[PATCH v6 0/1] rust: introduce abstractions for fwctl

[PATCH v6 0/1] rust: introduce abstractions for fwctl

[Zhi Wang]

In the NVIDIA vGPU RFC [1], the vGPU type blobs must be provided to the GSP
before userspace can enumerate available vGPU types and create vGPU
instances. The original design relied on the firmware loading interface,
but fwctl is a more natural fit for this use case, as it is designed for
uploading configuration or firmware data required before the device becomes
operational.

This series introduces a Rust abstraction over the fwctl subsystem,
providing safe and idiomatic bindings.

The series is now a single Rust patch. The fwctl core release hook from v4 is
no longer needed. Instead, `Registration` owns the callback-visible driver
data and ties it to the parent device binding lifetime using a GAT on
`Operations`. `Device<T>` remains the refcounted fwctl object, while
callbacks borrow registration data for the callback lifetime.

The Rust fwctl module allows Rust drivers to integrate with the existing
C-side fwctl core through a typed trait interface. It provides:

  - `Operations` trait: defines driver-specific callbacks: `open()`,
    `close()`, `info()`, and `fw_rpc()`. The implementing type itself
    serves as the per-FD user context, one instance per open().

  - `RegistrationData<'a>`: a GAT associated type owned by `Registration`
    and borrowed by callbacks under the fwctl registration lock.

  - `Device<T>`: wraps `struct fwctl_device` and stores the pointer to
    registration-owned driver data used during callbacks.

  - `Registration<T>`: registration and automatic unregistration of
    `struct fwctl_device` objects.

  - `RpcScope` / `FwRpcResponse`: type-safe enums for RPC scope and response
    handling, keeping unsafe pointer manipulation confined to the abstraction
    layer.

`rust/kernel/lib.rs` is updated to conditionally include this module under
`CONFIG_RUST_FWCTL_ABSTRACTIONS`.

v6:

- Use a GAT lifetime for `Operations::RegistrationData<'a>` instead of
  requiring `ForLt`. (Danilo)
- Require `FWCTL=y` for `RUST_FWCTL_ABSTRACTIONS` to avoid a built-in Rust
  abstraction depending on a modular fwctl core.
- Add `#[inline]` to small forwarding helpers.
- Return `EINVAL` for an unrecognized RPC scope instead of the internal
  `ENOTSUPP` errno.

Link to Danilo's GAT diff: [2]
Link to v4: [3]

v5:

- Rebase on top of the current drm-rust-next.
- Adopt Danilo's higher-ranked lifetime model for callback-visible
  registration data.
- Drop the fwctl core release hook patch.
- Reject `FwRpcResponse::InPlace(len)` when `len` is larger than the input
  buffer length.
- Add the missing BNXT fwctl device type to the Rust `DeviceType` enum.
- Document that `Registration::new()` requires an unregistered `Device` with
  no live `Registration`, and no concurrent registration attempt for that
  device.

v4:

- Rebase on top of the current drm-rust-next.
- Split out the fwctl core release hook before the Rust abstraction.
- Drop the fwctl init-ordering change from this series; it is already in
  drm-rust-next as commit a55f80233f38 ("fwctl: Fix class init ordering to
  avoid NULL pointer dereference on device removal").
- Add compile-time layout checks for the embedded `struct fwctl_device` and
  `struct fwctl_uctx` offset assumptions. (Jason)
- Use `const_assert!()` for generic layout assertions. (Zhi)
- Require `Operations` and its `DeviceData` to be `Send + Sync`. (Danilo)
- Pass pinned shared references to `info()` and `fw_rpc()`. (Danilo)
- Make `Operations::open()` return an initializer directly and report open
  failures through the initializer error path. (Danilo)
- Drop `DeviceData` from the fwctl device release hook.
- Fix clippy warnings for raw pointer casts and unsafe blocks. (Danilo)
- Fix the rustdoc broken link warning. (Danilo)

Link to v3: [4]

v3:

Quite some updates in this version. Here you can find the example
nova-core fwctl driver [5]. The interface is still WIP so it is just to
demonstrate the use of the rust fwctl abstractions.

Comments from folks:

- Use an enum for the return of fw_rpc. (Joel)
- Remove FWCTL_DEVICE_TYPE_RUST_FWCTL_TEST together with the sample
  driver. (Jason)
- Remove DeviceType:Error. (Gary)
- Add __rust_helper for fwctl_get/fwctl_put. (Gary)
- Refine the design of the device private data. Now it has a similar
  device private data structure as DRM. (Danilo)
- Separate fwctl alloc and register in the abstractions. (Jason)
- Registration::new() now takes &fwctl::Device<T> and the parent
  &Device<Bound> to align with other class device abstractions. (Danilo)
- Update the Registration SAFETY comments. (Danilo & Jason)
- Take self as per-FD user context in callbacks. (Danilo)
- {open, close}_uctx -> {open, close}(). open() now takes &Device<Self>.
  (Danilo)

Updates from me:

- Introduce enums for fwctl RPC scope.
- Introduce AlwaysRefCounted to avoid hacks after introducing the
  refined flow of device private data.
- Introduce default implementation of close()/info().
- Fix a leak: Drop T::UserCtx in the close_uctx_callback explicitly.

v2:

- Don't open fwctl_put(). Add a rust helper. (Jason/Danilo)
- Wrap Registration with Devres to guarantee proper lifetime management.
  (Jason/Danilo)
- Rename FwctlOps to Operations, FwctlUserCtx to UserCtx, FwctlDevice to
  Device. (Danilo)
- Use fwctl::DeviceType enum instead of raw u32 for DEVICE_TYPE. (Danilo)
- Change fwctl_uctx field in UserCtx to Opaque<bindings::fwctl_uctx> and
  make it private. (Danilo)
- Provide Deref and DerefMut implementations for UserCtx::uctx. (Danilo)
- Add UserCtx::parent_device_from_raw() to simplify parent device access.
- Use cast() and cast_mut() instead of manual pointer casts. (Danilo)
- Implement AlwaysRefCounted for Device and use ARef<Device> in
  Registration. (Danilo)
- Add rust_helper_fwctl_get() for reference counting.
- Improve safety comments for slice::from_raw_parts_mut() in
  fw_rpc_callback. (Danilo)
- Convert imports to vertical style.
- Fix all clippy warnings.

v1:

- Initial submission introducing fwctl Rust abstractions.

[1] https://lore.kernel.org/all/20250903221111.3866249-1-zhiw@nvidia.com/
[2] https://lore.kernel.org/all/DJJW7X4ESDSM.QCVYK2FC7ZR3@kernel.org/
[3] https://lore.kernel.org/all/20260624091758.1678092-1-zhiw@nvidia.com/
[4] https://lore.kernel.org/all/20260217204909.211793-1-zhiw@nvidia.com/
[5] https://github.com/zhiwang-nvidia/nova-core/commit/2068da7e8caf58da9584b0aa6c81fed8f547d59f

Zhi Wang (1):
  rust: introduce abstractions for fwctl

 drivers/fwctl/Kconfig           |  12 +
 rust/bindings/bindings_helper.h |   1 +
 rust/helpers/fwctl.c            |  17 +
 rust/helpers/helpers.c          |   3 +-
 rust/kernel/fwctl.rs            | 546 ++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |   2 +
 6 files changed, 580 insertions(+), 1 deletion(-)
 create mode 100644 rust/helpers/fwctl.c
 create mode 100644 rust/kernel/fwctl.rs

-- 
2.51.0

[PATCH v6 1/1] rust: introduce abstractions for fwctl

[Zhi Wang]

Introduce safe Rust wrappers around struct fwctl_device and
struct fwctl_uctx. This lets Rust drivers register fwctl devices and
implement firmware RPC callbacks through a typed trait interface.

The abstraction keeps lifetime and reference-count handling inside the
wrapper, exposes pinned per-FD user contexts to drivers, and validates the
layout assumptions required by the C fwctl allocation model.

Registration owns driver private data with a lifetime tied to the bound
parent device. fwctl callbacks access that data through the Registration
lifetime, while Device remains only the refcounted fwctl object. This
avoids requiring Rust drop glue from the fwctl_device release path after
unregister or module teardown.

