[PATCH 14/24] rust: auxiliary: generalize Registration over ForLt

[Danilo Krummrich <dakr@kernel.org>]

Generalize Registration<T> to Registration<F: ForLt> and
Device::registration_data<F: ForLt>() to return Pin<&F::Of<'_>>.

The stored 'static lifetime is shortened to the borrow lifetime of &self
via ForLt::cast_ref; ForLt's covariance guarantee makes this sound.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 drivers/gpu/nova-core/driver.rs       |  4 +-
 rust/kernel/auxiliary.rs              | 80 ++++++++++++++++++---------
 samples/rust/rust_driver_auxiliary.rs |  7 ++-
 3 files changed, 61 insertions(+), 30 deletions(-)

diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
index 815489dd92d0..2a17fc99d9b6 100644
--- a/drivers/gpu/nova-core/driver.rs
+++ b/drivers/gpu/nova-core/driver.rs
@@ -21,6 +21,7 @@
         },
         Arc,
     },
+    types::ForLt,
 };
 
 use crate::gpu::Gpu;
@@ -32,7 +33,8 @@
 pub(crate) struct NovaCore {
     #[pin]
     pub(crate) gpu: Gpu,
-    _reg: Devres<auxiliary::Registration<()>>,
+    #[allow(clippy::type_complexity)]
+    _reg: Devres<auxiliary::Registration<ForLt!(())>>,
 }
 
 const BAR0_SIZE: usize = SZ_16M;
diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
index f593a21a16be..e27de4be4d87 100644
--- a/rust/kernel/auxiliary.rs
+++ b/rust/kernel/auxiliary.rs
@@ -287,12 +287,19 @@ pub fn parent(&self) -> &device::Device<device::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
+    /// `F` is the [`ForLt`](trait@ForLt) encoding of the data type. The returned
+    /// reference has its lifetime shortened from `'static` to `&self`'s borrow lifetime via
+    /// [`ForLt::cast_ref`].
+    ///
+    /// Returns [`EINVAL`] if `F` 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>> {
+    pub fn registration_data<F: ForLt>(&self) -> Result<Pin<&F::Of<'_>>>
+    where
+        F::Of<'static>: 'static,
+    {
         // 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() {
@@ -305,18 +312,23 @@ pub fn registration_data<T: 'static>(&self) -> Result<Pin<&T>> {
 
         // 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`.
+        // at the start of the allocation is valid regardless of `F`.
         let type_id = unsafe { ptr.cast::<TypeId>().read() };
-        if type_id != TypeId::of::<T>() {
+        if type_id != TypeId::of::<F::Of<'static>>() {
             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: The `TypeId` check above confirms that the stored type matches
+        // `F::Of<'static>`; `ptr` remains valid until `Registration::drop()` calls
+        // `from_foreign()`.
+        let wrapper = unsafe { Pin::<KBox<RegistrationData<F::Of<'static>>>>::borrow(ptr) };
 
         // SAFETY: `data` is a structurally pinned field of `RegistrationData`.
-        Ok(unsafe { wrapper.map_unchecked(|w| &w.data) })
+        let pinned: Pin<&F::Of<'static>> = unsafe { wrapper.map_unchecked(|w| &w.data) };
+
+        // SAFETY: ForLt guarantees covariance, making it sound to shorten 'static to &self's
+        // lifetime via cast_ref.
+        Ok(unsafe { Pin::new_unchecked(F::cast_ref(pinned.get_ref())) })
     }
 }
 
@@ -405,43 +417,54 @@ struct RegistrationData<T> {
 /// 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()`].
+/// The type parameter `F` is a [`ForLt`](trait@ForLt) encoding of the registration
+/// data type. For non-lifetime-parameterized types, use [`ForLt!(T)`](macro@ForLt).
+/// The data can be accessed by the auxiliary driver through [`Device::registration_data()`].
 ///
 /// # Invariants
 ///
 /// `self.adev` always holds a valid pointer to an initialized and registered
 /// [`struct auxiliary_device`], and `registration_data` points to a valid
-/// `Pin<KBox<RegistrationData<T>>>`.
-pub struct Registration<T: 'static> {
+/// `Pin<KBox<RegistrationData<F::Of<'static>>>>`.
+pub struct Registration<F: ForLt>
+where
+    F::Of<'static>: 'static,
+{
     adev: NonNull<bindings::auxiliary_device>,
-    _data: PhantomData<T>,
+    _data: PhantomData<F>,
 }
 
