[PATCH v4 0/5] Auxiliary bus Rust abstractions

[PATCH v4 0/5] Auxiliary bus Rust abstractions

[Danilo Krummrich]

This series adds Rust abstractions for the auxiliary bus.

It implements the required abstractions to write auxiliary drivers and create
auxiliary device registrations. A driver sample illustrates how the
corresponding abstractions work.

The auxiliary abstractions are required for the Nova driver project, in order to
connect nova-core with the nova-drm driver.

A branch containing the patches can be found in [1].

[1] https://web.git.kernel.org/pub/scm/linux/kernel/git/dakr/linux.git/log/?h=rust/auxiliary

Changes in v4:
  - drop TryFrom<&Device> for &auxiliary::Device for now; we likely don't need
    it anytime soon
  - sample: rename PciDriver to ParentDriver; add connect() to ParentDriver
  - rebase onto https://lore.kernel.org/rust-for-linux/20250413173758.12068-1-dakr@kernel.org/

Changes in v3:
  - export auxiliary_bus_type
  - implement impl TryFrom<&Device> for &auxiliary::Device
  - implement Send + Sync for Registration and Device

Changes in v2:
  - implement bus device soundness improvements from [2]

Danilo Krummrich (5):
  rust: types: add `Opaque::zeroed`
  rust: device: implement Device::parent()
  rust: auxiliary: add auxiliary device / driver abstractions
  rust: auxiliary: add auxiliary registration
  samples: rust: add Rust auxiliary driver sample

 MAINTAINERS                           |   3 +
 rust/bindings/bindings_helper.h       |   1 +
 rust/helpers/auxiliary.c              |  23 ++
 rust/helpers/helpers.c                |   1 +
 rust/kernel/auxiliary.rs              | 360 ++++++++++++++++++++++++++
 rust/kernel/device.rs                 |  18 ++
 rust/kernel/lib.rs                    |   2 +
 rust/kernel/types.rs                  |   8 +
 samples/rust/Kconfig                  |  12 +
 samples/rust/Makefile                 |   1 +
 samples/rust/rust_driver_auxiliary.rs | 122 +++++++++
 11 files changed, 551 insertions(+)
 create mode 100644 rust/helpers/auxiliary.c
 create mode 100644 rust/kernel/auxiliary.rs
 create mode 100644 samples/rust/rust_driver_auxiliary.rs

-- 
2.49.0

[PATCH v4 1/5] rust: types: add `Opaque::zeroed`

[Danilo Krummrich]

Analogous to `Opaque::uninit` add `Opaque::zeroed`, which sets the
corresponding memory to zero. In contrast to `Opaque::uninit`, the
corresponding value, depending on its type, may be initialized.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/types.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 9d0471afc964..eee387727d1a 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -329,6 +329,14 @@ pub const fn uninit() -> Self {
         }
     }
 
+    /// Creates a new zeroed opaque value.
+    pub const fn zeroed() -> Self {
+        Self {
+            value: UnsafeCell::new(MaybeUninit::zeroed()),
+            _pin: PhantomPinned,
+        }
+    }
+
     /// Create an opaque pin-initializer from the given pin-initializer.
     pub fn pin_init(slot: impl PinInit<T>) -> impl PinInit<Self> {
         Self::ffi_init(|ptr: *mut T| {
-- 
2.49.0

[PATCH v4 2/5] rust: device: implement Device::parent()

[Danilo Krummrich]

Device::parent() returns a reference to the device' parent device, if
any.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/device.rs | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index 585a3fcfeea3..911045e32c34 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -67,6 +67,25 @@ pub(crate) fn as_raw(&self) -> *mut bindings::device {
         self.0.get()
     }
 
+    /// 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.
+        // - The parent device is only ever set at device creation.
+        let parent = unsafe { (*self.as_raw()).parent };
+
+        if parent.is_null() {
+            None
+        } else {
+            // SAFETY:
+            // - Since `parent` is not NULL, it must be a valid pointer to a `struct device`.
+            // - `parent` is valid for the lifetime of `self`, since a `struct device` holds a
+            //   reference count of its parent.
+            Some(unsafe { Self::as_ref(parent) })
+        }
+    }
+
     /// Convert a raw C `struct device` pointer to a `&'a Device`.
     ///
     /// # Safety
-- 
2.49.0

