed);

	// SAFETY:
	// - `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`, which has
	// been initialialized,
	// - `modname.as_char_ptr()` is a NULL terminated string.
	let ret = unsafe { bindings::__auxiliary_device_add(adev, modname.as_char_ptr()) };
	if ret != 0 {
	// SAFETY: `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`,
	// which has been initialialized.
	unsafe { bindings::auxiliary_device_uninit(adev) };

	return Err(Error::from_errno(ret));
	}

	// SAFETY: `adev` is guaranteed to be non-null, since the `KBox` was allocated successfully.
	//
	// INVARIANT: The device will remain registered until `auxiliary_device_delete()` is called,
	// which happens in `Self::drop()`.
	Ok(Self(unsafe { NonNull::new_unchecked(adev) }))
	}
	}

	impl Drop for Registration {
	fn drop(&mut self) {
	// SAFETY: By the type invariant of `Self`, `self.0.as_ptr()` is a valid registered
	// `struct auxiliary_device`.
	unsafe { bindings::auxiliary_device_delete(self.0.as_ptr()) };

	// This drops the reference we acquired through `auxiliary_device_init()`.
	//
	// SAFETY: By the type invariant of `Self`, `self.0.as_ptr()` is a valid registered
	// `struct auxiliary_device`.
	unsafe { bindings::auxiliary_device_uninit(self.0.as_ptr()) };
	}
	}

	// SAFETY: A `Registration` of a `struct auxiliary_device` can be released from any thread.
	unsafe impl Send for Registration {}

	// SAFETY: `Registration` does not expose any methods or fields that need synchronization.
	unsafe impl Sync for Registration {}