-impl<T: Send + 'static> Registration<T> {
+impl<F: ForLt> Registration<F>
+where
+    F::Of<'static>: Send + 'static,
+{
     /// 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>,
+    pub fn new<'a, E>(
+        parent: &'a device::Device<device::Bound>,
         name: &CStr,
         id: u32,
         modname: &CStr,
-        data: impl PinInit<T, E>,
+        data: impl PinInit<F::Of<'a>, E>,
     ) -> Result<Devres<Self>>
     where
         Error: From<E>,
     {
         let data = KBox::pin_init::<Error>(
             try_pin_init!(RegistrationData {
-                type_id: TypeId::of::<T>(),
+                type_id: TypeId::of::<F::Of<'static>>(),
                 data <- data,
             }),
             GFP_KERNEL,
         )?;
 
+        // SAFETY: Lifetimes are erased and do not affect layout, so RegistrationData<F::Of<'a>>
+        // and RegistrationData<F::Of<'static>> have identical representation.
+        let data: Pin<KBox<RegistrationData<F::Of<'static>>>> =
+            unsafe { core::mem::transmute(data) };
+
         let boxed = KBox::new(Opaque::<bindings::auxiliary_device>::zeroed(), GFP_KERNEL)?;
         let adev = boxed.get();
 
@@ -470,9 +493,11 @@ pub fn new<E>(
         let ret = unsafe { bindings::__auxiliary_device_add(adev, modname.as_char_ptr()) };
         if ret != 0 {
             // SAFETY: `registration_data` was set above via `into_foreign()`.
-            let _ = unsafe {
-                Pin::<KBox<RegistrationData<T>>>::from_foreign((*adev).registration_data_rust)
-            };
+            drop(unsafe {
+                Pin::<KBox<RegistrationData<F::Of<'static>>>>::from_foreign(
+                    (*adev).registration_data_rust,
+                )
+            });
 
             // SAFETY: `adev` is guaranteed to be a valid pointer to a
             // `struct auxiliary_device`, which has been initialized.
@@ -494,7 +519,10 @@ pub fn new<E>(
     }
 }
 
-impl<T: 'static> Drop for Registration<T> {
+impl<F: ForLt> Drop for Registration<F>
+where
+    F::Of<'static>: 'static,
+{
     fn drop(&mut self) {
         // SAFETY: By the type invariant of `Self`, `self.adev.as_ptr()` is a valid registered
         // `struct auxiliary_device`.
@@ -502,7 +530,7 @@ fn drop(&mut self) {
 
         // SAFETY: `registration_data` was set in `new()` via `into_foreign()`.
         drop(unsafe {
-            Pin::<KBox<RegistrationData<T>>>::from_foreign(
+            Pin::<KBox<RegistrationData<F::Of<'static>>>>::from_foreign(
                 (*self.adev.as_ptr()).registration_data_rust,
             )
         });
@@ -516,7 +544,7 @@ fn drop(&mut self) {
 }
 
 // SAFETY: A `Registration` of a `struct auxiliary_device` can be released from any thread.
-unsafe impl<T: Send> Send for Registration<T> {}
+unsafe impl<F: ForLt> Send for Registration<F> where F::Of<'static>: Send {}
 
 // SAFETY: `Registration` does not expose any methods or fields that need synchronization.
-unsafe impl<T: Send> Sync for Registration<T> {}
+unsafe impl<F: ForLt> Sync for Registration<F> where F::Of<'static>: Send {}
diff --git a/samples/rust/rust_driver_auxiliary.rs b/samples/rust/rust_driver_auxiliary.rs
index d35963ac7fa4..4ad619c5731e 100644
--- a/samples/rust/rust_driver_auxiliary.rs
+++ b/samples/rust/rust_driver_auxiliary.rs
@@ -55,9 +55,10 @@ struct Data {
     index: u32,
 }
 
+#[allow(clippy::type_complexity)]
 struct ParentDriver {
-    _reg0: Devres<auxiliary::Registration<Data>>,
-    _reg1: Devres<auxiliary::Registration<Data>>,
+    _reg0: Devres<auxiliary::Registration<ForLt!(Data)>>,
+    _reg1: Devres<auxiliary::Registration<ForLt!(Data)>>,
 }
 
 kernel::pci_device_table!(
@@ -100,7 +101,7 @@ fn connect(adev: &auxiliary::Device<Bound>) -> Result {
         let dev = adev.parent();
         let pdev: &pci::Device<Bound> = dev.try_into()?;
 
-        let data = adev.registration_data::<Data>()?;
+        let data = adev.registration_data::<ForLt!(Data)>()?;
 
         dev_info!(
             dev,
-- 
2.54.0