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

[Danilo Krummrich]

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

[PATCH v4 4/5] rust: auxiliary: add auxiliary registration

[Danilo Krummrich]

Implement the `auxiliary::Registration` type, which provides an API to
create and register new auxiliary devices in the system.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/auxiliary.rs | 88 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 87 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
index 75423737032a..b40d663b42eb 100644
--- a/rust/kernel/auxiliary.rs
+++ b/rust/kernel/auxiliary.rs
@@ -5,7 +5,7 @@
 //! C header: [`include/linux/auxiliary_bus.h`](srctree/include/linux/auxiliary_bus.h)
 
 use crate::{
-    bindings, device,
+    bindings, container_of, device,
     device_id::RawDeviceId,
     driver,
     error::{to_result, Result},
@@ -230,6 +230,18 @@ pub fn parent(&self) -> Option<&device::Device> {
     }
 }
 
+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) }.cast_mut();
+
+        // 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 });
@@ -272,3 +284,77 @@ 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 {}
-- 
2.49.0

[PATCH v4 5/5] samples: rust: add Rust auxiliary driver sample

[Danilo Krummrich]

Add a sample Rust auxiliary driver based on a PCI driver for QEMU's
"pci-testdev" device.

The PCI driver only registers an auxiliary device, in order to make the
corresponding auxiliary driver probe.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 MAINTAINERS                           |   1 +
 samples/rust/Kconfig                  |  12 +++
 samples/rust/Makefile                 |   1 +
 samples/rust/rust_driver_auxiliary.rs | 122 ++++++++++++++++++++++++++
 4 files changed, 136 insertions(+)
 create mode 100644 samples/rust/rust_driver_auxiliary.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index a7bc29ca37d4..23c6ca105471 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7246,6 +7246,7 @@ F:	rust/kernel/devres.rs
 F:	rust/kernel/driver.rs
 F:	rust/kernel/faux.rs
 F:	rust/kernel/platform.rs
+F:	samples/rust/rust_driver_auxiliary.rs
 F:	samples/rust/rust_driver_platform.rs
 F:	samples/rust/rust_driver_faux.rs
 
diff --git a/samples/rust/Kconfig b/samples/rust/Kconfig
index cad52b7120b5..43cb72d72631 100644
--- a/samples/rust/Kconfig
+++ b/samples/rust/Kconfig
@@ -82,6 +82,18 @@ config SAMPLE_RUST_DRIVER_FAUX
 
 	  If unsure, say N.
 
+config SAMPLE_RUST_DRIVER_AUXILIARY
+	tristate "Auxiliary Driver"
+	depends on AUXILIARY_BUS
+	depends on PCI
+	help
+	  This option builds the Rust auxiliary driver sample.
+
+	  To compile this as a module, choose M here:
+	  the module will be called rust_driver_auxiliary.
+
+	  If unsure, say N.
+
 config SAMPLE_RUST_HOSTPROGS
 	bool "Host programs"
 	help
diff --git a/samples/rust/Makefile b/samples/rust/Makefile
index c6a2479f7d9c..6a466afd2a21 100644
--- a/samples/rust/Makefile
+++ b/samples/rust/Makefile
@@ -8,6 +8,7 @@ obj-$(CONFIG_SAMPLE_RUST_DMA)			+= rust_dma.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_PCI)		+= rust_driver_pci.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_PLATFORM)	+= rust_driver_platform.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_FAUX)		+= rust_driver_faux.o
+obj-$(CONFIG_SAMPLE_RUST_DRIVER_AUXILIARY)	+= rust_driver_auxiliary.o
 
