Re: [PATCH v6 1/2] rust: core abstractions for HID drivers

["Gary Guo" <gary@garyguo.net>]

On Sun Feb 22, 2026 at 9:56 PM GMT, Rahul Rameshbabu wrote:
> These abstractions enable the development of HID drivers in Rust by binding
> with the HID core C API. They provide Rust types that map to the
> equivalents in C. In this initial draft, only hid_device and hid_device_id
> are provided direct Rust type equivalents. hid_driver is specially wrapped
> with a custom Driver type. The module_hid_driver! macro provides analogous
> functionality to its C equivalent. Only the .report_fixup callback is
> binded to Rust so far.
>
> Future work for these abstractions would include more bindings for common
> HID-related types, such as hid_field, hid_report_enum, and hid_report as
> well as more bus callbacks. Providing Rust equivalents to useful core HID
> functions will also be necessary for HID driver development in Rust.
>
> Signed-off-by: Rahul Rameshbabu <sergeantsagara@protonmail.com>
> ---
>
> Notes:
>     Changelog:
>     
>         v5->v6:
>           * Converted From<u16> for Group to TryFrom<u16> to properly handle
>             error case
>           * Renamed into method for Group to into_u16 to not conflate with the
>             From trait
>           * Refactored new upstream changes to RegistrationOps
>           * Implemented DriverLayout trait for hid::Adapter<T>
>         v4->v5:
>           * Add rust/ to drivers/hid/Makefile
>           * Implement RawDeviceIdIndex trait
>         v3->v4:
>           * Removed specifying tree in MAINTAINERS file since that is up for
>             debate
>           * Minor rebase cleanup
>           * Moved driver logic under drivers/hid/rust
>         v2->v3:
>           * Implemented AlwaysRefCounted trait using embedded struct device's
>             reference counts instead of the separate reference counter in struct
>             hid_device
>           * Used &raw mut as appropriate
>           * Binded include/linux/device.h for get_device and put_device
>           * Cleaned up various comment related formatting
>           * Minified dev_err! format string
>           * Updated Group enum to be repr(u16)
>           * Implemented From<u16> trait for Group
>           * Added TODO comment when const_trait_impl stabilizes
>           * Made group getter functions return a Group variant instead of a raw
>             number
>           * Made sure example code builds
>         v1->v2:
>           * Binded drivers/hid/hid-ids.h for use in Rust drivers
>           * Remove pre-emptive referencing of a C HID driver instance before
>             it is fully initialized in the driver registration path
>           * Moved static getters to generic Device trait implementation, so
>             they can be used by all device::DeviceContext
>           * Use core macros for supporting DeviceContext transitions
>           * Implemented the AlwaysRefCounted and AsRef traits
>           * Make use for dev_err! as appropriate
>         RFC->v1:
>           * Use Danilo's core infrastructure
>           * Account for HID device groups
>           * Remove probe and remove callbacks
>           * Implement report_fixup support
>           * Properly comment code including SAFETY comments
>
>  MAINTAINERS                     |   8 +
>  drivers/hid/Kconfig             |   2 +
>  drivers/hid/Makefile            |   2 +
>  drivers/hid/rust/Kconfig        |  12 +
>  rust/bindings/bindings_helper.h |   3 +
>  rust/kernel/hid.rs              | 530 ++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs              |   2 +
