[RFC PATCH 08/11] rust: add devres abstraction

[Danilo Krummrich <dakr@redhat.com>]

Add a Rust abstraction for the kernel's devres (device resource
management) implementation.

The Devres type acts as a container to manage the lifetime and
accessibility of device bound resources. Therefore it registers a
devres callback and revokes access to the resource on invocation.

Users of the Devres abstraction can simply free the corresponding
resources in their Drop implementation, which is invoked when either the
Devres instance goes out of scope or the devres callback leads to the
resource being revoked, which implies a call to drop_in_place().

Co-developed-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
---
 rust/helpers.c        |   5 ++
 rust/kernel/devres.rs | 151 ++++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs    |   1 +
 3 files changed, 157 insertions(+)
 create mode 100644 rust/kernel/devres.rs

diff --git a/rust/helpers.c b/rust/helpers.c
index 1d3e800140fc..34061eca05a0 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -173,6 +173,11 @@ void rust_helper_rcu_read_unlock(void)
 EXPORT_SYMBOL_GPL(rust_helper_rcu_read_unlock);
 /* end rcu */
 
+int rust_helper_devm_add_action(struct device *dev, void (*action)(void *), void *data)
+{
+	return devm_add_action(dev, action, data);
+}
+
 /*
  * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
  * use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
new file mode 100644
index 000000000000..bf7bd304cd9b
--- /dev/null
+++ b/rust/kernel/devres.rs
@@ -0,0 +1,151 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Devres abstraction
+//!
+//! [`Devres`] represents an abstraction for the kernel devres (device resource management)
+//! implementation.
+
+use crate::{
+    alloc::Flags,
+    bindings,
+    device::Device,
+    error::{Error, Result},
+    prelude::*,
+    revocable::Revocable,
+    types::ARef,
+};
+
+use core::ffi::c_void;
+use core::ops::Deref;
+
+#[pin_data]
+struct DevresInner<T> {
+    dev: ARef<Device>,
+    #[pin]
+    data: Revocable<T>,
+}
+
+/// This abstraction is meant to be used by subsystems to containerize [`Device`] bound resources to
+/// manage their lifetime.
+///
+/// [`Device`] bound resources should be freed when either the resource goes out of scope or the
+/// [`Device`] is unbound respectively, depending on what happens first.
+///
+/// To achieve that [`Devres`] registers a devres callback on creation, which is called once the
+/// [`Device`] is unbound, revoking access to the encapsulated resource (see also [`Revocable`]).
+///
+/// After the [`Devres`] has been unbound it is not possible to access the encapsulated resource
+/// anymore.
+///
+/// [`Devres`] users should make sure to simply free the corresponding backing resource in `T`'s
+/// [`Drop`] implementation.
+///
+/// # Example
+///
+/// ```
+/// use kernel::devres::Devres;
+///
+/// // See also [`pci::Bar`] for a real example.
+/// struct IoRemap(IoMem);
+///
+/// impl IoRemap {
+///     fn new(usize paddr, usize len) -> Result<Self>{
+///         // assert success
+///         let addr = unsafe { bindings::ioremap(paddr as _); };
+///         let iomem = IoMem::new(addr, len)?;
+///
+///         Ok(IoRemap(iomem))
+///     }
+/// }
+///
+/// impl Drop for IoRemap {
+///     fn drop(&mut self) {
+///         unsafe { bindings::iounmap(self.0.ioptr as _); };
+///     }
+/// }
+///
+/// impl Deref for IoRemap {
+///    type Target = IoMem;
+///
+///    fn deref(&self) -> &Self::Target {
+///        &self.0
+///    }
+/// }
+///
+/// let devres = Devres::new(dev, IoRemap::new(0xBAAAAAAD, 0x4)?, GFP_KERNEL)?;
+///
+/// let res = devres.try_access().ok_or(ENXIO)?;
+/// res.writel(0xBAD);
+/// ```
+///
+pub struct Devres<T> {
+    inner: Pin<Box<DevresInner<T>>>,
+    callback: unsafe extern "C" fn(*mut c_void),
+}
+
+impl<T> DevresInner<T> {
+    fn as_ptr(&self) -> *const DevresInner<T> {
+        self as *const DevresInner<T>
+    }
+
+    fn as_cptr(&self) -> *mut c_void {
+        self.as_ptr() as *mut c_void
+    }
+}
+
+unsafe extern "C" fn devres_callback<T>(inner: *mut c_void) {
+    let inner = inner as *const DevresInner<T>;
+    let inner = unsafe { &*inner };
+
+    inner.data.revoke();
+}
+
+impl<T> Devres<T> {
+    /// Creates a new [`Devres`] instance of the give data.
+    pub fn new(dev: ARef<Device>, data: T, flags: Flags) -> Result<Self> {
+        let callback = devres_callback::<T>;
+
+        let inner = Box::pin_init(
+            pin_init!( DevresInner {
+                dev: dev,
+                data <- Revocable::new(data),
+            }),
+            flags,
+        )?;
+
+        let ret = unsafe {
+            bindings::devm_add_action(inner.dev.as_raw(), Some(callback), inner.as_cptr())
+        };
+
+        if ret != 0 {
+            return Err(Error::from_errno(ret));
+        }
+
+        // We have to store the exact callback function pointer used with
+        // `bindings::devm_add_action` for `bindings::devm_remove_action`. There compiler might put
+        // multiple definitions of `devres_callback<T>` for the same `T` in both the kernel itself
+        // and modules. Hence, we might see different pointer values depending on whether we look
+        // at `devres_callback<T>`'s address from `Devres::new` or `Devres::drop`.
+        Ok(Devres { inner, callback })
+    }
+}
+
+impl<T> Deref for Devres<T> {
+    type Target = Revocable<T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.inner.data
+    }
+}
+
+impl<T> Drop for Devres<T> {
+    fn drop(&mut self) {
+        unsafe {
+            bindings::devm_remove_action(
+                self.inner.dev.as_raw(),
+                Some(self.callback),
+                self.inner.as_cptr(),
+            )
+        }
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index d7d415429517..11645060b444 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -30,6 +30,7 @@
 pub mod alloc;
 mod build_assert;
 pub mod device;
+pub mod devres;
 pub mod driver;
 pub mod error;
 pub mod init;
-- 
2.45.1