 rust_print-y := rust_print_main.o rust_print_events.o
 
diff --git a/samples/rust/rust_driver_auxiliary.rs b/samples/rust/rust_driver_auxiliary.rs
new file mode 100644
index 000000000000..8373c9a2337b
--- /dev/null
+++ b/samples/rust/rust_driver_auxiliary.rs
@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust auxiliary driver sample (based on a PCI driver for QEMU's `pci-testdev`).
+//!
+//! To make this driver probe, QEMU must be run with `-device pci-testdev`.
+
+use kernel::{
+    auxiliary, bindings, c_str, device::Core, driver, error::Error, pci, prelude::*, str::CStr,
+    InPlaceModule,
+};
+
+use pin_init::PinInit;
+
+const MODULE_NAME: &CStr = <LocalModule as kernel::ModuleMetadata>::NAME;
+const AUXILIARY_NAME: &CStr = c_str!("auxiliary");
+
+struct AuxiliaryDriver;
+
+kernel::auxiliary_device_table!(
+    AUX_TABLE,
+    MODULE_AUX_TABLE,
+    <AuxiliaryDriver as auxiliary::Driver>::IdInfo,
+    [(auxiliary::DeviceId::new(MODULE_NAME, AUXILIARY_NAME), ())]
+);
+
+impl auxiliary::Driver for AuxiliaryDriver {
+    type IdInfo = ();
+
+    const ID_TABLE: auxiliary::IdTable<Self::IdInfo> = &AUX_TABLE;
+
+    fn probe(adev: &auxiliary::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self>>> {
+        dev_info!(
+            adev.as_ref(),
+            "Probing auxiliary driver for auxiliary device with id={}\n",
+            adev.id()
+        );
+
+        ParentDriver::connect(adev)?;
+
+        let this = KBox::new(Self, GFP_KERNEL)?;
+
+        Ok(this.into())
+    }
+}
+
+struct ParentDriver {
+    _reg: [auxiliary::Registration; 2],
+}
+
+kernel::pci_device_table!(
+    PCI_TABLE,
+    MODULE_PCI_TABLE,
+    <ParentDriver as pci::Driver>::IdInfo,
+    [(
+        pci::DeviceId::from_id(bindings::PCI_VENDOR_ID_REDHAT, 0x5),
+        ()
+    )]
+);
+
+impl pci::Driver for ParentDriver {
+    type IdInfo = ();
+
+    const ID_TABLE: pci::IdTable<Self::IdInfo> = &PCI_TABLE;
+
+    fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self>>> {
+        let this = KBox::new(
+            Self {
+                _reg: [
+                    auxiliary::Registration::new(pdev.as_ref(), AUXILIARY_NAME, 0, MODULE_NAME)?,
+                    auxiliary::Registration::new(pdev.as_ref(), AUXILIARY_NAME, 1, MODULE_NAME)?,
+                ],
+            },
+            GFP_KERNEL,
+        )?;
+
+        Ok(this.into())
+    }
+}
+
+impl ParentDriver {
+    fn connect(adev: &auxiliary::Device) -> Result<()> {
+        if let Some(parent) = adev.parent() {
+            // Validate that we're the parent of the given auxiliary device.
+            let pdev: &pci::Device = parent.try_into()?;
+
+            dev_info!(
+                adev.as_ref(),
+                "Connect auxiliary {} with parent: VendorID={:#x}, DeviceID={:#x}\n",
+                adev.id(),
+                pdev.vendor_id(),
+                pdev.device_id()
+            );
+        }
+
+        Ok(())
+    }
+}
+
+#[pin_data]
+struct SampleModule {
+    #[pin]
+    _pci_driver: driver::Registration<pci::Adapter<ParentDriver>>,
+    #[pin]
+    _aux_driver: driver::Registration<auxiliary::Adapter<AuxiliaryDriver>>,
+}
+
+impl InPlaceModule for SampleModule {
+    fn init(module: &'static kernel::ThisModule) -> impl PinInit<Self, Error> {
+        try_pin_init!(Self {
+            _pci_driver <- driver::Registration::new(MODULE_NAME, module),
+            _aux_driver <- driver::Registration::new(MODULE_NAME, module),
+        })
+    }
+}
+
+module! {
+    type: SampleModule,
+    name: "rust_driver_auxiliary",
+    author: "Danilo Krummrich",
+    description: "Rust auxiliary driver",
+    license: "GPL v2",
+}
-- 
2.49.0

Re: [PATCH v4 0/5] Auxiliary bus Rust abstractions

[Greg KH]

