rust: auxiliary: add registration data to auxiliary devices

Add a registration_data pointer to struct auxiliary_device, allowing the
registering (parent) driver to attach private data to the device at
registration time and retrieve it later when called back by the
auxiliary (child) driver.

By tying the data to the device's registration, Rust drivers can bind
the lifetime of device resources to it, since the auxiliary bus
guarantees that the parent driver remains bound while the auxiliary
device is bound.

On the Rust side, Registration<T> takes ownership of the data via
ForeignOwnable. A TypeId is stored alongside the data for runtime type
checking, making Device::registration_data<T>() a safe method.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260505152400.3905096-3-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

4 files changed, 173 insertions, 75 deletions

diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
index 84b0e1703150..8fe484d357f6 100644
--- a/drivers/gpu/nova-core/driver.rs
+++ b/drivers/gpu/nova-core/driver.rs
@@ -32,8 +32,7 @@ static AUXILIARY_ID_COUNTER: Atomic<u32> = Atomic::new(0);
 pub(crate) struct NovaCore {
     #[pin]
     pub(crate) gpu: Gpu,
-    #[pin]
-    _reg: Devres<auxiliary::Registration>,
+    _reg: Devres<auxiliary::Registration<()>>,
 }
 
 const BAR0_SIZE: usize = SZ_16M;
@@ -96,14 +95,15 @@ impl pci::Driver for NovaCore {
 
             Ok(try_pin_init!(Self {
                 gpu <- Gpu::new(pdev, bar.clone(), bar.access(pdev.as_ref())?),
-                _reg <- auxiliary::Registration::new(
+                _reg: auxiliary::Registration::new(
                     pdev.as_ref(),
                     c"nova-drm",
                     // TODO[XARR]: Use XArray or perhaps IDA for proper ID allocation/recycling. For
                     // now, use a simple atomic counter that never recycles IDs.
                     AUXILIARY_ID_COUNTER.fetch_add(1, Relaxed),
-                    crate::MODULE_NAME
-                ),
+                    crate::MODULE_NAME,
+                    (),
+                )?,
             }))
         })
     }
diff --git a/include/linux/auxiliary_bus.h b/include/linux/auxiliary_bus.h
index bc09b55e3682..4e1ad8ccbcdd 100644
--- a/include/linux/auxiliary_bus.h
+++ b/include/linux/auxiliary_bus.h
@@ -62,6 +62,9 @@
  * @sysfs.irqs: irqs xarray contains irq indices which are used by the device,
  * @sysfs.lock: Synchronize irq sysfs creation,
  * @sysfs.irq_dir_exists: whether "irqs" directory exists,
+ * @registration_data_rust: private data owned by the registering (parent)
+ *                          driver; valid for as long as the device is
+ *                          registered with the driver core,
  *
  * An auxiliary_device represents a part of its parent device's functionality.
  * It is given a name that, combined with the registering drivers
@@ -148,6 +151,7 @@ struct auxiliary_device {
 		struct mutex lock; /* Synchronize irq sysfs creation */
 		bool irq_dir_exists;
 	} sysfs;
+	void *registration_data_rust;
 };
 
 /**
diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
index 93c0db1f6655..19aec94aa95b 100644
--- a/rust/kernel/auxiliary.rs
+++ b/rust/kernel/auxiliary.rs
@@ -19,12 +19,17 @@ use crate::{
         to_result, //
     },
     prelude::*,
-    types::Opaque,
+    types::{
+        ForeignOwnable,
+        Opaque, //
+    },
     ThisModule, //
 };
 use core::{
+    any::TypeId,
     marker::PhantomData,
     mem::offset_of,
+    pin::Pin,
     ptr::{
         addr_of_mut,
         NonNull, //
@@ -257,6 +262,40 @@ impl Device<device::Bound> {
         // SAFETY: A bound auxiliary device always has a bound parent device.
         unsafe { parent.as_bound() }
     }
+
+    /// Returns a pinned reference to the registration data set by the registering (parent) driver.
+    ///
+    /// Returns [`EINVAL`] if `T` does not match the type used by the parent driver when calling
+    /// [`Registration::new()`].
+    ///
+    /// Returns [`ENOENT`] if no registration data has been set, e.g. when the device was
+    /// registered by a C driver.
+    pub fn registration_data<T: 'static>(&self) -> Result<Pin<&T>> {
+        // SAFETY: By the type invariant, `self.as_raw()` is a valid `struct auxiliary_device`.
+        let ptr = unsafe { (*self.as_raw()).registration_data_rust };
+        if ptr.is_null() {
+            dev_warn!(
+                self.as_ref(),
+                "No registration data set; parent is not a Rust driver.\n"
+            );
+            return Err(ENOENT);
+        }
+
+        // SAFETY: `ptr` is non-null and was set via `into_foreign()` in `Registration::new()`;
+        // `RegistrationData` is `#[repr(C)]` with `type_id` at offset 0, so reading a `TypeId`
+        // at the start of the allocation is valid regardless of `T`.
+        let type_id = unsafe { ptr.cast::<TypeId>().read() };
+        if type_id != TypeId::of::<T>() {
+            return Err(EINVAL);
+        }
+
+        // SAFETY: The `TypeId` check above confirms that the stored type is `T`; `ptr` remains
+        // valid until `Registration::drop()` calls `from_foreign()`.
+        let wrapper = unsafe { Pin::<KBox<RegistrationData<T>>>::borrow(ptr) };
+
+        // SAFETY: `data` is a structurally pinned field of `RegistrationData`.
+        Ok(unsafe { wrapper.map_unchecked(|w| &w.data) })
+    }
 }
 