Co-developed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/DJJW7X4ESDSM.QCVYK2FC7ZR3@kernel.org
Signed-off-by: Zhi Wang <zhiw@nvidia.com>
---
 drivers/fwctl/Kconfig           |  12 +
 rust/bindings/bindings_helper.h |   1 +
 rust/helpers/fwctl.c            |  17 +
 rust/helpers/helpers.c          |   3 +-
 rust/kernel/fwctl.rs            | 546 ++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |   2 +
 6 files changed, 580 insertions(+), 1 deletion(-)
 create mode 100644 rust/helpers/fwctl.c
 create mode 100644 rust/kernel/fwctl.rs

diff --git a/drivers/fwctl/Kconfig b/drivers/fwctl/Kconfig
index d1b1925bdaec..cac38cf79f30 100644
--- a/drivers/fwctl/Kconfig
+++ b/drivers/fwctl/Kconfig
@@ -9,6 +9,18 @@ menuconfig FWCTL
 	  fit neatly into an existing subsystem.
 
 if FWCTL
+
+config RUST_FWCTL_ABSTRACTIONS
+	bool "Rust fwctl abstractions"
+	depends on RUST && FWCTL=y
+	help
+	  This enables the Rust abstractions for the fwctl device firmware
+	  access framework. It provides safe wrappers around struct fwctl_device
+	  and struct fwctl_uctx, allowing Rust drivers to register fwctl devices
+	  and implement their control and RPC logic in safe Rust.
+
+	  If unsure, say N.
+
 config FWCTL_BNXT
 	tristate "bnxt control fwctl driver"
 	depends on BNXT
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 1124785e210b..3d0511e4ab4f 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -60,6 +60,7 @@
 #include <linux/fdtable.h>
 #include <linux/file.h>
 #include <linux/firmware.h>
+#include <linux/fwctl.h>
 #include <linux/fs.h>
 #include <linux/i2c.h>
 #include <linux/interrupt.h>
diff --git a/rust/helpers/fwctl.c b/rust/helpers/fwctl.c
new file mode 100644
index 000000000000..c7eecd4336a7
--- /dev/null
+++ b/rust/helpers/fwctl.c
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/fwctl.h>
+
+#if IS_ENABLED(CONFIG_RUST_FWCTL_ABSTRACTIONS)
+
+__rust_helper struct fwctl_device *rust_helper_fwctl_get(struct fwctl_device *fwctl)
+{
+	return fwctl_get(fwctl);
+}
+
+__rust_helper void rust_helper_fwctl_put(struct fwctl_device *fwctl)
+{
+	fwctl_put(fwctl);
+}
+
+#endif
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 998e31052e66..b7d9512da9a6 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -62,10 +62,11 @@
 #include "drm.c"
 #include "drm_gpuvm.c"
 #include "err.c"
-#include "irq.c"
 #include "fs.c"
+#include "fwctl.c"
 #include "gpu.c"
 #include "io.c"
+#include "irq.c"
 #include "jump_label.c"
 #include "kunit.c"
 #include "list.c"
diff --git a/rust/kernel/fwctl.rs b/rust/kernel/fwctl.rs
new file mode 100644
index 000000000000..f59ed1ae6c7a
--- /dev/null
+++ b/rust/kernel/fwctl.rs
@@ -0,0 +1,546 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+//! Abstractions for the fwctl subsystem.
+//!
+//! C header: `include/linux/fwctl.h`
+
+use crate::{
+    bindings,
+    container_of,
+    device,
+    prelude::*,
+    sync::aref::{
+        ARef,
+        AlwaysRefCounted, //
+    },
+    types::Opaque, //
+};
+use core::{
+    cell::UnsafeCell,
+    marker::PhantomData,
+    mem,
+    ptr::NonNull,
+    slice, //
+};
+
+/// Represents a fwctl device type.
+///
+/// Corresponds to the C `enum fwctl_device_type`.
+#[repr(u32)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum DeviceType {
+    /// Mellanox ConnectX (mlx5) device.
+    Mlx5 = bindings::fwctl_device_type_FWCTL_DEVICE_TYPE_MLX5,
+    /// CXL (Compute Express Link) device.
+    Cxl = bindings::fwctl_device_type_FWCTL_DEVICE_TYPE_CXL,
+    /// AMD/Pensando PDS device.
+    Pds = bindings::fwctl_device_type_FWCTL_DEVICE_TYPE_PDS,
+    /// Broadcom NetXtreme (bnxt) device.
+    Bnxt = bindings::fwctl_device_type_FWCTL_DEVICE_TYPE_BNXT,
+}
+
+impl From<DeviceType> for u32 {
+    fn from(device_type: DeviceType) -> Self {
+        device_type as u32
+    }
+}
+
+/// Scope of access for an RPC request.
+///
+/// Corresponds to the C `enum fwctl_rpc_scope`.
+#[repr(u32)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum RpcScope {
+    /// Read/write access to device configuration.
+    Configuration = bindings::fwctl_rpc_scope_FWCTL_RPC_CONFIGURATION,
+    /// Read-only access to debug information.
+    DebugReadOnly = bindings::fwctl_rpc_scope_FWCTL_RPC_DEBUG_READ_ONLY,
+    /// Write access to lockdown-compatible debug information.
+    DebugWrite = bindings::fwctl_rpc_scope_FWCTL_RPC_DEBUG_WRITE,
+    /// Full read/write access to all debug information (requires `CAP_SYS_RAWIO`).
+    DebugWriteFull = bindings::fwctl_rpc_scope_FWCTL_RPC_DEBUG_WRITE_FULL,
+}
+
+impl TryFrom<u32> for RpcScope {
+    type Error = Error;
+
+    #[inline]
+    fn try_from(value: u32) -> Result<Self, Error> {
+        match value {
+            v if v == Self::Configuration as u32 => Ok(Self::Configuration),
+            v if v == Self::DebugReadOnly as u32 => Ok(Self::DebugReadOnly),
+            v if v == Self::DebugWrite as u32 => Ok(Self::DebugWrite),
+            v if v == Self::DebugWriteFull as u32 => Ok(Self::DebugWriteFull),
+            _ => Err(EINVAL),
+        }
+    }
+}
+
+/// Response from a [`Operations::fw_rpc`] call.
+pub enum FwRpcResponse {
+    /// Reuse the input buffer as the output, with the given output length.
+    InPlace(usize),
+    /// Return a newly allocated buffer as the output.
+    NewBuffer(KVec<u8>),
+}
+
+/// Trait implemented by each Rust driver that integrates with the fwctl subsystem.
+///
+/// The implementing type **is** the per-FD user context: one instance is
+/// created for each `open()` call and dropped when the FD is closed.
+///
+/// Each implementation corresponds to a specific device type and provides the
+/// vtable used by the core `fwctl` layer to manage per-FD user contexts and
+/// handle RPC requests.
+pub trait Operations: Sized + Send + Sync + 'static {
+    /// Data owned by the [`Registration`] and accessible during callbacks.
+    ///
+    /// The lifetime `'a` is tied to the [`Registration`] scope (which lives within the parent bus
+    /// device binding scope). Drivers use it to store references to resources bound to this scope,
+    /// such as PCI BARs or typed bus device references.
+    type RegistrationData<'a>: Send + Sync + 'a
+    where
+        Self: 'a;
+
+    /// fwctl device type identifier.
+    const DEVICE_TYPE: DeviceType;
+
+    /// Called when a new user context is opened.
+    ///
+    /// Returns a [`PinInit`] initializer for `Self`. The instance is dropped
+    /// automatically when the FD is closed (after [`close`](Self::close)).
+    fn open<'a>(
+        device: &'a Device<Self>,
+        reg_data: &'a Self::RegistrationData<'a>,
+    ) -> impl PinInit<Self, Error>;
+
+    /// Called when the user context is closed.
+    ///
+    /// The driver may perform additional cleanup here that requires access
+    /// to the owning [`Device`]. `Self` is dropped automatically after this
+    /// returns.
+    fn close<'a>(
+        _this: Pin<&mut Self>,
+        _device: &'a Device<Self>,
+        _reg_data: &'a Self::RegistrationData<'a>,
+    ) {
+    }
+
+    /// Return device information to userspace.
+    ///
+    /// The default implementation returns no device-specific data.
+    fn info<'a>(
+        _this: Pin<&Self>,
+        _device: &'a Device<Self>,
+        _reg_data: &'a Self::RegistrationData<'a>,
+    ) -> Result<KVec<u8>, Error> {
+        Ok(KVec::new())
+    }
+
+    /// Handle a userspace RPC request.
+    fn fw_rpc<'a>(
+        this: Pin<&Self>,
+        device: &'a Device<Self>,
+        reg_data: &'a Self::RegistrationData<'a>,
+        scope: RpcScope,
+        rpc_in: &mut [u8],
+    ) -> Result<FwRpcResponse, Error>;
+}
+
+/// A fwctl device.
+///
+/// `#[repr(C)]` with the `fwctl_device` at offset 0, matching the C `fwctl_alloc_device()` layout
+/// convention. Contains a pointer to the [`Registration`]'s data, set at registration time and
+/// cleared on unregistration.
+///
+/// # Invariants
+///
+/// - `dev` is embedded at offset 0 and is initialised by fwctl.
+/// - The fwctl refcount owns the allocation lifetime.