On Mon, Apr 14, 2025 at 03:18:03PM +0200, Danilo Krummrich wrote:
> This series adds Rust abstractions for the auxiliary bus.
> 
> It implements the required abstractions to write auxiliary drivers and create
> auxiliary device registrations. A driver sample illustrates how the
> corresponding abstractions work.
> 
> The auxiliary abstractions are required for the Nova driver project, in order to
> connect nova-core with the nova-drm driver.
> 
> A branch containing the patches can be found in [1].
> 
> [1] https://web.git.kernel.org/pub/scm/linux/kernel/git/dakr/linux.git/log/?h=rust/auxiliary

Looks good to me, want to take this through a gpu tree so you can build
on top of it, or do you want it in the driver-core.git tree?

thanks,

greg k-h

Re: [PATCH v4 1/5] rust: types: add `Opaque::zeroed`

[Alice Ryhl]

On Mon, Apr 14, 2025 at 03:18:04PM +0200, Danilo Krummrich wrote:
> Analogous to `Opaque::uninit` add `Opaque::zeroed`, which sets the
> corresponding memory to zero. In contrast to `Opaque::uninit`, the
> corresponding value, depending on its type, may be initialized.
> 
> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

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

[Alice Ryhl]

On Mon, Apr 14, 2025 at 03:18:06PM +0200, Danilo Krummrich wrote:
> 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>

Overall looks okay to me, but I do have some comments.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

>  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 {

We shouldn't use core::ffi.

> +        // 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

Typo: missing ]

> +        // 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) };

Nit: isn't this usually written

    drop(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

Typo: missing ]

> +//   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
> 

Re: [PATCH v4 4/5] rust: auxiliary: add auxiliary registration

[Alice Ryhl]

On Mon, Apr 14, 2025 at 03:18:07PM +0200, Danilo Krummrich wrote:
> Implement the `auxiliary::Registration` type, which provides an API to
> create and register new auxiliary devices in the system.
> 
> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/auxiliary.rs | 88 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 87 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
> index 75423737032a..b40d663b42eb 100644
> --- a/rust/kernel/auxiliary.rs
> +++ b/rust/kernel/auxiliary.rs
> @@ -5,7 +5,7 @@
>  //! C header: [`include/linux/auxiliary_bus.h`](srctree/include/linux/auxiliary_bus.h)
>  
>  use crate::{
> -    bindings, device,
> +    bindings, container_of, device,
>      device_id::RawDeviceId,
>      driver,
>      error::{to_result, Result},
> @@ -230,6 +230,18 @@ pub fn parent(&self) -> Option<&device::Device> {
>      }
>  }
>  
> +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) }.cast_mut();

FYI, Tamir's patch makes this cast_mut() unnecessary.

> +        // 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 });
> @@ -272,3 +284,77 @@ 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) };

Does this error-path actually free the box?

Alice

> +            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 {}
> -- 
> 2.49.0
> 

Re: [PATCH v4 0/5] Auxiliary bus Rust abstractions

[Danilo Krummrich]

On Tue, Apr 15, 2025 at 01:38:29PM +0200, Greg KH wrote:
> On Mon, Apr 14, 2025 at 03:18:03PM +0200, Danilo Krummrich wrote:
> > This series adds Rust abstractions for the auxiliary bus.
> > 
> > It implements the required abstractions to write auxiliary drivers and create
> > auxiliary device registrations. A driver sample illustrates how the
> > corresponding abstractions work.
> > 
> > The auxiliary abstractions are required for the Nova driver project, in order to
> > connect nova-core with the nova-drm driver.
> > 
> > A branch containing the patches can be found in [1].
> > 
> > [1] https://web.git.kernel.org/pub/scm/linux/kernel/git/dakr/linux.git/log/?h=rust/auxiliary
> 
> Looks good to me, want to take this through a gpu tree so you can build
> on top of it, or do you want it in the driver-core.git tree?

Yes, I'd like to take this one through the nova tree.

