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

[Rahul Rameshbabu <sergeantsagara@protonmail.com>]

On Fri, 27 Feb, 2026 15:27:59 +0000 "Gary Guo" <gary@garyguo.net> wrote:
> 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?
>

I think the problem is when a driver decides to replace the _rdesc with
a static lifetime report descriptor declared in the device driver. My
example driver does not do this, but this is another common pattern in C
HID drivers. This is why two separate lifetimes were required.

>> +        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
>

Thanks for the feedback. Aside from my concern regarding the two
separate lifetimes, I think all the feedback you provided made sense to
me. Applying towards my v7.

Thanks,
Rahul Rameshbabu

>> +                    "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;