>  7 files changed, 559 insertions(+)
>  create mode 100644 drivers/hid/rust/Kconfig
>  create mode 100644 rust/kernel/hid.rs
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index b8d8a5c41597..1fee14024fa2 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -11319,6 +11319,14 @@ F:	include/uapi/linux/hid*
>  F:	samples/hid/
>  F:	tools/testing/selftests/hid/
>  
> +HID CORE LAYER [RUST]
> +M:	Rahul Rameshbabu <sergeantsagara@protonmail.com>
> +R:	Benjamin Tissoires <bentiss@kernel.org>
> +L:	linux-input@vger.kernel.org
> +S:	Maintained
> +F:	drivers/hid/rust/*.rs
> +F:	rust/kernel/hid.rs
> +
>  HID LOGITECH DRIVERS
>  R:	Filipe Laíns <lains@riseup.net>
>  L:	linux-input@vger.kernel.org
> diff --git a/drivers/hid/Kconfig b/drivers/hid/Kconfig
> index c1d9f7c6a5f2..750c2d49a806 100644
> --- a/drivers/hid/Kconfig
> +++ b/drivers/hid/Kconfig
> @@ -1439,6 +1439,8 @@ endmenu
>  
>  source "drivers/hid/bpf/Kconfig"
>  
> +source "drivers/hid/rust/Kconfig"
> +
>  source "drivers/hid/i2c-hid/Kconfig"
>  
>  source "drivers/hid/intel-ish-hid/Kconfig"
> diff --git a/drivers/hid/Makefile b/drivers/hid/Makefile
> index e01838239ae6..b78ab84c47b4 100644
> --- a/drivers/hid/Makefile
> +++ b/drivers/hid/Makefile
> @@ -8,6 +8,8 @@ hid-$(CONFIG_HID_HAPTIC)	+= hid-haptic.o
>  
>  obj-$(CONFIG_HID_BPF)		+= bpf/
>  
> +obj-$(CONFIG_RUST_HID_ABSTRACTIONS)		+= rust/
> +
>  obj-$(CONFIG_HID)		+= hid.o
>  obj-$(CONFIG_UHID)		+= uhid.o
>  
> diff --git a/drivers/hid/rust/Kconfig b/drivers/hid/rust/Kconfig
> new file mode 100644
> index 000000000000..d3247651829e
> --- /dev/null
> +++ b/drivers/hid/rust/Kconfig
> @@ -0,0 +1,12 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +menu "Rust HID support"
> +
> +config RUST_HID_ABSTRACTIONS
> +	bool "Rust HID abstractions support"
> +	depends on RUST
> +	depends on HID=y
> +	help
> +	  Adds support needed for HID drivers written in Rust. It provides a
> +	  wrapper around the C hid core.
> +
> +endmenu # Rust HID support
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 083cc44aa952..200e58af27a3 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -48,6 +48,7 @@
>  #include <linux/cpumask.h>
>  #include <linux/cred.h>
>  #include <linux/debugfs.h>
> +#include <linux/device.h>
>  #include <linux/device/faux.h>
>  #include <linux/dma-direction.h>
>  #include <linux/dma-mapping.h>
> @@ -60,6 +61,8 @@
>  #include <linux/i2c.h>
>  #include <linux/interrupt.h>
>  #include <linux/io-pgtable.h>
> +#include <linux/hid.h>
> +#include "../../drivers/hid/hid-ids.h"
>  #include <linux/ioport.h>
>  #include <linux/jiffies.h>
>  #include <linux/jump_label.h>
> diff --git a/rust/kernel/hid.rs b/rust/kernel/hid.rs
> new file mode 100644
> index 000000000000..b9db542d923a
> --- /dev/null
> +++ b/rust/kernel/hid.rs
> @@ -0,0 +1,530 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2025 Rahul Rameshbabu <sergeantsagara@protonmail.com>
> +
> +//! Abstractions for the HID interface.
> +//!
> +//! C header: [`include/linux/hid.h`](srctree/include/linux/hid.h)
> +
> +use crate::{
> +    device,
> +    device_id::{
> +        RawDeviceId,
> +        RawDeviceIdIndex, //
> +    },
> +    driver,
> +    error::*,
> +    prelude::*,
> +    types::Opaque, //
> +};
> +use core::{
> +    marker::PhantomData,
> +    ptr::{
> +        addr_of_mut,
> +        NonNull, //
> +    } //
> +};
> +
> +/// Indicates the item is static read-only.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_CONSTANT: u8 = bindings::HID_MAIN_ITEM_CONSTANT as u8;