Re: [PATCH v4 1/5] rust: types: add `Opaque::zeroed`

[Danilo Krummrich]

On Tue, Apr 15, 2025 at 11:59:25AM +0000, Alice Ryhl wrote:
> On Mon, Apr 14, 2025 at 03:18:04PM +0200, Danilo Krummrich wrote:
> > Analogous to `Opaque::uninit` add `Opaque::zeroed`, which sets the
> > corresponding memory to zero. In contrast to `Opaque::uninit`, the
> > corresponding value, depending on its type, may be initialized.
> > 
> > Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> 
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>

@Miguel: Can I get an ACK for taking this one through the nova tree?

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

[Danilo Krummrich]

On Tue, Apr 15, 2025 at 12:08:24PM +0000, Alice Ryhl wrote:
> On Mon, Apr 14, 2025 at 03:18:06PM +0200, Danilo Krummrich wrote:
> > 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>
> 
> Overall looks okay to me, but I do have some comments.

Thanks for reviewing; unless there are more comments, I'll address those when
applying the series.

> Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Re: [PATCH v4 4/5] rust: auxiliary: add auxiliary registration

[Danilo Krummrich]

On Tue, Apr 15, 2025 at 12:11:16PM +0000, Alice Ryhl wrote:
> On Mon, Apr 14, 2025 at 03:18:07PM +0200, Danilo Krummrich wrote:
> > +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) };
> 
> Does this error-path actually free the box?

Yes, auxiliary_device_uninit() calls put_device() on the underlying struct
device, hence the release() callback is called at this point, which frees the
Box.

Re: [PATCH v4 0/5] Auxiliary bus Rust abstractions

[Greg KH]

On Tue, Apr 15, 2025 at 02:42:54PM +0200, Danilo Krummrich wrote:
> On Tue, Apr 15, 2025 at 01:38:29PM +0200, Greg KH wrote:
> > On Mon, Apr 14, 2025 at 03:18:03PM +0200, Danilo Krummrich wrote:
> > > This series adds Rust abstractions for the auxiliary bus.
> > > 
> > > It implements the required abstractions to write auxiliary drivers and create
> > > auxiliary device registrations. A driver sample illustrates how the
> > > corresponding abstractions work.
> > > 
> > > The auxiliary abstractions are required for the Nova driver project, in order to
> > > connect nova-core with the nova-drm driver.
> > > 
> > > A branch containing the patches can be found in [1].
> > > 
> > > [1] https://web.git.kernel.org/pub/scm/linux/kernel/git/dakr/linux.git/log/?h=rust/auxiliary
> > 
> > Looks good to me, want to take this through a gpu tree so you can build
> > on top of it, or do you want it in the driver-core.git tree?
> 
> Yes, I'd like to take this one through the nova tree.
> 

Please do!

Re: [PATCH v4 1/5] rust: types: add `Opaque::zeroed`

[Miguel Ojeda]

On Tue, Apr 15, 2025 at 2:44 PM Danilo Krummrich <dakr@kernel.org> wrote:
>
> @Miguel: Can I get an ACK for taking this one through the nova tree?

Sure, please go ahead.

Acked-by: Miguel Ojeda <ojeda@kernel.org>

Thanks!

Cheers,
Miguel

Re: [PATCH v4 4/5] rust: auxiliary: add auxiliary registration

[Alice Ryhl]

On Tue, Apr 15, 2025 at 02:50:02PM +0200, Danilo Krummrich wrote:
> On Tue, Apr 15, 2025 at 12:11:16PM +0000, Alice Ryhl wrote:
> > On Mon, Apr 14, 2025 at 03:18:07PM +0200, Danilo Krummrich wrote:
> > > +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) };
> > 
> > Does this error-path actually free the box?
> 
> Yes, auxiliary_device_uninit() calls put_device() on the underlying struct
> device, hence the release() callback is called at this point, which frees the
> Box.

In that case:
Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Re: [PATCH v4 0/5] Auxiliary bus Rust abstractions

[Danilo Krummrich]

On Mon, Apr 14, 2025 at 03:18:03PM +0200, Danilo Krummrich wrote:
> This series adds Rust abstractions for the auxiliary bus.

With the following changes, applied to nova-next, thanks!

  * abstraction: fix typos, `let _ =` => `drop()`, use `kernel::ffi`
  * sample: use `ok_or()` when accessing auxiliary::Device::parent()

- Danilo