[PATCH v2 2/4] rust: devres: replace Devres::new_foreign_owned()

Danilo Krummrich
2025-06-22T16:41:30Z

Replace Devres::new_foreign_owned() with devres::register().

The current implementation of Devres::new_foreign_owned() creates a full
Devres container instance, including the internal Revocable and
completion.

However, none of that is necessary for the intended use of giving full
ownership of an object to devres and getting it dropped once the given
device is unbound.

Hence, implement devres::register(), which is limited to consume the
given data, wrap it in a KBox and drop the KBox once the given device is
unbound, without any other synchronization.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
rust/helpers/device.c | 7 ++++
rust/kernel/cpufreq.rs | 11 +++---
rust/kernel/devres.rs | 70 +++++++++++++++++++++++++++++++++------
rust/kernel/drm/driver.rs | 14 ++++----
4 files changed, 82 insertions(+), 20 deletions(-)

diff --git a/rust/helpers/device.c b/rust/helpers/device.c
index b2135c6686b0..502fef7e9ae8 100644
--- a/rust/helpers/device.c
+++ b/rust/helpers/device.c
@@ -8,3 +8,10 @@ int rust_helper_devm_add_action(struct device *dev,
{
return devm_add_action(dev, action, data);
}
+
+int rust_helper_devm_add_action_or_reset(struct device *dev,
+ void (*action)(void *),
+ void *data)
+{
+ return devm_add_action_or_reset(dev, action, data);
+}
diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
index 11b03e9d7e89..dd84e2b4d7ae 100644
--- a/rust/kernel/cpufreq.rs
+++ b/rust/kernel/cpufreq.rs
@@ -13,7 +13,7 @@
cpu::CpuId,
cpumask,
device::{Bound, Device},
- devres::Devres,
+ devres,
error::{code::*, from_err_ptr, from_result, to_result, Result, VTABLE_DEFAULT_ERROR},
ffi::{c_char, c_ulong},
prelude::*,
@@ -1046,10 +1046,13 @@ pub fn new() -> Result<Self> {

/// Same as [`Registration::new`], but does not return a [`Registration`] instance.
///
- /// Instead the [`Registration`] is owned by [`Devres`] and will be revoked / dropped, once the
+ /// Instead the [`Registration`] is owned by [`devres::register`] and will be dropped, once the
/// device is detached.
- pub fn new_foreign_owned(dev: &Device<Bound>) -> Result {
- Devres::new_foreign_owned(dev, Self::new()?, GFP_KERNEL)
+ pub fn new_foreign_owned(dev: &Device<Bound>) -> Result
+ where
+ T: 'static,
+ {
+ devres::register(dev, Self::new()?, GFP_KERNEL)
}
}

diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 544e50efab43..250073749279 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -9,12 +9,12 @@
alloc::Flags,
bindings,
device::{Bound, Device},
- error::{Error, Result},
+ error::{to_result, Error, Result},
ffi::c_void,
prelude::*,
revocable::{Revocable, RevocableGuard},
sync::{rcu, Arc, Completion},
- types::ARef,
+ types::{ARef, ForeignOwnable},
};

#[pin_data]
@@ -184,14 +184,6 @@ pub fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Self> {
Ok(Devres(inner))
}

- /// Same as [`Devres::new`], but does not return a `Devres` instance. Instead the given `data`
- /// is owned by devres and will be revoked / dropped, once the device is detached.
- pub fn new_foreign_owned(dev: &Device<Bound>, data: T, flags: Flags) -> Result {
- let _ = DevresInner::new(dev, data, flags)?;
-
- Ok(())
- }
-
/// Obtain `&'a T`, bypassing the [`Revocable`].
///
/// This method allows to directly obtain a `&'a T`, bypassing the [`Revocable`], by presenting
@@ -261,3 +253,61 @@ fn drop(&mut self) {
}
}
}
+
+/// Consume `data` and [`Drop::drop`] `data` once `dev` is unbound.
+fn register_foreign<P: ForeignOwnable>(dev: &Device<Bound>, data: P) -> Result {
+ let ptr = data.into_foreign();
+
+ #[allow(clippy::missing_safety_doc)]
+ unsafe extern "C" fn callback<P: ForeignOwnable>(ptr: *mut kernel::ffi::c_void) {
+ // SAFETY: `ptr` is the pointer to the `ForeignOwnable` leaked above and hence valid.
+ let _ = unsafe { P::from_foreign(ptr.cast()) };
+ }
+
+ // SAFETY:
+ // - `dev.as_raw()` is a pointer to a valid and bound device.
+ // - `ptr` is a valid pointer the `ForeignOwnable` devres takes ownership of.
+ to_result(unsafe {
+ // `devm_add_action_or_reset()` also calls `callback` on failure, such that the
+ // `ForeignOwnable` is released eventually.
+ bindings::devm_add_action_or_reset(dev.as_raw(), Some(callback:: ), ptr.cast())
+ })
+}
+
+/// Encapsulate `data` in a [`KBox`] and [`Drop::drop`] `data` once `dev` is unbound.
+///
+/// # Examples
+///
+/// ```no_run
+/// use kernel::{device::{Bound, Device}, devres};
+///
+/// /// Registration of e.g. a class device, IRQ, etc.
+/// struct Registration;
+///
+/// impl Registration {
+/// fn new() -> Self {
+/// // register
+///
+/// Self
+/// }
+/// }
+///
+/// impl Drop for Registration {
+/// fn drop(&mut self) {
+/// // unregister
+/// }
+/// }
+///
+/// fn from_bound_context(dev: &Device<Bound>) -> Result {
+/// devres::register(dev, Registration::new(), GFP_KERNEL)
+/// }
+/// ```
+pub fn register<T, E>(dev: &Device<Bound>, data: impl PinInit<T, E>, flags: Flags) -> Result
+where
+ T: 'static,
+ Error: From<E>,
+{
+ let data = KBox::pin_init(data, flags)?;
+
+ register_foreign(dev, data)
+}
diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
index acb638086131..f63addaf7235 100644
--- a/rust/kernel/drm/driver.rs
+++ b/rust/kernel/drm/driver.rs
@@ -5,9 +5,7 @@
//! C header: [`include/linux/drm/drm_drv.h`](srctree/include/linux/drm/drm_drv.h)

use crate::{
- bindings, device,
- devres::Devres,
- drm,
+ bindings, device, devres, drm,
error::{to_result, Result},
prelude::*,
str::CStr,
@@ -130,18 +128,22 @@ fn new(drm: &drm::Device<T>, flags: usize) -> Result<Self> {
}

/// Same as [`Registration::new`}, but transfers ownership of the [`Registration`] to
- /// [`Devres`].
+ /// [`devres::register`].
pub fn new_foreign_owned(
drm: &drm::Device<T>,
dev: &device::Device<device::Bound>,
flags: usize,
- ) -> Result {
+ ) -> Result
+ where
+ T: 'static,
+ {
if drm.as_ref().as_raw() != dev.as_raw() {
return Err(EINVAL);
}

let reg = Registration::<T>::new(drm, flags)?;
- Devres::new_foreign_owned(dev, reg, GFP_KERNEL)
+
+ devres::register(dev, reg, GFP_KERNEL)
}

/// Returns a reference to the `Device` instance for this registration.
--
2.49.0