These can be bitflags with `impl_flags!`?

> +
> +/// Indicates the item represents data from a physical control.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_VARIABLE: u8 = bindings::HID_MAIN_ITEM_VARIABLE as u8;
> +
> +/// Indicates the item should be treated as a relative change from the previous
> +/// report.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_RELATIVE: u8 = bindings::HID_MAIN_ITEM_RELATIVE as u8;
> +
> +/// Indicates the item should wrap around when reaching the extreme high or
> +/// extreme low values.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_WRAP: u8 = bindings::HID_MAIN_ITEM_WRAP as u8;
> +
> +/// Indicates the item should wrap around when reaching the extreme high or
> +/// extreme low values.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_NONLINEAR: u8 = bindings::HID_MAIN_ITEM_NONLINEAR as u8;
> +
> +/// Indicates whether the control has a preferred state it will physically
> +/// return to without user intervention.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_NO_PREFERRED: u8 = bindings::HID_MAIN_ITEM_NO_PREFERRED as u8;
> +
> +/// Indicates whether the control has a physical state where it will not send
> +/// any reports.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_NULL_STATE: u8 = bindings::HID_MAIN_ITEM_NULL_STATE as u8;
> +
> +/// Indicates whether the control requires host system logic to change state.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_VOLATILE: u8 = bindings::HID_MAIN_ITEM_VOLATILE as u8;
> +
> +/// Indicates whether the item is fixed size or a variable buffer of bytes.
> +///
> +/// Refer to [Device Class Definition for HID 1.11]
> +/// Section 6.2.2.5 Input, Output, and Feature Items.
> +///
> +/// [Device Class Definition for HID 1.11]: https://www.usb.org/sites/default/files/hid1_11.pdf
> +pub const MAIN_ITEM_BUFFERED_BYTE: u8 = bindings::HID_MAIN_ITEM_BUFFERED_BYTE as u8;
> +
> +/// HID device groups are intended to help categories HID devices based on a set
> +/// of common quirks and logic that they will require to function correctly.
> +#[repr(u16)]
> +pub enum Group {
> +    /// Used to match a device against any group when probing.
> +    Any = bindings::HID_GROUP_ANY as u16,
> +
> +    /// Indicates a generic device that should need no custom logic from the
> +    /// core HID stack.
> +    Generic = bindings::HID_GROUP_GENERIC as u16,
> +
> +    /// Maps multitouch devices to hid-multitouch instead of hid-generic.
> +    Multitouch = bindings::HID_GROUP_MULTITOUCH as u16,
> +
> +    /// Used for autodetecing and mapping of HID sensor hubs to
> +    /// hid-sensor-hub.
> +    SensorHub = bindings::HID_GROUP_SENSOR_HUB as u16,
> +
> +    /// Used for autodetecing and mapping Win 8 multitouch devices to set the
> +    /// needed quirks.
> +    MultitouchWin8 = bindings::HID_GROUP_MULTITOUCH_WIN_8 as u16,
> +
> +    // Vendor-specific device groups.
> +    /// Used to distinguish Synpatics touchscreens from other products. The
> +    /// touchscreens will be handled by hid-multitouch instead, while everything
> +    /// else will be managed by hid-rmi.
> +    RMI = bindings::HID_GROUP_RMI as u16,
> +
> +    /// Used for hid-core handling to automatically identify Wacom devices and
> +    /// have them probed by hid-wacom.
> +    Wacom = bindings::HID_GROUP_WACOM as u16,
> +
> +    /// Used by logitech-djreceiver and logitech-djdevice to autodetect if
> +    /// devices paied to the DJ receivers are DJ devices and handle them with
> +    /// the device driver.
> +    LogitechDJDevice = bindings::HID_GROUP_LOGITECH_DJ_DEVICE as u16,
> +
> +    /// Since the Valve Steam Controller only has vendor-specific usages,
> +    /// prevent hid-generic from parsing its reports since there would be
> +    /// nothing hid-generic could do for the device.
> +    Steam = bindings::HID_GROUP_STEAM as u16,
> +
> +    /// Used to differentiate 27 Mhz frequency Logitech DJ devices from other
> +    /// Logitech DJ devices.
> +    Logitech27MHzDevice = bindings::HID_GROUP_LOGITECH_27MHZ_DEVICE as u16,
> +
> +    /// Used for autodetecting and mapping Vivaldi devices to hid-vivaldi.
> +    Vivaldi = bindings::HID_GROUP_VIVALDI as u16,
> +}
> +
> +// TODO: use `const_trait_impl` once stabilized:
> +//
> +// ```
> +// impl const From<Group> for u16 {
> +//     /// [`Group`] variants are represented by [`u16`] values.
> +//     fn from(value: Group) -> Self {
> +//         value as Self
> +//     }
> +// }
> +// ```
> +impl Group {
> +    /// Internal function used to convert [`Group`] variants into [`u16`].
> +    const fn into_u16(self) -> u16 {

This and many functions in the abstraction can be `#[inline]`.