 impl Device {
@@ -326,87 +365,132 @@ unsafe impl Send for Device {}
 // (i.e. `Device<Normal>) are thread safe.
 unsafe impl Sync for Device {}
 
+/// Wrapper that stores a [`TypeId`] alongside the registration data for runtime type checking.
+#[repr(C)]
+#[pin_data]
+struct RegistrationData<T> {
+    type_id: TypeId,
+    #[pin]
+    data: T,
+}
+
 /// The registration of an auxiliary device.
 ///
 /// This type represents the registration of a [`struct auxiliary_device`]. When its parent device
 /// is unbound, the corresponding auxiliary device will be unregistered from the system.
 ///
+/// The type parameter `T` is the type of the registration data owned by the registering (parent)
+/// driver. It can be accessed by the auxiliary driver through
+/// [`Device::registration_data()`].
+///
 /// # Invariants
 ///
-/// `self.0` always holds a valid pointer to an initialized and registered
-/// [`struct auxiliary_device`].
-pub struct Registration(NonNull<bindings::auxiliary_device>);
+/// `self.adev` always holds a valid pointer to an initialized and registered
+/// [`struct auxiliary_device`] whose `registration_data_rust` field points to a
+/// valid `Pin<KBox<RegistrationData<T>>>`.
+pub struct Registration<T: 'static> {
+    adev: NonNull<bindings::auxiliary_device>,
+    _data: PhantomData<T>,
+}
 
-impl Registration {
-    /// Create and register a new auxiliary device.
-    pub fn new<'a>(
-        parent: &'a device::Device<device::Bound>,
-        name: &'a CStr,
+impl<T: Send + Sync + 'static> Registration<T> {
+    /// Create and register a new auxiliary device with the given registration data.
+    ///
+    /// The `data` is owned by the registration and can be accessed through the auxiliary device
+    /// via [`Device::registration_data()`].
+    pub fn new<E>(
+        parent: &device::Device<device::Bound>,
+        name: &CStr,
         id: u32,
-        modname: &'a CStr,
-    ) -> impl PinInit<Devres<Self>, Error> + 'a {
-        pin_init::pin_init_scope(move || {
-            let boxed = KBox::new(Opaque::<bindings::auxiliary_device>::zeroed(), GFP_KERNEL)?;
-            let adev = boxed.get();
+        modname: &CStr,
+        data: impl PinInit<T, E>,
+    ) -> Result<Devres<Self>>
+    where
+        Error: From<E>,
+    {
+        let data = KBox::pin_init::<Error>(
+            try_pin_init!(RegistrationData {
+                type_id: TypeId::of::<T>(),
+                data <- data,
+            }),
+            GFP_KERNEL,
+        )?;
 
-            // 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;
-            }
+        let boxed: KBox<Opaque<bindings::auxiliary_device>> = KBox::zeroed(GFP_KERNEL)?;
+        let adev = boxed.get();
 
-            // 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) };
+        // 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;
+            (*adev).registration_data_rust = data.into_foreign();
+        }
 
-            // 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 not been initialized yet.
+        unsafe { bindings::auxiliary_device_init(adev) };
 
-            // SAFETY:
-            // - `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`, which
-            //   has been initialized,
-            // - `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 initialized.
-                unsafe { bindings::auxiliary_device_uninit(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);
 
-                return Err(Error::from_errno(ret));
-            }
+        // SAFETY:
+        // - `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`, which
+        //   has been initialized,
+        // - `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: `registration_data` was set above via `into_foreign()`.
+            drop(unsafe {
+                Pin::<KBox<RegistrationData<T>>>::from_foreign((*adev).registration_data_rust)
+            });
 
-            // INVARIANT: The device will remain registered until `auxiliary_device_delete()` is
-            // called, which happens in `Self::drop()`.
-            Ok(Devres::new(
-                parent,
-                // SAFETY: `adev` is guaranteed to be non-null, since the `KBox` was allocated
-                // successfully.
-                Self(unsafe { NonNull::new_unchecked(adev) }),
-            ))
-        })
+            // SAFETY: `adev` is guaranteed to be a valid pointer to a
+            // `struct auxiliary_device`, which has been initialized.
+            unsafe { bindings::auxiliary_device_uninit(adev) };
+
+            return Err(Error::from_errno(ret));
+        }
+
+        // INVARIANT: The device will remain registered until `auxiliary_device_delete()` is
+        // called, which happens in `Self::drop()`.
+        let reg = Self {
+            // SAFETY: `adev` is guaranteed to be non-null, since the `KBox` was allocated
+            // successfully.
+            adev: unsafe { NonNull::new_unchecked(adev) },
+            _data: PhantomData,
+        };
+
+        Devres::new::<core::convert::Infallible>(parent, reg)
     }
 }
 