+/// - `registration_data` is either `NonNull::dangling()` (before registration / after
+///   unregistration) or points to valid data owned by the [`Registration`].
+#[repr(C)]
+pub struct Device<T: Operations> {
+    dev: Opaque<bindings::fwctl_device>,
+    registration_data: UnsafeCell<NonNull<T::RegistrationData<'static>>>,
+}
+
+impl<T: Operations> Device<T> {
+    /// Allocate a new fwctl device.
+    ///
+    /// Returns an [`ARef`] that can be passed to [`Registration::new()`]
+    /// to make the device visible to userspace.
+    pub fn new(parent: &device::Device<device::Bound>) -> Result<ARef<Self>> {
+        const_assert!(
+            core::mem::offset_of!(Self, dev) == 0,
+            "struct fwctl_device must be at offset 0"
+        );
+
+        let ops = core::ptr::from_ref::<bindings::fwctl_ops>(&VTable::<T>::VTABLE).cast_mut();
+
+        // SAFETY: `ops` is static, `parent` is bound, and `size` covers the full `Device<T>`.
+        let raw = unsafe {
+            bindings::_fwctl_alloc_device(parent.as_raw(), ops, core::mem::size_of::<Self>())
+        };
+
+        if raw.is_null() {
+            return Err(ENOMEM);
+        }
+
+        let this = raw.cast::<Self>();
+
+        // INVARIANT: Set `registration_data` to dangling (no registration yet).
+        // SAFETY: `this` points to the allocation just returned by fwctl.
+        unsafe {
+            core::ptr::addr_of_mut!((*this).registration_data)
+                .write(UnsafeCell::new(NonNull::dangling()));
+        };
+
+        // SAFETY: `raw` owns the initial reference.
+        Ok(unsafe { ARef::from_raw(NonNull::new_unchecked(this)) })
+    }
+
+    #[inline]
+    fn as_raw(&self) -> *mut bindings::fwctl_device {
+        self.dev.get()
+    }
+
+    /// # Safety
+    ///
+    /// `ptr` must point to a valid `fwctl_device` embedded in a `Device<T>`.
+    #[inline]
+    unsafe fn from_raw<'a>(ptr: *mut bindings::fwctl_device) -> &'a Self {
+        // SAFETY: The caller upholds the offset-0 `Device<T>` invariant.
+        unsafe { &*ptr.cast() }
+    }
+
+    /// Returns a reference to the registration data.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure the device is registered, i.e. that this is called within a fwctl
+    /// callback protected by `registration_lock`. The pointer cast from
+    /// `RegistrationData<'static>` to `RegistrationData<'_>` is sound because lifetimes do not
+    /// affect layout, and the data is guaranteed alive for the duration of the borrow.
+    #[inline]
+    unsafe fn registration_data_unchecked(&self) -> &T::RegistrationData<'_> {
+        // SAFETY: Caller guarantees the device is registered, so the pointer is valid.
+        // Lifetimes do not affect layout, so the cast is sound.
+        unsafe { (*self.registration_data.get()).cast::<_>().as_ref() }
+    }
+}
+
+impl<T: Operations> AsRef<device::Device> for Device<T> {
+    #[inline]
+    fn as_ref(&self) -> &device::Device {
+        // SAFETY: `self` contains a live fwctl_device.
+        let dev = unsafe { core::ptr::addr_of_mut!((*self.as_raw()).dev) };
+        // SAFETY: The embedded device is initialised by fwctl.
+        unsafe { device::Device::from_raw(dev) }
+    }
+}
+
+// SAFETY: `fwctl_get` increments the refcount of a valid fwctl_device.
+// `fwctl_put` decrements it and frees the device when it reaches zero.
+unsafe impl<T: Operations> AlwaysRefCounted for Device<T> {
+    #[inline]
+    fn inc_ref(&self) {
+        // SAFETY: `self` holds a live reference.
+        unsafe { bindings::fwctl_get(self.as_raw()) };
+    }
+
+    #[inline]
+    unsafe fn dec_ref(obj: NonNull<Self>) {
+        // SAFETY: The caller owns a live reference.
+        unsafe { bindings::fwctl_put(obj.cast().as_ptr()) };
+    }
+}
+
+// SAFETY: `Device<T>` is refcounted by the fwctl core and may be released from any thread.
+unsafe impl<T: Operations> Send for Device<T> {}
+
+// SAFETY: Shared access to the embedded `fwctl_device` is protected by the fwctl core. The
+// `registration_data` field is only mutated before registration and after unregistration (both
+// single-threaded with respect to callbacks).
+unsafe impl<T: Operations> Sync for Device<T> {}
+
+/// A registered fwctl device.
+///
+/// Carries the lifetime `'a` of the parent device to ensure that [`fwctl_unregister`] runs (via
+/// [`Drop`]) before the parent driver unbinds. Owns the
+/// [`RegistrationData`](Operations::RegistrationData) which is accessible during callbacks via the
+/// pointer stored in [`Device`].
+///
+/// On drop the device is unregistered (all user contexts are closed and `ops` is set to `NULL`)
+/// and the registration data is dropped.
+///
+/// [`fwctl_unregister`]: srctree/drivers/fwctl/main.c
+pub struct Registration<'a, T: Operations> {
+    dev: ARef<Device<T>>,
+    _reg_data: Pin<KBox<T::RegistrationData<'a>>>,
+}
+
+impl<'a, T: Operations> Registration<'a, T> {
+    /// Register a previously allocated fwctl device with the given registration data.
+    ///
+    /// The `reg_data` is owned by the registration and accessible during callbacks via
+    /// `Device::registration_data_unchecked()`.
+    ///
+    /// # Safety
+    ///
+    /// Callers must not `mem::forget()` the returned [`Registration`] or otherwise prevent its
+    /// [`Drop`] implementation from running, since `fwctl_unregister` must be called before the
+    /// parent device is unbound.
+    ///
+    /// `dev` must be an unregistered [`Device`] that is not associated with any live
+    /// [`Registration`], and no other thread may attempt to register the same device concurrently.
+    pub unsafe fn new(
+        _parent: &'a device::Device<device::Bound>,
+        dev: &Device<T>,
+        reg_data: impl PinInit<T::RegistrationData<'a>, Error>,
+    ) -> Result<Self> {
+        let reg_data: Pin<KBox<T::RegistrationData<'a>>> = KBox::pin_init(reg_data, GFP_KERNEL)?;
+
+        // Store the registration data pointer in the device before registration, so that it is
+        // visible once callbacks can be invoked.
+        //
+        // SAFETY: Lifetimes do not affect layout, so the pointer cast is sound.
+        let ptr: NonNull<T::RegistrationData<'static>> =
+            unsafe { mem::transmute(NonNull::from(Pin::get_ref(reg_data.as_ref()))) };
+
+        // SAFETY: No concurrent access; the device is not yet registered.
+        unsafe { *dev.registration_data.get() = ptr };
+
+        // SAFETY: `dev` is a valid fwctl_device backed by an ARef.
+        let ret = unsafe { bindings::fwctl_register(dev.as_raw()) };
+        if ret != 0 {
+            // SAFETY: No concurrent readers; registration failed.
+            unsafe { *dev.registration_data.get() = NonNull::dangling() };
+            return Err(Error::from_errno(ret));
+        }
+
+        Ok(Self {
+            dev: dev.into(),
+            _reg_data: reg_data,
+        })
+    }
+}
+
+impl<T: Operations> Drop for Registration<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: The Registration lifetime guarantees that the parent device is still bound.
+        // `fwctl_unregister` takes the write lock, closes all user contexts, and sets ops=NULL.
+        // After it returns, no callbacks can be running or will run.
+        unsafe { bindings::fwctl_unregister(self.dev.as_raw()) };
+
+        // SAFETY: `fwctl_unregister` guarantees no concurrent readers.
+        unsafe { *self.dev.registration_data.get() = NonNull::dangling() };
+
+        // `self._reg_data` is dropped here, after callbacks have stopped.
+    }
+}
+
+// SAFETY: `Registration` can be sent between threads; the underlying fwctl_device uses internal
+// locking.
+unsafe impl<T: Operations> Send for Registration<'_, T> {}
+
+// SAFETY: `Registration` provides no mutable access; the underlying fwctl_device is protected by
+// internal locking.
+unsafe impl<T: Operations> Sync for Registration<'_, T> {}
+
+/// Internal per-FD user context wrapping `struct fwctl_uctx` and `T`.
+///
+/// Not exposed to drivers; they work with `&T` / `Pin<&mut T>` directly.
