Merge patch series "ForLt/CovariantForLt split, auxiliary closure API and DevresLt"

Danilo Krummrich <dakr@kernel.org> says:

The ForLt trait currently guarantees covariance, which allows safe
lifetime shortening via cast_ref(). However, some types (e.g. those
containing Mutex<&'bound T>) are invariant over their lifetime parameter
and cannot safely use cast_ref().

This series splits ForLt into two traits:

  - ForLt: base trait for all lifetime-parameterized types, providing
    only the Of<'a> GAT.

  - CovariantForLt: unsafe subtrait that guarantees covariance,
    providing a safe cast_ref() method.

For invariant types, a closure-based API (registration_data_with()) is
added to the auxiliary subsystem. The closure's HRTB prevents the caller
from choosing a concrete lifetime, which would be unsound for invariant
types.

On top of that, this series adds DevresLt<F: ForLt>, a thin wrapper
around Devres<F::Of<'static>> that shortens the stored 'static lifetime
back to the caller's borrow scope. DevresLt provides both closure-based
access (access_with/try_access_with for ForLt types) and direct
reference access (access/try_access for CovariantForLt types).

Also implement ForLt and CovariantForLt for Bar, IoMem and
ExclusiveIoMem, and update their into_devres() methods to return
DevresLt. Provide convenience type aliases DevresBar, DevresIoMem and
DevresExclusiveIoMem.

Link: https://patch.msgid.link/20260626183630.2585057-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 106 insertions, 0 deletions

diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 11ce500e9b76..b7c075a39ba4 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -24,6 +24,8 @@ use crate::{
         Arc, //
     },
     types::{
+        CovariantForLt,
+        ForLt,
         ForeignOwnable,
         Opaque, //
     },
@@ -365,6 +367,110 @@ impl<T: Send + 'static> Drop for Devres<T> {
     }
 }
 
+/// Guard returned by [`DevresLt::try_access`].
+///
+/// Dereferences to `F::Of<'a>`, shortening the lifetime of the stored data to the guard's borrow
+/// lifetime.
+pub struct DevresGuard<'a, F: CovariantForLt>(RevocableGuard<'a, F::Of<'static>>);
+
+impl<'a, F: CovariantForLt> core::ops::Deref for DevresGuard<'a, F> {
+    type Target = F::Of<'a>;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        F::cast_ref(&*self.0)
+    }
+}
+
+/// Device-managed resource with [`ForLt`](trait@ForLt)-aware access.
+///
+/// `DevresLt` wraps [`Devres`] and shortens the stored `'static` lifetime to the caller's borrow
+/// lifetime in all access methods.
+///
+/// Types that implement [`trait@CovariantForLt`] get direct-reference accessors ([`Self::access`],
+/// [`Self::try_access`]). Plain [`ForLt`](trait@ForLt) types use closure-based accessors
+/// ([`Self::access_with`], [`Self::try_access_with`]).
+pub struct DevresLt<F: ForLt>(Devres<F::Of<'static>>)
+where
+    for<'a> F::Of<'a>: Send;
+
+impl<F: ForLt> DevresLt<F>
+where
+    for<'a> F::Of<'a>: Send,
+{
+    /// Creates a new [`DevresLt`] instance of the given `data`.
+    ///
+    /// # Safety
+    ///
+    /// The data must remain valid for the device's full bound scope. [`DevresLt`] allows
+    /// access until the device is unbound, which may outlast `'a`.
+    pub unsafe fn new<'a, E>(
+        dev: &'a Device<Bound>,
+        data: impl PinInit<F::Of<'a>, E>,
+    ) -> Result<Self>
+    where
+        Error: From<E>,
+    {
+        // SAFETY: The caller guarantees the data is valid for the device's full bound scope.
+        // Lifetimes do not affect layout, so F::Of<'a> and F::Of<'static> have identical
+        // representation; casting the slot pointer is sound.
+        let data = unsafe {
+            pin_init::pin_init_from_closure::<F::Of<'static>, E>(move |slot| {
+                data.__pinned_init(slot.cast())
+            })
+        };
+
+        Ok(Self(Devres::new(dev, data)?))
+    }
+
+    /// Return a reference of the [`Device`] this [`DevresLt`] instance has been created with.
+    #[inline]
+    pub fn device(&self) -> &Device {
+        self.0.device()
+    }
+
+    /// Obtain `&F::Of<'_>`, bypassing the [`Revocable`], through a closure.
+    ///
+    /// This method works like [`DevresLt::access`](DevresLt::access) but accepts any
+    /// [`trait@ForLt`] type, not just [`trait@CovariantForLt`].
+    #[inline]
+    pub fn access_with<R, G>(&self, dev: &Device<Bound>, f: G) -> Result<R>
+    where
+        G: for<'a> FnOnce(&F::Of<'a>) -> R,
+    {
+        self.0.access(dev).map(f)
+    }
+
+    /// [`DevresLt`] accessor for [`Revocable::try_access_with`].
+    #[inline]
+    pub fn try_access_with<R, G>(&self, f: G) -> Option<R>
+    where
+        G: for<'a> FnOnce(&F::Of<'a>) -> R,
+    {
+        self.0.data().try_access_with(f)
+    }
+}
+
+impl<F: CovariantForLt> DevresLt<F>
+where
+    for<'a> F::Of<'a>: Send,
+{
+    /// Obtain `&'a F::Of<'a>`, bypassing the [`Revocable`].
+    ///
+    /// This method works like [`Devres::access`], but shortens the returned reference's lifetime
+    /// from `'static` to `'a` via [`CovariantForLt::cast_ref`].
+    #[inline]
+    pub fn access<'a>(&'a self, dev: &'a Device<Bound>) -> Result<&'a F::Of<'a>> {
+        self.0.access(dev).map(F::cast_ref)
+    }
+
+    /// [`DevresLt`] accessor for [`Revocable::try_access`].
+    #[inline]
+    pub fn try_access(&self) -> Option<DevresGuard<'_, F>> {
+        self.0.data().try_access().map(DevresGuard)
+    }
+}
+
 /// Consume `data` and [`Drop::drop`] `data` once `dev` is unbound.
 fn register_foreign<P>(dev: &Device<Bound>, data: P) -> Result
 where