-impl Drop for Registration {
+impl<T: 'static> Drop for Registration<T> {
     fn drop(&mut self) {
-        // SAFETY: By the type invariant of `Self`, `self.0.as_ptr()` is a valid registered
+        // SAFETY: By the type invariant of `Self`, `self.adev.as_ptr()` is a valid registered
         // `struct auxiliary_device`.
-        unsafe { bindings::auxiliary_device_delete(self.0.as_ptr()) };
+        unsafe { bindings::auxiliary_device_delete(self.adev.as_ptr()) };
+
+        // SAFETY: `registration_data` was set in `new()` via `into_foreign()`.
+        drop(unsafe {
+            Pin::<KBox<RegistrationData<T>>>::from_foreign(
+                (*self.adev.as_ptr()).registration_data_rust,
+            )
+        });
 
         // 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
+        // SAFETY: By the type invariant of `Self`, `self.adev.as_ptr()` is a valid registered
         // `struct auxiliary_device`.
-        unsafe { bindings::auxiliary_device_uninit(self.0.as_ptr()) };
+        unsafe { bindings::auxiliary_device_uninit(self.adev.as_ptr()) };
     }
 }
 
 // SAFETY: A `Registration` of a `struct auxiliary_device` can be released from any thread.
-unsafe impl Send for Registration {}
+unsafe impl<T: Send + Sync> Send for Registration<T> {}
 
 // SAFETY: `Registration` does not expose any methods or fields that need synchronization.
-unsafe impl Sync for Registration {}
+unsafe impl<T: Send + Sync> Sync for Registration<T> {}
diff --git a/samples/rust/rust_driver_auxiliary.rs b/samples/rust/rust_driver_auxiliary.rs
index 5c5a5105a3ff..319ef734c02b 100644
--- a/samples/rust/rust_driver_auxiliary.rs
+++ b/samples/rust/rust_driver_auxiliary.rs
@@ -17,8 +17,6 @@ use kernel::{
     InPlaceModule, //
 };
 
-use core::any::TypeId;
-
 const MODULE_NAME: &CStr = <LocalModule as kernel::ModuleMetadata>::NAME;
 const AUXILIARY_NAME: &CStr = c"auxiliary";
 
@@ -49,13 +47,13 @@ impl auxiliary::Driver for AuxiliaryDriver {
     }
 }
 
-#[pin_data]
+struct Data {
+    index: u32,
+}
+
 struct ParentDriver {
-    private: TypeId,
-    #[pin]
-    _reg0: Devres<auxiliary::Registration>,
-    #[pin]
-    _reg1: Devres<auxiliary::Registration>,
+    _reg0: Devres<auxiliary::Registration<Data>>,
+    _reg1: Devres<auxiliary::Registration<Data>>,
 }
 
 kernel::pci_device_table!(
@@ -71,10 +69,21 @@ impl pci::Driver for ParentDriver {
     const ID_TABLE: pci::IdTable<Self::IdInfo> = &PCI_TABLE;
 
     fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> impl PinInit<Self, Error> {
-        try_pin_init!(Self {
-            private: TypeId::of::<Self>(),
-            _reg0 <- auxiliary::Registration::new(pdev.as_ref(), AUXILIARY_NAME, 0, MODULE_NAME),
-            _reg1 <- auxiliary::Registration::new(pdev.as_ref(), AUXILIARY_NAME, 1, MODULE_NAME),
+        Ok(Self {
+            _reg0: auxiliary::Registration::new(
+                pdev.as_ref(),
+                AUXILIARY_NAME,
+                0,
+                MODULE_NAME,
+                Data { index: 0 },
+            )?,
+            _reg1: auxiliary::Registration::new(
+                pdev.as_ref(),
+                AUXILIARY_NAME,
+                1,
+                MODULE_NAME,
+                Data { index: 1 },
+            )?,
         })
     }
 }
@@ -83,7 +92,8 @@ impl ParentDriver {
     fn connect(adev: &auxiliary::Device<Bound>) -> Result {
         let dev = adev.parent();
         let pdev: &pci::Device<Bound> = dev.try_into()?;
-        let drvdata = dev.drvdata::<Self>()?;
+
+        let data = adev.registration_data::<Data>()?;
 
         dev_info!(
             dev,
@@ -95,8 +105,8 @@ impl ParentDriver {
 
         dev_info!(
             dev,
-            "We have access to the private data of {:?}.\n",
-            drvdata.private
+            "Connected to auxiliary device with index {}.\n",
+            data.index
         );
 
         Ok(())