+#[repr(C)]
+#[pin_data]
+struct UserCtx<T: Operations> {
+    #[pin]
+    fwctl_uctx: Opaque<bindings::fwctl_uctx>,
+    #[pin]
+    uctx: T,
+}
+
+impl<T: Operations> UserCtx<T> {
+    /// # Safety
+    ///
+    /// `ptr` must point to a `fwctl_uctx` embedded in a live `UserCtx<T>`.
+    #[inline]
+    unsafe fn from_raw<'a>(ptr: *mut bindings::fwctl_uctx) -> &'a Self {
+        // SAFETY: The caller upholds the `UserCtx<T>` embedding invariant.
+        unsafe { &*container_of!(Opaque::cast_from(ptr), Self, fwctl_uctx) }
+    }
+
+    /// # Safety
+    ///
+    /// `ptr` must point to a `fwctl_uctx` embedded in a live `UserCtx<T>`.
+    /// The caller must ensure exclusive access to the `UserCtx<T>`.
+    #[inline]
+    unsafe fn from_raw_mut<'a>(ptr: *mut bindings::fwctl_uctx) -> &'a mut Self {
+        // SAFETY: The caller upholds the embedding and exclusivity invariants.
+        unsafe { &mut *container_of!(Opaque::cast_from(ptr), Self, fwctl_uctx).cast_mut() }
+    }
+
+    /// Returns a reference to the fwctl [`Device`] that owns this context.
+    #[inline]
+    fn device(&self) -> &Device<T> {
+        // SAFETY: fwctl initialises this pointer before any driver callback.
+        let raw_fwctl = unsafe { (*self.fwctl_uctx.get()).fwctl };
+        // SAFETY: Rust fwctl devices use the offset-0 `Device<T>` layout.
+        unsafe { Device::from_raw(raw_fwctl) }
+    }
+}
+
+/// Static vtable mapping Rust trait methods to C callbacks.
+pub struct VTable<T: Operations>(PhantomData<T>);
+
+impl<T: Operations> VTable<T> {
+    /// The fwctl operations vtable for this driver type.
+    pub const VTABLE: bindings::fwctl_ops = bindings::fwctl_ops {
+        device_type: T::DEVICE_TYPE as u32,
+        uctx_size: core::mem::size_of::<UserCtx<T>>(),
+        open_uctx: Some(Self::open_uctx_callback),
+        close_uctx: Some(Self::close_uctx_callback),
+        info: Some(Self::info_callback),
+        fw_rpc: Some(Self::fw_rpc_callback),
+    };
+
+    /// # Safety
+    ///
+    /// `uctx` must be a valid `fwctl_uctx` embedded in a `UserCtx<T>` with
+    /// sufficient allocated space for the uctx field.
+    unsafe extern "C" fn open_uctx_callback(uctx: *mut bindings::fwctl_uctx) -> ffi::c_int {
+        const_assert!(
+            core::mem::offset_of!(UserCtx<T>, fwctl_uctx) == 0,
+            "struct fwctl_uctx must be at offset 0"
+        );
+
+        // SAFETY: fwctl sets this pointer before calling `open_uctx`.
+        let raw_fwctl = unsafe { (*uctx).fwctl };
+        // SAFETY: Rust fwctl devices use the offset-0 `Device<T>` layout.
+        let device = unsafe { Device::<T>::from_raw(raw_fwctl) };
+
+        // SAFETY: open_uctx is called under registration_lock read; the device is registered.
+        let reg_data = unsafe { device.registration_data_unchecked() };
+
+        let initializer = T::open(device, reg_data);
+
+        let uctx_offset = core::mem::offset_of!(UserCtx<T>, uctx);
+        // SAFETY: `uctx_size` reserves space for the full `UserCtx<T>`.
+        let uctx_ptr: *mut T = unsafe { uctx.cast::<u8>().add(uctx_offset).cast() };
+
+        // SAFETY: `uctx_ptr` addresses the uninitialised pinned context.
+        match unsafe { initializer.__pinned_init(uctx_ptr.cast()) } {
+            Ok(()) => 0,
+            Err(e) => e.to_errno(),
+        }
+    }
+
+    /// # Safety
+    ///
+    /// `uctx` must point to a fully initialised `UserCtx<T>`.
+    unsafe extern "C" fn close_uctx_callback(uctx: *mut bindings::fwctl_uctx) {
+        // SAFETY: fwctl keeps the owning device live for this callback.
+        let device = unsafe { Device::<T>::from_raw((*uctx).fwctl) };
+
+        // SAFETY: close_uctx is called under registration_lock write (from fwctl_unregister) or
+        // under registration_lock read (from fwctl_fops_release); the device is registered.
+        let reg_data = unsafe { device.registration_data_unchecked() };
+
+        // SAFETY: close is called for an opened Rust user context.
+        let ctx = unsafe { UserCtx::<T>::from_raw_mut(uctx) };
+
+        {
+            // SAFETY: fwctl never moves an opened user context.
+            let pinned = unsafe { Pin::new_unchecked(&mut ctx.uctx) };
+            T::close(pinned, device, reg_data);
+        }
+
+        // SAFETY: close is the last callback before fwctl frees the allocation.
+        unsafe { core::ptr::drop_in_place(&mut ctx.uctx) };
+    }
+
+    /// # Safety
+    ///
+    /// `uctx` must point to a fully initialised `UserCtx<T>`.
+    /// `length` must be a valid pointer.
+    unsafe extern "C" fn info_callback(
+        uctx: *mut bindings::fwctl_uctx,
+        length: *mut usize,
+    ) -> *mut ffi::c_void {
+        // SAFETY: info is called for an opened Rust user context.
+        let ctx = unsafe { UserCtx::<T>::from_raw(uctx) };
+        let device = ctx.device();
+
+        // SAFETY: info is called under registration_lock read; the device is registered.
+        let reg_data = unsafe { device.registration_data_unchecked() };
+
+        // SAFETY: fwctl never moves an opened user context.
+        let pinned = unsafe { Pin::new_unchecked(&ctx.uctx) };
+
+        match T::info(pinned, device, reg_data) {
+            Ok(kvec) if kvec.is_empty() => {
+                // SAFETY: `length` is a valid out-parameter.
+                unsafe { *length = 0 };
+                // Return NULL for empty data; kfree(NULL) is safe.
+                core::ptr::null_mut()
+            }
+            Ok(kvec) => {
+                let (ptr, len, _cap) = kvec.into_raw_parts();
+                // SAFETY: `length` is a valid out-parameter.
+                unsafe { *length = len };
+                ptr.cast::<ffi::c_void>()
+            }
+            Err(e) => Error::to_ptr(e),
+        }
+    }
+
+    /// # Safety
+    ///
+    /// `uctx` must point to a fully initialised `UserCtx<T>`.
+    /// `rpc_in` must be valid for `in_len` bytes. `out_len` must be valid.
+    unsafe extern "C" fn fw_rpc_callback(
+        uctx: *mut bindings::fwctl_uctx,
+        scope: u32,
+        rpc_in: *mut ffi::c_void,
+        in_len: usize,
+        out_len: *mut usize,
+    ) -> *mut ffi::c_void {
+        let scope = match RpcScope::try_from(scope) {
+            Ok(s) => s,
+            Err(e) => return Error::to_ptr(e),
+        };
+
+        // SAFETY: RPC is called for an opened Rust user context.
+        let ctx = unsafe { UserCtx::<T>::from_raw(uctx) };
+        let device = ctx.device();
+
+        // SAFETY: fw_rpc is called under registration_lock read; the device is registered.
+        let reg_data = unsafe { device.registration_data_unchecked() };
+
+        // SAFETY: fwctl passes a valid in/out buffer for this callback.
+        let rpc_in_slice: &mut [u8] =
+            unsafe { slice::from_raw_parts_mut(rpc_in.cast::<u8>(), in_len) };
+
+        // SAFETY: fwctl never moves an opened user context.
+        let pinned = unsafe { Pin::new_unchecked(&ctx.uctx) };
+
+        match T::fw_rpc(pinned, device, reg_data, scope, rpc_in_slice) {
+            Ok(FwRpcResponse::InPlace(len)) => {
+                if len > in_len {
+                    return Error::to_ptr(EINVAL);
+                }
+
+                // SAFETY: `out_len` is valid.
+                unsafe { *out_len = len };
+                rpc_in
+            }
+            Ok(FwRpcResponse::NewBuffer(kvec)) => {
+                let (ptr, len, _cap) = kvec.into_raw_parts();
+                // SAFETY: `out_len` is valid.
+                unsafe { *out_len = len };
+                ptr.cast::<ffi::c_void>()
+            }
+            Err(e) => Error::to_ptr(e),
+        }
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 9512af7156df..35094e2131d3 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -73,6 +73,8 @@
 pub mod firmware;
 pub mod fmt;
 pub mod fs;
+#[cfg(CONFIG_RUST_FWCTL_ABSTRACTIONS)]
+pub mod fwctl;
 #[cfg(CONFIG_GPU_BUDDY = "y")]
 pub mod gpu;
 #[cfg(CONFIG_I2C = "y")]
-- 
2.51.0