> +        self as u16
> +    }
> +}
> +
> +impl TryFrom<u16> for Group {
> +    type Error = &'static str;
> +
> +    /// [`u16`] values can be safely converted to [`Group`] variants.
> +    fn try_from(value: u16) -> Result<Group, Self::Error> {
> +        match value.into() {
> +            bindings::HID_GROUP_GENERIC => Ok(Group::Generic),
> +            bindings::HID_GROUP_MULTITOUCH => Ok(Group::Multitouch),
> +            bindings::HID_GROUP_SENSOR_HUB => Ok(Group::SensorHub),
> +            bindings::HID_GROUP_MULTITOUCH_WIN_8 => Ok(Group::MultitouchWin8),
> +            bindings::HID_GROUP_RMI => Ok(Group::RMI),
> +            bindings::HID_GROUP_WACOM => Ok(Group::Wacom),
> +            bindings::HID_GROUP_LOGITECH_DJ_DEVICE => Ok(Group::LogitechDJDevice),
> +            bindings::HID_GROUP_STEAM => Ok(Group::Steam),
> +            bindings::HID_GROUP_LOGITECH_27MHZ_DEVICE => Ok(Group::Logitech27MHzDevice),
> +            bindings::HID_GROUP_VIVALDI => Ok(Group::Vivaldi),
> +            _ => Err("Unknown HID group encountered!"),

Please define a custom error type or just use a kernel error code.

> +        }
> +    }
> +}
> +
> +/// The HID device representation.
> +///
> +/// This structure represents the Rust abstraction for a C `struct hid_device`.
> +/// The implementation abstracts the usage of an already existing C `struct
> +/// hid_device` within Rust code that we get passed from the C side.
> +///
> +/// # Invariants
> +///
> +/// A [`Device`] instance represents a valid `struct hid_device` created by the
> +/// C portion of the kernel.
> +#[repr(transparent)]
> +pub struct Device<Ctx: device::DeviceContext = device::Normal>(
> +    Opaque<bindings::hid_device>,
> +    PhantomData<Ctx>,
> +);
> +
> +impl<Ctx: device::DeviceContext> Device<Ctx> {
> +    fn as_raw(&self) -> *mut bindings::hid_device {
> +        self.0.get()
> +    }
> +
> +    /// Returns the HID transport bus ID.
> +    pub fn bus(&self) -> u16 {
> +        // SAFETY: `self.as_raw` is a valid pointer to a `struct hid_device`
> +        unsafe { *self.as_raw() }.bus
> +    }
> +
> +    /// Returns the HID report group.
> +    pub fn group(&self) -> Result<Group, &'static str> {
> +        // SAFETY: `self.as_raw` is a valid pointer to a `struct hid_device`
> +        unsafe { *self.as_raw() }.group.try_into()
> +    }
> +
> +    /// Returns the HID vendor ID.
> +    pub fn vendor(&self) -> u32 {
> +        // SAFETY: `self.as_raw` is a valid pointer to a `struct hid_device`
> +        unsafe { *self.as_raw() }.vendor
> +    }
> +
> +    /// Returns the HID product ID.
> +    pub fn product(&self) -> u32 {
> +        // SAFETY: `self.as_raw` is a valid pointer to a `struct hid_device`
> +        unsafe { *self.as_raw() }.product
> +    }
> +}
> +
> +// SAFETY: `Device` is a transparent wrapper of a type that doesn't depend on `Device`'s generic
> +// argument.
> +kernel::impl_device_context_deref!(unsafe { Device });
> +kernel::impl_device_context_into_aref!(Device);
> +
> +// SAFETY: Instances of `Device` are always reference-counted.
> +unsafe impl crate::types::AlwaysRefCounted for Device {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
> +        unsafe { bindings::get_device(&raw mut (*self.as_raw()).dev) };
> +    }
> +
> +    unsafe fn dec_ref(obj: NonNull<Self>) {
> +        // SAFETY: The safety requirements guarantee that the refcount is non-zero.
> +        unsafe { bindings::put_device(&raw mut (*obj.cast::<bindings::hid_device>().as_ptr()).dev) }
> +    }
> +}
> +
> +impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
> +    fn as_ref(&self) -> &device::Device<Ctx> {
> +        // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> +        // `struct hid_device`.
> +        let dev = unsafe { addr_of_mut!((*self.as_raw()).dev) };
> +
> +        // SAFETY: `dev` points to a valid `struct device`.
> +        unsafe { device::Device::from_raw(dev) }
> +    }
> +}
> +
> +/// Abstraction for the HID device ID structure `struct hid_device_id`.
> +#[repr(transparent)]
> +#[derive(Clone, Copy)]
> +pub struct DeviceId(bindings::hid_device_id);
> +
> +impl DeviceId {
> +    /// Equivalent to C's `HID_USB_DEVICE` macro.
> +    ///
> +    /// Create a new `hid::DeviceId` from a group, vendor ID, and device ID
> +    /// number.