Re: [PATCH v6 1/1] rust: introduce abstractions for fwctl

[Danilo Krummrich]

On Mon Jun 29, 2026 at 5:01 PM CEST, Zhi Wang wrote:
> Introduce safe Rust wrappers around struct fwctl_device and
> struct fwctl_uctx. This lets Rust drivers register fwctl devices and
> implement firmware RPC callbacks through a typed trait interface.
>
> The abstraction keeps lifetime and reference-count handling inside the
> wrapper, exposes pinned per-FD user contexts to drivers, and validates the
> layout assumptions required by the C fwctl allocation model.
>
> Registration owns driver private data with a lifetime tied to the bound
> parent device. fwctl callbacks access that data through the Registration
> lifetime, while Device remains only the refcounted fwctl object. This
> avoids requiring Rust drop glue from the fwctl_device release path after
> unregister or module teardown.
>
> Co-developed-by: Danilo Krummrich <dakr@kernel.org>

This also needs:

Signed-off-by: Danilo Krummrich <dakr@kernel.org>

> Link: https://lore.kernel.org/r/DJJW7X4ESDSM.QCVYK2FC7ZR3@kernel.org
> Signed-off-by: Zhi Wang <zhiw@nvidia.com>

Few remaining nits below.

Jason, would you be fine if I take this through the DRM tree, so Nova can build
on top of it right away, or do you want to take it through your fwctl tree?

> ---
>  drivers/fwctl/Kconfig           |  12 +
>  rust/bindings/bindings_helper.h |   1 +
>  rust/helpers/fwctl.c            |  17 +
>  rust/helpers/helpers.c          |   3 +-
>  rust/kernel/fwctl.rs            | 546 ++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs              |   2 +
>  6 files changed, 580 insertions(+), 1 deletion(-)
>  create mode 100644 rust/helpers/fwctl.c
>  create mode 100644 rust/kernel/fwctl.rs


The MAINTAINERS file needs an update.

> +/// Represents a fwctl device type.
> +///
> +/// Corresponds to the C `enum fwctl_device_type`.
> +#[repr(u32)]
> +#[derive(Copy, Clone, Debug, Eq, PartialEq)]
> +pub enum DeviceType {
> +    /// Mellanox ConnectX (mlx5) device.
> +    Mlx5 = bindings::fwctl_device_type_FWCTL_DEVICE_TYPE_MLX5,
> +    /// CXL (Compute Express Link) device.
> +    Cxl = bindings::fwctl_device_type_FWCTL_DEVICE_TYPE_CXL,
> +    /// AMD/Pensando PDS device.
> +    Pds = bindings::fwctl_device_type_FWCTL_DEVICE_TYPE_PDS,
> +    /// Broadcom NetXtreme (bnxt) device.
> +    Bnxt = bindings::fwctl_device_type_FWCTL_DEVICE_TYPE_BNXT,
> +}

NIT: Do we want to add all device types even though they do not (plan to) have a
Rust driver?

> +/// Trait implemented by each Rust driver that integrates with the fwctl subsystem.
> +///
> +/// The implementing type **is** the per-FD user context: one instance is
> +/// created for each `open()` call and dropped when the FD is closed.
> +///
> +/// Each implementation corresponds to a specific device type and provides the
> +/// vtable used by the core `fwctl` layer to manage per-FD user contexts and
> +/// handle RPC requests.
> +pub trait Operations: Sized + Send + Sync + 'static {
> +    /// Data owned by the [`Registration`] and accessible during callbacks.
> +    ///
> +    /// The lifetime `'a` is tied to the [`Registration`] scope (which lives within the parent bus
> +    /// device binding scope). Drivers use it to store references to resources bound to this scope,
> +    /// such as PCI BARs or typed bus device references.
> +    type RegistrationData<'a>: Send + Sync + 'a
> +    where
> +        Self: 'a;
> +
> +    /// fwctl device type identifier.
> +    const DEVICE_TYPE: DeviceType;
> +
> +    /// Called when a new user context is opened.
> +    ///
> +    /// Returns a [`PinInit`] initializer for `Self`. The instance is dropped
> +    /// automatically when the FD is closed (after [`close`](Self::close)).
> +    fn open<'a>(
> +        device: &'a Device<Self>,
> +        reg_data: &'a Self::RegistrationData<'a>,
> +    ) -> impl PinInit<Self, Error>;

I think this can just be

	fn open<'a>(
	    device: &Device<Self>,
	    reg_data: &Self::RegistrationData<'a>,
	) -> impl PinInit<Self, Error>;

> +impl<T: Operations> Device<T> {
> +    /// Allocate a new fwctl device.
> +    ///
> +    /// Returns an [`ARef`] that can be passed to [`Registration::new()`]
> +    /// to make the device visible to userspace.
> +    pub fn new(parent: &device::Device<device::Bound>) -> Result<ARef<Self>> {
> +        const_assert!(
> +            core::mem::offset_of!(Self, dev) == 0,
> +            "struct fwctl_device must be at offset 0"
> +        );
> +
> +        let ops = core::ptr::from_ref::<bindings::fwctl_ops>(&VTable::<T>::VTABLE).cast_mut();
> +
> +        // SAFETY: `ops` is static, `parent` is bound, and `size` covers the full `Device<T>`.
> +        let raw = unsafe {
> +            bindings::_fwctl_alloc_device(parent.as_raw(), ops, core::mem::size_of::<Self>())
> +        };
> +
> +        if raw.is_null() {
> +            return Err(ENOMEM);
> +        }
> +
> +        let this = raw.cast::<Self>();
> +
> +        // INVARIANT: Set `registration_data` to dangling (no registration yet).
> +        // SAFETY: `this` points to the allocation just returned by fwctl.
> +        unsafe {
> +            core::ptr::addr_of_mut!((*this).registration_data)

Here and in a few other places, this can use &raw mut instead.

> +/// Static vtable mapping Rust trait methods to C callbacks.
> +pub struct VTable<T: Operations>(PhantomData<T>);

Why does this need public visibility?

> +
> +impl<T: Operations> VTable<T> {
> +    /// The fwctl operations vtable for this driver type.
> +    pub const VTABLE: bindings::fwctl_ops = bindings::fwctl_ops {
> +        device_type: T::DEVICE_TYPE as u32,
> +        uctx_size: core::mem::size_of::<UserCtx<T>>(),
> +        open_uctx: Some(Self::open_uctx_callback),
> +        close_uctx: Some(Self::close_uctx_callback),
> +        info: Some(Self::info_callback),
> +        fw_rpc: Some(Self::fw_rpc_callback),
> +    };

Same here.

> +
> +    /// # Safety
> +    ///
> +    /// `uctx` must be a valid `fwctl_uctx` embedded in a `UserCtx<T>` with
> +    /// sufficient allocated space for the uctx field.
> +    unsafe extern "C" fn open_uctx_callback(uctx: *mut bindings::fwctl_uctx) -> ffi::c_int {
> +        const_assert!(
> +            core::mem::offset_of!(UserCtx<T>, fwctl_uctx) == 0,
> +            "struct fwctl_uctx must be at offset 0"
> +        );
> +
> +        // SAFETY: fwctl sets this pointer before calling `open_uctx`.
> +        let raw_fwctl = unsafe { (*uctx).fwctl };
> +        // SAFETY: Rust fwctl devices use the offset-0 `Device<T>` layout.
> +        let device = unsafe { Device::<T>::from_raw(raw_fwctl) };
> +
> +        // SAFETY: open_uctx is called under registration_lock read; the device is registered.
> +        let reg_data = unsafe { device.registration_data_unchecked() };
> +
> +        let initializer = T::open(device, reg_data);
> +
> +        let uctx_offset = core::mem::offset_of!(UserCtx<T>, uctx);
> +        // SAFETY: `uctx_size` reserves space for the full `UserCtx<T>`.
> +        let uctx_ptr: *mut T = unsafe { uctx.cast::<u8>().add(uctx_offset).cast() };

This can use byte_add() instead.

> +            Ok(FwRpcResponse::NewBuffer(kvec)) => {
> +                let (ptr, len, _cap) = kvec.into_raw_parts();
> +                // SAFETY: `out_len` is valid.
> +                unsafe { *out_len = len };
> +                ptr.cast::<ffi::c_void>()

This probably needs a kvec.is_empty() check, since an empty vector contains a
dangling pointer.

Re: [PATCH v6 1/1] rust: introduce abstractions for fwctl

[Alexandre Courbot]

Hi Zhi,

This is looking pretty good to me overall, a few remarks/questions
below. None of these should require big changes.

On Tue Jun 30, 2026 at 12:01 AM JST, Zhi Wang wrote:
<...>
> +/// A fwctl device.
> +///
> +/// `#[repr(C)]` with the `fwctl_device` at offset 0, matching the C `fwctl_alloc_device()` layout
> +/// convention. Contains a pointer to the [`Registration`]'s data, set at registration time and
> +/// cleared on unregistration.
> +///
> +/// # Invariants
> +///
> +/// - `dev` is embedded at offset 0 and is initialised by fwctl.
> +/// - The fwctl refcount owns the allocation lifetime.
> +/// - `registration_data` is either `NonNull::dangling()` (before registration / after
> +///   unregistration) or points to valid data owned by the [`Registration`].
> +#[repr(C)]
> +pub struct Device<T: Operations> {
> +    dev: Opaque<bindings::fwctl_device>,
> +    registration_data: UnsafeCell<NonNull<T::RegistrationData<'static>>>,
> +}
> +
> +impl<T: Operations> Device<T> {
> +    /// Allocate a new fwctl device.
> +    ///
> +    /// Returns an [`ARef`] that can be passed to [`Registration::new()`]
> +    /// to make the device visible to userspace.
> +    pub fn new(parent: &device::Device<device::Bound>) -> Result<ARef<Self>> {
> +        const_assert!(
> +            core::mem::offset_of!(Self, dev) == 0,
> +            "struct fwctl_device must be at offset 0"
> +        );
> +
> +        let ops = core::ptr::from_ref::<bindings::fwctl_ops>(&VTable::<T>::VTABLE).cast_mut();
> +
> +        // SAFETY: `ops` is static, `parent` is bound, and `size` covers the full `Device<T>`.
> +        let raw = unsafe {
> +            bindings::_fwctl_alloc_device(parent.as_raw(), ops, core::mem::size_of::<Self>())
> +        };
> +
> +        if raw.is_null() {
> +            return Err(ENOMEM);
> +        }
> +
> +        let this = raw.cast::<Self>();
> +
> +        // INVARIANT: Set `registration_data` to dangling (no registration yet).
> +        // SAFETY: `this` points to the allocation just returned by fwctl.
> +        unsafe {
> +            core::ptr::addr_of_mut!((*this).registration_data)
> +                .write(UnsafeCell::new(NonNull::dangling()));
> +        };
> +
> +        // SAFETY: `raw` owns the initial reference.
> +        Ok(unsafe { ARef::from_raw(NonNull::new_unchecked(this)) })

How about replacing from `if raw.is_null() ...` with something more
functional-style:

    NonNull::new(raw.cast::<Self>())
        .map(|this| {
            // INVARIANT: Set `registration_data` to dangling (no registration yet).
            // SAFETY: `this` points to the allocation just returned by fwctl.
            unsafe {
                (&raw mut (*this.as_ptr()).registration_data)
                    .write(UnsafeCell::new(NonNull::dangling()))
            };
            // SAFETY: `this` owns the initial reference.
            unsafe { ARef::from_raw(this) }
        })
        .ok_or(ENOMEM)

It's not just cosmetic: it removes one call to an unsafe method
(`NonNull::new_unchecked`), carries the fact that `this` is not NULL in
the type system from the get-go (instead of relying on an explicit
check), and makes the transition steps from the raw pointer to the
`ARef` more explicit.

> +    }
> +
> +    #[inline]
> +    fn as_raw(&self) -> *mut bindings::fwctl_device {
> +        self.dev.get()
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// `ptr` must point to a valid `fwctl_device` embedded in a `Device<T>`.

nit: missing doclink: [`Device<T>`].

> +    #[inline]
> +    unsafe fn from_raw<'a>(ptr: *mut bindings::fwctl_device) -> &'a Self {
> +        // SAFETY: The caller upholds the offset-0 `Device<T>` invariant.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    /// Returns a reference to the registration data.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure the device is registered, i.e. that this is called within a fwctl
> +    /// callback protected by `registration_lock`. The pointer cast from
> +    /// `RegistrationData<'static>` to `RegistrationData<'_>` is sound because lifetimes do not
> +    /// affect layout, and the data is guaranteed alive for the duration of the borrow.

The last sentence is not part of the safety contract with callers, and
is also repeated in the `SAFETY` comment (where it does belong), so it
should be removed imho.

> +    #[inline]
> +    unsafe fn registration_data_unchecked(&self) -> &T::RegistrationData<'_> {
> +        // SAFETY: Caller guarantees the device is registered, so the pointer is valid.
> +        // Lifetimes do not affect layout, so the cast is sound.
> +        unsafe { (*self.registration_data.get()).cast::<_>().as_ref() }
> +    }
> +}
> +
> +impl<T: Operations> AsRef<device::Device> for Device<T> {
> +    #[inline]
> +    fn as_ref(&self) -> &device::Device {
> +        // SAFETY: `self` contains a live fwctl_device.
> +        let dev = unsafe { core::ptr::addr_of_mut!((*self.as_raw()).dev) };

This can be

    let dev = unsafe { &raw mut (*self.as_raw()).dev };

> +        // SAFETY: The embedded device is initialised by fwctl.
> +        unsafe { device::Device::from_raw(dev) }
> +    }
> +}
> +
> +// SAFETY: `fwctl_get` increments the refcount of a valid fwctl_device.
> +// `fwctl_put` decrements it and frees the device when it reaches zero.
> +unsafe impl<T: Operations> AlwaysRefCounted for Device<T> {
> +    #[inline]
> +    fn inc_ref(&self) {
> +        // SAFETY: `self` holds a live reference.
> +        unsafe { bindings::fwctl_get(self.as_raw()) };
> +    }
> +
> +    #[inline]
> +    unsafe fn dec_ref(obj: NonNull<Self>) {
> +        // SAFETY: The caller owns a live reference.
> +        unsafe { bindings::fwctl_put(obj.cast().as_ptr()) };
> +    }
> +}
> +
> +// SAFETY: `Device<T>` is refcounted by the fwctl core and may be released from any thread.
> +unsafe impl<T: Operations> Send for Device<T> {}
> +
> +// SAFETY: Shared access to the embedded `fwctl_device` is protected by the fwctl core. The
> +// `registration_data` field is only mutated before registration and after unregistration (both
> +// single-threaded with respect to callbacks).
> +unsafe impl<T: Operations> Sync for Device<T> {}
> +
> +/// A registered fwctl device.
> +///
> +/// Carries the lifetime `'a` of the parent device to ensure that [`fwctl_unregister`] runs (via
> +/// [`Drop`]) before the parent driver unbinds. Owns the
> +/// [`RegistrationData`](Operations::RegistrationData) which is accessible during callbacks via the
> +/// pointer stored in [`Device`].

This paragraph sounds like an implementation detail and not necessarily
useful information to users.

> +///
> +/// On drop the device is unregistered (all user contexts are closed and `ops` is set to `NULL`)
> +/// and the registration data is dropped.
> +///
> +/// [`fwctl_unregister`]: srctree/drivers/fwctl/main.c
> +pub struct Registration<'a, T: Operations> {
> +    dev: ARef<Device<T>>,
> +    _reg_data: Pin<KBox<T::RegistrationData<'a>>>,
> +}
> +
> +impl<'a, T: Operations> Registration<'a, T> {
> +    /// Register a previously allocated fwctl device with the given registration data.
> +    ///
> +    /// The `reg_data` is owned by the registration and accessible during callbacks via
> +    /// `Device::registration_data_unchecked()`.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must not `mem::forget()` the returned [`Registration`] or otherwise prevent its
> +    /// [`Drop`] implementation from running, since `fwctl_unregister` must be called before the
> +    /// parent device is unbound.
> +    ///
> +    /// `dev` must be an unregistered [`Device`] that is not associated with any live
> +    /// [`Registration`], and no other thread may attempt to register the same device concurrently.
> +    pub unsafe fn new(
> +        _parent: &'a device::Device<device::Bound>,

Why do we need `_parent`? Is it to provide a proof that the parent
device is bound by the time of registration? If so, there is an obvious
loophole: this method will accept any bound device, not necessarily the
actual parent that was passed to `Device::new`.

We should either store an `ARef<device::Device>` in `fwctl::Device` to
perform the check at runtime, or add a requirement in the `Safety`
paragraph that `_parent` must be the actual parent, since the method is
already `unsafe` anyway.