The main description should be the first sentence, and C equivalent sentence
follows.

> +    pub const fn new_usb(group: Group, vendor: u32, product: u32) -> Self {
> +        Self(bindings::hid_device_id {
> +            bus: 0x3, // BUS_USB
> +            group: group.into_u16(),
> +            vendor,
> +            product,
> +            driver_data: 0,
> +        })
> +    }
> +
> +    /// Returns the HID transport bus ID.
> +    pub fn bus(&self) -> u16 {
> +        self.0.bus
> +    }
> +
> +    /// Returns the HID report group.
> +    pub fn group(&self) -> Result<Group, &'static str> {
> +        self.0.group.try_into()
> +    }
> +
> +    /// Returns the HID vendor ID.
> +    pub fn vendor(&self) -> u32 {
> +        self.0.vendor
> +    }
> +
> +    /// Returns the HID product ID.
> +    pub fn product(&self) -> u32 {
> +        self.0.product
> +    }
> +}
> +
> +// SAFETY:
> +// * `DeviceId` is a `#[repr(transparent)` wrapper of `hid_device_id` and does not add
> +//   additional invariants, so it's safe to transmute to `RawType`.
> +// * `DRIVER_DATA_OFFSET` is the offset to the `driver_data` field.
> +unsafe impl RawDeviceId for DeviceId {
> +    type RawType = bindings::hid_device_id;
> +}
> +
> +// SAFETY: `DRIVER_DATA_OFFSET` is the offset to the `driver_data` field.
> +unsafe impl RawDeviceIdIndex for DeviceId {
> +    const DRIVER_DATA_OFFSET: usize = core::mem::offset_of!(bindings::hid_device_id, driver_data);
> +
> +    fn index(&self) -> usize {
> +        self.0.driver_data
> +    }
> +}
> +
> +/// [`IdTable`] type for HID.
> +pub type IdTable<T> = &'static dyn kernel::device_id::IdTable<DeviceId, T>;
> +
> +/// Create a HID [`IdTable`] with its alias for modpost.
> +#[macro_export]
> +macro_rules! hid_device_table {
> +    ($table_name:ident, $module_table_name:ident, $id_info_type: ty, $table_data: expr) => {
> +        const $table_name: $crate::device_id::IdArray<
> +            $crate::hid::DeviceId,
> +            $id_info_type,
> +            { $table_data.len() },
> +        > = $crate::device_id::IdArray::new($table_data);
> +
> +        $crate::module_device_table!("hid", $module_table_name, $table_name);
> +    };
> +}
> +
> +/// The HID driver trait.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::{bindings, device, hid};
> +///
> +/// struct MyDriver;
> +///
> +/// kernel::hid_device_table!(
> +///     HID_TABLE,
> +///     MODULE_HID_TABLE,
> +///     <MyDriver as hid::Driver>::IdInfo,
> +///     [(
> +///         hid::DeviceId::new_usb(
> +///             hid::Group::Steam,
> +///             bindings::USB_VENDOR_ID_VALVE,
> +///             bindings::USB_DEVICE_ID_STEAM_DECK,
> +///         ),
> +///         (),
> +///     )]
> +/// );
> +///
> +/// #[vtable]
> +/// impl hid::Driver for MyDriver {
> +///     type IdInfo = ();
> +///     const ID_TABLE: hid::IdTable<Self::IdInfo> = &HID_TABLE;
> +///
> +///     /// This function is optional to implement.
> +///     fn report_fixup<'a, 'b: 'a>(_hdev: &hid::Device<device::Core>, rdesc: &'b mut [u8]) -> &'a [u8] {
> +///         // Perform some report descriptor fixup.
> +///         rdesc
> +///     }
> +/// }
> +/// ```
> +/// Drivers must implement this trait in order to get a HID driver registered.
> +/// Please refer to the `Adapter` documentation for an example.
> +#[vtable]
> +pub trait Driver: Send {
> +    /// The type holding information about each device id supported by the driver.
> +    // TODO: Use `associated_type_defaults` once stabilized:
> +    //
> +    // ```
> +    // type IdInfo: 'static = ();
> +    // ```
> +    type IdInfo: 'static;
> +
> +    /// The table of device ids supported by the driver.
> +    const ID_TABLE: IdTable<Self::IdInfo>;
> +
> +    /// Called before report descriptor parsing. Can be used to mutate the
> +    /// report descriptor before the core HID logic processes the descriptor.
> +    /// Useful for problematic report descriptors that prevent HID devices from
> +    /// functioning correctly.
> +    ///
> +    /// Optional to implement.
> +    fn report_fixup<'a, 'b: 'a>(_hdev: &Device<device::Core>, _rdesc: &'b mut [u8]) -> &'a [u8] {

I think this can just use a single lifetime?

> +        build_error!(VTABLE_DEFAULT_ERROR)
> +    }
> +}
> +
> +/// An adapter for the registration of HID drivers.
> +pub struct Adapter<T: Driver>(T);
> +
> +// SAFETY:
> +// - `bindings::hid_driver` is a C type declared as `repr(C)`.
> +// - `T` is the type of the driver's device private data.
> +// - `struct hid_driver` embeds a `struct device_driver`.
> +// - `DEVICE_DRIVER_OFFSET` is the correct byte offset to the embedded `struct device_driver`.
> +unsafe impl<T: Driver + 'static> driver::DriverLayout for Adapter<T> {
> +    type DriverType = bindings::hid_driver;
> +    type DriverData = T;
> +    const DEVICE_DRIVER_OFFSET: usize = core::mem::offset_of!(Self::DriverType, driver);
> +}
> +
> +// SAFETY: A call to `unregister` for a given instance of `DriverType` is guaranteed to be valid if
> +// a preceding call to `register` has been successful.
> +unsafe impl<T: Driver + 'static> driver::RegistrationOps for Adapter<T> {
> +    unsafe fn register(
> +        hdrv: &Opaque<Self::DriverType>,
> +        name: &'static CStr,
> +        module: &'static ThisModule,
> +    ) -> Result {
> +        // SAFETY: It's safe to set the fields of `struct hid_driver` on initialization.
> +        unsafe {
> +            (*hdrv.get()).name = name.as_char_ptr();
> +            (*hdrv.get()).id_table = T::ID_TABLE.as_ptr();
> +            (*hdrv.get()).report_fixup = if T::HAS_REPORT_FIXUP {
> +                Some(Self::report_fixup_callback)
> +            } else {
> +                None
> +            };
> +        }
> +
> +        // SAFETY: `hdrv` is guaranteed to be a valid `DriverType`
> +        to_result(unsafe {
> +            bindings::__hid_register_driver(hdrv.get(), module.0, name.as_char_ptr())
> +        })
> +    }
> +
> +    unsafe fn unregister(hdrv: &Opaque<Self::DriverType>) {
> +        // SAFETY: `hdrv` is guaranteed to be a valid `DriverType`
> +        unsafe { bindings::hid_unregister_driver(hdrv.get()) }
> +    }
> +}
> +
> +impl<T: Driver + 'static> Adapter<T> {
> +    extern "C" fn report_fixup_callback(
> +        hdev: *mut bindings::hid_device,
> +        buf: *mut u8,
> +        size: *mut kernel::ffi::c_uint,
> +    ) -> *const u8 {
> +        // SAFETY: The HID subsystem only ever calls the report_fixup callback
> +        // with a valid pointer to a `struct hid_device`.
> +        //
> +        // INVARIANT: `hdev` is valid for the duration of
> +        // `report_fixup_callback()`.
> +        let hdev = unsafe { &*hdev.cast::<Device<device::Core>>() };
> +
> +        // SAFETY: The HID subsystem only ever calls the report_fixup callback
> +        // with a valid pointer to a `kernel::ffi::c_uint`.
> +        //
> +        // INVARIANT: `size` is valid for the duration of
> +        // `report_fixup_callback()`.
> +        let buf_len: usize = match unsafe { *size }.try_into() {