> +        dev: &Device<T>,
> +        reg_data: impl PinInit<T::RegistrationData<'a>, Error>,
> +    ) -> Result<Self> {
> +        let reg_data: Pin<KBox<T::RegistrationData<'a>>> = KBox::pin_init(reg_data, GFP_KERNEL)?;
> +
> +        // Store the registration data pointer in the device before registration, so that it is
> +        // visible once callbacks can be invoked.
> +        //
> +        // SAFETY: Lifetimes do not affect layout, so the pointer cast is sound.
> +        let ptr: NonNull<T::RegistrationData<'static>> =
> +            unsafe { mem::transmute(NonNull::from(Pin::get_ref(reg_data.as_ref()))) };

The only unsafe part of this statement is the `mem::transmute`. In such
cases, I think it is worth using an intermediate variable for the
non-unsafe part so non-checked unsafe operations don't slip in if the
code is modified, e.g.:

    let r = NonNull::from(Pin::get_ref(reg_data.as_ref()));
    ...
    let ptr: NonNull<T::RegistrationData<'static>> = unsafe { mem::transmute(r) };
 
> +
> +        // SAFETY: No concurrent access; the device is not yet registered.
> +        unsafe { *dev.registration_data.get() = ptr };
> +
> +        // SAFETY: `dev` is a valid fwctl_device backed by an ARef.
> +        let ret = unsafe { bindings::fwctl_register(dev.as_raw()) };
> +        if ret != 0 {
> +            // SAFETY: No concurrent readers; registration failed.
> +            unsafe { *dev.registration_data.get() = NonNull::dangling() };
> +            return Err(Error::from_errno(ret));
> +        }
> +
> +        Ok(Self {
> +            dev: dev.into(),
> +            _reg_data: reg_data,
> +        })
> +    }
> +}
> +
> +impl<T: Operations> Drop for Registration<'_, T> {
> +    fn drop(&mut self) {
> +        // SAFETY: The Registration lifetime guarantees that the parent device is still bound.
> +        // `fwctl_unregister` takes the write lock, closes all user contexts, and sets ops=NULL.
> +        // After it returns, no callbacks can be running or will run.
> +        unsafe { bindings::fwctl_unregister(self.dev.as_raw()) };
> +
> +        // SAFETY: `fwctl_unregister` guarantees no concurrent readers.
> +        unsafe { *self.dev.registration_data.get() = NonNull::dangling() };
> +
> +        // `self._reg_data` is dropped here, after callbacks have stopped.
> +    }
> +}
> +
> +// SAFETY: `Registration` can be sent between threads; the underlying fwctl_device uses internal
> +// locking.
> +unsafe impl<T: Operations> Send for Registration<'_, T> {}
> +
> +// SAFETY: `Registration` provides no mutable access; the underlying fwctl_device is protected by
> +// internal locking.
> +unsafe impl<T: Operations> Sync for Registration<'_, T> {}

IIUC all the fields of `Registration` are already `Send` and `Sync`, so
we don't need these manual implementations.

> +/// Internal per-FD user context wrapping `struct fwctl_uctx` and `T`.
> +///
> +/// Not exposed to drivers; they work with `&T` / `Pin<&mut T>` directly.
> +#[repr(C)]
> +#[pin_data]
> +struct UserCtx<T: Operations> {
> +    #[pin]
> +    fwctl_uctx: Opaque<bindings::fwctl_uctx>,
> +    #[pin]
> +    uctx: T,
> +}
> +
> +impl<T: Operations> UserCtx<T> {
> +    /// # Safety
> +    ///
> +    /// `ptr` must point to a `fwctl_uctx` embedded in a live `UserCtx<T>`.
> +    #[inline]
> +    unsafe fn from_raw<'a>(ptr: *mut bindings::fwctl_uctx) -> &'a Self {
> +        // SAFETY: The caller upholds the `UserCtx<T>` embedding invariant.
> +        unsafe { &*container_of!(Opaque::cast_from(ptr), Self, fwctl_uctx) }
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// `ptr` must point to a `fwctl_uctx` embedded in a live `UserCtx<T>`.
> +    /// The caller must ensure exclusive access to the `UserCtx<T>`.
> +    #[inline]
> +    unsafe fn from_raw_mut<'a>(ptr: *mut bindings::fwctl_uctx) -> &'a mut Self {
> +        // SAFETY: The caller upholds the embedding and exclusivity invariants.
> +        unsafe { &mut *container_of!(Opaque::cast_from(ptr), Self, fwctl_uctx).cast_mut() }
> +    }
> +
> +    /// Returns a reference to the fwctl [`Device`] that owns this context.
> +    #[inline]
> +    fn device(&self) -> &Device<T> {
> +        // SAFETY: fwctl initialises this pointer before any driver callback.
> +        let raw_fwctl = unsafe { (*self.fwctl_uctx.get()).fwctl };
> +        // SAFETY: Rust fwctl devices use the offset-0 `Device<T>` layout.
> +        unsafe { Device::from_raw(raw_fwctl) }
> +    }
> +}
> +
> +/// Static vtable mapping Rust trait methods to C callbacks.
> +pub struct VTable<T: Operations>(PhantomData<T>);
> +
> +impl<T: Operations> VTable<T> {
> +    /// The fwctl operations vtable for this driver type.
> +    pub const VTABLE: bindings::fwctl_ops = bindings::fwctl_ops {
> +        device_type: T::DEVICE_TYPE as u32,
> +        uctx_size: core::mem::size_of::<UserCtx<T>>(),

Aren't the alignment concerns raised on [1] still valid? Since the user
context is allocated by the C side, we need to make sure that Rust's
alignment expectations are properly met.

[1] https://lore.kernel.org/all/DGUZ4A6W87JU.36R2KDR7YGSEX@kernel.org/

> +        open_uctx: Some(Self::open_uctx_callback),
> +        close_uctx: Some(Self::close_uctx_callback),
> +        info: Some(Self::info_callback),
> +        fw_rpc: Some(Self::fw_rpc_callback),
> +    };
> +
> +    /// # Safety
> +    ///
> +    /// `uctx` must be a valid `fwctl_uctx` embedded in a `UserCtx<T>` with
> +    /// sufficient allocated space for the uctx field.
> +    unsafe extern "C" fn open_uctx_callback(uctx: *mut bindings::fwctl_uctx) -> ffi::c_int {
> +        const_assert!(
> +            core::mem::offset_of!(UserCtx<T>, fwctl_uctx) == 0,
> +            "struct fwctl_uctx must be at offset 0"
> +        );
> +
> +        // SAFETY: fwctl sets this pointer before calling `open_uctx`.
> +        let raw_fwctl = unsafe { (*uctx).fwctl };
> +        // SAFETY: Rust fwctl devices use the offset-0 `Device<T>` layout.
> +        let device = unsafe { Device::<T>::from_raw(raw_fwctl) };
> +
> +        // SAFETY: open_uctx is called under registration_lock read; the device is registered.
> +        let reg_data = unsafe { device.registration_data_unchecked() };

This `from_raw`..`registration_data_unchecked` sequence is performed by
all hooks, how about factoring them out in a helper function?

> +
> +        let initializer = T::open(device, reg_data);
> +
> +        let uctx_offset = core::mem::offset_of!(UserCtx<T>, uctx);
> +        // SAFETY: `uctx_size` reserves space for the full `UserCtx<T>`.
> +        let uctx_ptr: *mut T = unsafe { uctx.cast::<u8>().add(uctx_offset).cast() };
> +
> +        // SAFETY: `uctx_ptr` addresses the uninitialised pinned context.
> +        match unsafe { initializer.__pinned_init(uctx_ptr.cast()) } {

This `cast` is unneeded here IIUC.

> +            Ok(()) => 0,
> +            Err(e) => e.to_errno(),
> +        }
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// `uctx` must point to a fully initialised `UserCtx<T>`.
> +    unsafe extern "C" fn close_uctx_callback(uctx: *mut bindings::fwctl_uctx) {
> +        // SAFETY: fwctl keeps the owning device live for this callback.
> +        let device = unsafe { Device::<T>::from_raw((*uctx).fwctl) };
> +
> +        // SAFETY: close_uctx is called under registration_lock write (from fwctl_unregister) or
> +        // under registration_lock read (from fwctl_fops_release); the device is registered.
> +        let reg_data = unsafe { device.registration_data_unchecked() };
> +
> +        // SAFETY: close is called for an opened Rust user context.
> +        let ctx = unsafe { UserCtx::<T>::from_raw_mut(uctx) };
> +
> +        {
> +            // SAFETY: fwctl never moves an opened user context.
> +            let pinned = unsafe { Pin::new_unchecked(&mut ctx.uctx) };
> +            T::close(pinned, device, reg_data);
> +        }
> +
> +        // SAFETY: close is the last callback before fwctl frees the allocation.
> +        unsafe { core::ptr::drop_in_place(&mut ctx.uctx) };
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// `uctx` must point to a fully initialised `UserCtx<T>`.
> +    /// `length` must be a valid pointer.
> +    unsafe extern "C" fn info_callback(
> +        uctx: *mut bindings::fwctl_uctx,
> +        length: *mut usize,
> +    ) -> *mut ffi::c_void {
> +        // SAFETY: info is called for an opened Rust user context.
> +        let ctx = unsafe { UserCtx::<T>::from_raw(uctx) };
> +        let device = ctx.device();
> +
> +        // SAFETY: info is called under registration_lock read; the device is registered.
> +        let reg_data = unsafe { device.registration_data_unchecked() };
> +
> +        // SAFETY: fwctl never moves an opened user context.
> +        let pinned = unsafe { Pin::new_unchecked(&ctx.uctx) };
> +
> +        match T::info(pinned, device, reg_data) {
> +            Ok(kvec) if kvec.is_empty() => {
> +                // SAFETY: `length` is a valid out-parameter.
> +                unsafe { *length = 0 };
> +                // Return NULL for empty data; kfree(NULL) is safe.
> +                core::ptr::null_mut()
> +            }
> +            Ok(kvec) => {
> +                let (ptr, len, _cap) = kvec.into_raw_parts();
> +                // SAFETY: `length` is a valid out-parameter.
> +                unsafe { *length = len };
> +                ptr.cast::<ffi::c_void>()
> +            }
> +            Err(e) => Error::to_ptr(e),
> +        }
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// `uctx` must point to a fully initialised `UserCtx<T>`.
> +    /// `rpc_in` must be valid for `in_len` bytes. `out_len` must be valid.
> +    unsafe extern "C" fn fw_rpc_callback(
> +        uctx: *mut bindings::fwctl_uctx,
> +        scope: u32,
> +        rpc_in: *mut ffi::c_void,
> +        in_len: usize,
> +        out_len: *mut usize,
> +    ) -> *mut ffi::c_void {
> +        let scope = match RpcScope::try_from(scope) {
> +            Ok(s) => s,
> +            Err(e) => return Error::to_ptr(e),
> +        };
> +
> +        // SAFETY: RPC is called for an opened Rust user context.
> +        let ctx = unsafe { UserCtx::<T>::from_raw(uctx) };
> +        let device = ctx.device();
> +
> +        // SAFETY: fw_rpc is called under registration_lock read; the device is registered.
> +        let reg_data = unsafe { device.registration_data_unchecked() };
> +
> +        // SAFETY: fwctl passes a valid in/out buffer for this callback.
> +        let rpc_in_slice: &mut [u8] =
> +            unsafe { slice::from_raw_parts_mut(rpc_in.cast::<u8>(), in_len) };