This can just be a cast. In all kernel envs `usize` is at least as large as `u32`.

> +            Ok(len) => len,
> +            Err(e) => {
> +                dev_err!(
> +                    hdev.as_ref(),
> +                    "Cannot fix report description due to {:?}!\n",
> +                    e
> +                );
> +
> +                return buf;
> +            }
> +        };
> +
> +        // Build a mutable Rust slice from `buf` and `size`.
> +        //
> +        // SAFETY: The HID subsystem only ever calls the `report_fixup callback`
> +        // with a valid pointer to a `u8` buffer.
> +        //
> +        // INVARIANT: `buf` is valid for the duration of
> +        // `report_fixup_callback()`.
> +        let rdesc_slice = unsafe { core::slice::from_raw_parts_mut(buf, buf_len) };
> +        let rdesc_slice = T::report_fixup(hdev, rdesc_slice);
> +
> +        match rdesc_slice.len().try_into() {

I'm somewhat inclined to just `BUG()` (i.e. `.expect()`) in this case. A driver
that hits this error condition would need to leak >= 4G of memory to satisfy the
lifetime bound.

> +            // SAFETY: The HID subsystem only ever calls the report_fixup
> +            // callback with a valid pointer to a `kernel::ffi::c_uint`.
> +            //
> +            // INVARIANT: `size` is valid for the duration of
> +            // `report_fixup_callback()`.
> +            Ok(len) => unsafe { *size = len },
> +            Err(e) => {
> +                dev_err!(
> +                    hdev.as_ref(),

as_ref() shouldn't be needed now.

Best,
Gary

> +                    "Fixed report description will not be used due to {:?}!\n",
> +                    e
> +                );
> +
> +                return buf;
> +            }
> +        }
> +
> +        rdesc_slice.as_ptr()
> +    }
> +}
> +
> +/// Declares a kernel module that exposes a single HID driver.
> +///
> +/// # Examples
> +///
> +/// ```ignore
> +/// kernel::module_hid_driver! {
> +///     type: MyDriver,
> +///     name: "Module name",
> +///     authors: ["Author name"],
> +///     description: "Description",
> +///     license: "GPL",
> +/// }
> +/// ```
> +#[macro_export]
> +macro_rules! module_hid_driver {
> +    ($($f:tt)*) => {
> +        $crate::module_driver!(<T>, $crate::hid::Adapter<T>, { $($f)* });
> +    };
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 3da92f18f4ee..e2dcacd9369e 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -102,6 +102,8 @@
>  pub mod id_pool;
>  #[doc(hidden)]
>  pub mod impl_flags;
> +#[cfg(CONFIG_RUST_HID_ABSTRACTIONS)]
> +pub mod hid;
>  pub mod init;
>  pub mod io;
>  pub mod ioctl;