Name nit: technically this is not just an `in` slice, since the reply
might be written into it.

> +
> +        // SAFETY: fwctl never moves an opened user context.
> +        let pinned = unsafe { Pin::new_unchecked(&ctx.uctx) };
> +
> +        match T::fw_rpc(pinned, device, reg_data, scope, rpc_in_slice) {

The C side passes `out_len` to its callback, don't we want to do the
same here?

> +            Ok(FwRpcResponse::InPlace(len)) => {
> +                if len > in_len {
> +                    return Error::to_ptr(EINVAL);
> +                }
> +
> +                // SAFETY: `out_len` is valid.
> +                unsafe { *out_len = len };
> +                rpc_in
> +            }
> +            Ok(FwRpcResponse::NewBuffer(kvec)) => {
> +                let (ptr, len, _cap) = kvec.into_raw_parts();
> +                // SAFETY: `out_len` is valid.
> +                unsafe { *out_len = len };

Both arms are writing `*out_len` and require an `unsafe` statement. We
could factor these out into a single one by returning a `(len, res)`
tuple from the match statement, and writing `out_len` once out of it.

> +                ptr.cast::<ffi::c_void>()
> +            }
> +            Err(e) => Error::to_ptr(e),

(if you follow my recommendation above, then this needs to become a
return statement).

Re: [PATCH v6 1/1] rust: introduce abstractions for fwctl

[Danilo Krummrich]

On Mon Jul 6, 2026 at 6:19 AM CEST, Alexandre Courbot wrote:
>> +impl<'a, T: Operations> Registration<'a, T> {
>> +    /// Register a previously allocated fwctl device with the given registration data.
>> +    ///
>> +    /// The `reg_data` is owned by the registration and accessible during callbacks via
>> +    /// `Device::registration_data_unchecked()`.
>> +    ///
>> +    /// # Safety
>> +    ///
>> +    /// Callers must not `mem::forget()` the returned [`Registration`] or otherwise prevent its
>> +    /// [`Drop`] implementation from running, since `fwctl_unregister` must be called before the
>> +    /// parent device is unbound.
>> +    ///
>> +    /// `dev` must be an unregistered [`Device`] that is not associated with any live
>> +    /// [`Registration`], and no other thread may attempt to register the same device concurrently.
>> +    pub unsafe fn new(
>> +        _parent: &'a device::Device<device::Bound>,
>
> Why do we need `_parent`? Is it to provide a proof that the parent
> device is bound by the time of registration? If so, there is an obvious
> loophole: this method will accept any bound device, not necessarily the
> actual parent that was passed to `Device::new`.
>
> We should either store an `ARef<device::Device>` in `fwctl::Device` to
> perform the check at runtime, or add a requirement in the `Safety`
> paragraph that `_parent` must be the actual parent, since the method is
> already `unsafe` anyway.

Neither is needed, the fwctl::Device already carries a parent device pointer
(available through AsRef in Rust).

This function should just compare dev.as_ref() with parent (which I thought was
already the case in a previous version).

Re: [PATCH v6 1/1] rust: introduce abstractions for fwctl

[Gary Guo]

On Mon Jul 6, 2026 at 5:19 AM BST, Alexandre Courbot wrote:
> Hi Zhi,
>
> This is looking pretty good to me overall, a few remarks/questions
> below. None of these should require big changes.
>
> On Tue Jun 30, 2026 at 12:01 AM JST, Zhi Wang wrote:
> <...>
>> +/// A fwctl device.
>> +///
>> +/// `#[repr(C)]` with the `fwctl_device` at offset 0, matching the C `fwctl_alloc_device()` layout
>> +/// convention. Contains a pointer to the [`Registration`]'s data, set at registration time and
>> +/// cleared on unregistration.
>> +///
>> +/// # Invariants
>> +///
>> +/// - `dev` is embedded at offset 0 and is initialised by fwctl.
>> +/// - The fwctl refcount owns the allocation lifetime.
>> +/// - `registration_data` is either `NonNull::dangling()` (before registration / after
>> +///   unregistration) or points to valid data owned by the [`Registration`].
>> +#[repr(C)]
>> +pub struct Device<T: Operations> {
>> +    dev: Opaque<bindings::fwctl_device>,
>> +    registration_data: UnsafeCell<NonNull<T::RegistrationData<'static>>>,
>> +}
>> +
>> +impl<T: Operations> Device<T> {
>> +    /// Allocate a new fwctl device.
>> +    ///
>> +    /// Returns an [`ARef`] that can be passed to [`Registration::new()`]
>> +    /// to make the device visible to userspace.
>> +    pub fn new(parent: &device::Device<device::Bound>) -> Result<ARef<Self>> {
>> +        const_assert!(
>> +            core::mem::offset_of!(Self, dev) == 0,
>> +            "struct fwctl_device must be at offset 0"
>> +        );
>> +
>> +        let ops = core::ptr::from_ref::<bindings::fwctl_ops>(&VTable::<T>::VTABLE).cast_mut();
>> +
>> +        // SAFETY: `ops` is static, `parent` is bound, and `size` covers the full `Device<T>`.
>> +        let raw = unsafe {
>> +            bindings::_fwctl_alloc_device(parent.as_raw(), ops, core::mem::size_of::<Self>())
>> +        };
>> +
>> +        if raw.is_null() {
>> +            return Err(ENOMEM);
>> +        }
>> +
>> +        let this = raw.cast::<Self>();
>> +
>> +        // INVARIANT: Set `registration_data` to dangling (no registration yet).
>> +        // SAFETY: `this` points to the allocation just returned by fwctl.
>> +        unsafe {
>> +            core::ptr::addr_of_mut!((*this).registration_data)
>> +                .write(UnsafeCell::new(NonNull::dangling()));
>> +        };
>> +
>> +        // SAFETY: `raw` owns the initial reference.
>> +        Ok(unsafe { ARef::from_raw(NonNull::new_unchecked(this)) })
>
> How about replacing from `if raw.is_null() ...` with something more
> functional-style:
>
>     NonNull::new(raw.cast::<Self>())
>         .map(|this| {
>             // INVARIANT: Set `registration_data` to dangling (no registration yet).
>             // SAFETY: `this` points to the allocation just returned by fwctl.
>             unsafe {
>                 (&raw mut (*this.as_ptr()).registration_data)
>                     .write(UnsafeCell::new(NonNull::dangling()))
>             };
>             // SAFETY: `this` owns the initial reference.
>             unsafe { ARef::from_raw(this) }
>         })
>         .ok_or(ENOMEM)

I would rather do

    let this = NonNull::new(raw.cast::<Self>()).ok_or(ENOMEM);
    // use this

for a more linear style. Map shouldn't be needed.

Best,
Gary

>
> It's not just cosmetic: it removes one call to an unsafe method
> (`NonNull::new_unchecked`), carries the fact that `this` is not NULL in
> the type system from the get-go (instead of relying on an explicit
> check), and makes the transition steps from the raw pointer to the
> `ARef` more explicit.