Message-ID: <b4092d2693a06526e2b39fd7642e80dd2a7cd0e8.camel@posteo.de>
Date: Mon, 17 Nov 2025 16:12:35 +0000
From: Markus Probst <markus.probst@...teo.de>
To: Igor Korotin <igor.korotin.linux@...il.com>, Miguel Ojeda
 <ojeda@...nel.org>,  Alex Gaynor <alex.gaynor@...il.com>, Wolfram Sang
 <wsa+renesas@...g-engineering.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
 Björn Roy Baron	 <bjorn3_gh@...tonmail.com>, Benno
 Lossin <lossin@...nel.org>, Andreas Hindborg	 <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross	 <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>, Greg Kroah-Hartman	
 <gregkh@...uxfoundation.org>, Viresh Kumar <viresh.kumar@...aro.org>, Asahi
 Lina <lina+kernel@...hilina.net>, Wedson Almeida Filho
 <wedsonaf@...il.com>, Alex Hung	 <alex.hung@....com>, Tamir Duberstein
 <tamird@...il.com>, Xiangfei Ding	 <dingxiangfei2009@...il.com>,
 linux-kernel@...r.kernel.org, 	rust-for-linux@...r.kernel.org,
 linux-i2c@...r.kernel.org
Subject: Re: [PATCH v8 1/4] rust: i2c: add basic I2C device and driver
 abstractions

On Sun, 2025-11-16 at 16:21 +0000, Igor Korotin wrote:
> Implement the core abstractions needed for I2C drivers, including:
> 
> * `i2c::Driver` — the trait drivers must implement, including `probe`
> 
> * `i2c::I2cClient` — a safe wrapper around `struct i2c_client`
> 
> * `i2c::Adapter` — implements `driver::RegistrationOps` to hook into the
>   generic `driver::Registration` machinery
> 
> * `i2c::DeviceId` — a `RawDeviceIdIndex` implementation for I2C device IDs
> 
> Signed-off-by: Igor Korotin <igor.korotin.linux@...il.com>
> ---
>  MAINTAINERS                     |   8 +
>  rust/bindings/bindings_helper.h |   1 +
>  rust/kernel/i2c.rs              | 436 ++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs              |   2 +
>  4 files changed, 447 insertions(+)
>  create mode 100644 rust/kernel/i2c.rs
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 3da2c26a796b..a8dfde2f5633 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -11743,6 +11743,14 @@ F:	include/linux/i2c.h
>  F:	include/uapi/linux/i2c-*.h
>  F:	include/uapi/linux/i2c.h
>  
> +I2C SUBSYSTEM [RUST]
> +M:	Igor Korotin <igor.korotin.linux@...il.com>
> +R:	Danilo Krummrich <dakr@...nel.org>
> +R:	Daniel Almeida <daniel.almeida@...labora.com>
> +L:	rust-for-linux@...r.kernel.org
> +S:	Maintained
> +F:	rust/kernel/i2c.rs
> +
>  I2C SUBSYSTEM HOST DRIVERS
>  M:	Andi Shyti <andi.shyti@...nel.org>
>  L:	linux-i2c@...r.kernel.org
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index a79fd111f886..f92abb578b56 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -58,6 +58,7 @@
>  #include <linux/firmware.h>
>  #include <linux/interrupt.h>
>  #include <linux/fs.h>
> +#include <linux/i2c.h>
>  #include <linux/ioport.h>
>  #include <linux/jiffies.h>
>  #include <linux/jump_label.h>
> diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
> new file mode 100644
> index 000000000000..a5ad8213d9ac
> --- /dev/null
> +++ b/rust/kernel/i2c.rs
> @@ -0,0 +1,436 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! I2C Driver subsystem
> +
> +// I2C Driver abstractions.
> +use crate::{
> +    acpi,
> +    container_of,
> +    device,
> +    device_id::{
> +        RawDeviceId,
> +        RawDeviceIdIndex, //
> +    },
> +    driver,
> +    error::*,
> +    of,
> +    prelude::*,
> +    types::{
> +        AlwaysRefCounted,
> +        Opaque, //
> +    }, //
> +};
> +
> +use core::{
> +    marker::PhantomData,
> +    ptr::NonNull, //
> +};
> +
> +/// An I2C device id table.
> +#[repr(transparent)]
> +#[derive(Clone, Copy)]
> +pub struct DeviceId(bindings::i2c_device_id);
> +
> +impl DeviceId {
> +    const I2C_NAME_SIZE: usize = 20;
> +
> +    /// Create a new device id from an I2C 'id' string.
> +    #[inline(always)]
> +    pub const fn new(id: &'static CStr) -> Self {
> +        build_assert!(
> +            id.len_with_nul() <= Self::I2C_NAME_SIZE,
> +            "ID exceeds 20 bytes"
> +        );
> +        let src = id.as_bytes_with_nul();
> +        // Replace with `bindings::i2c_device_id::default()` once stabilized for `const`.
> +        // SAFETY: FFI type is valid to be zero-initialized.
> +        let mut i2c: bindings::i2c_device_id = pin_init::zeroed();
Clippy throws the error:

error: statement has unnecessary safety comment
  --> rust/kernel/i2c.rs:54:9
   |
54 |         let mut i2c: bindings::i2c_device_id = pin_init::zeroed();
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: consider removing the safety comment
  --> rust/kernel/i2c.rs:53:9
   |
53 |         // SAFETY: FFI type is valid to be zero-initialized.
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = help: for further information visit
https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#unnecessary_safety_comment
   = note: `-D clippy::unnecessary-safety-comment` implied by `-D
warnings`
   = help: to override `-D warnings` add
`#[allow(clippy::unnecessary_safety_comment)]`

Thanks
- Markus Probst

> +        let mut i = 0;
> +        while i < src.len() {
> +            i2c.name[i] = src[i];
> +            i += 1;
> +        }
> +
> +        Self(i2c)
> +    }
> +}
> +
> +// SAFETY: `DeviceId` is a `#[repr(transparent)]` wrapper of `i2c_device_id` and does not add
> +// additional invariants, so it's safe to transmute to `RawType`.
> +unsafe impl RawDeviceId for DeviceId {
> +    type RawType = bindings::i2c_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::i2c_device_id, driver_data);
> +
> +    fn index(&self) -> usize {
> +        self.0.driver_data
> +    }
> +}
> +
> +/// IdTable type for I2C
> +pub type IdTable<T> = &'static dyn kernel::device_id::IdTable<DeviceId, T>;
> +
> +/// Create a I2C `IdTable` with its alias for modpost.
> +#[macro_export]
> +macro_rules! i2c_device_table {
> +    ($table_name:ident, $module_table_name:ident, $id_info_type: ty, $table_data: expr) => {
> +        const $table_name: $crate::device_id::IdArray<
> +            $crate::i2c::DeviceId,
> +            $id_info_type,
> +            { $table_data.len() },
> +        > = $crate::device_id::IdArray::new($table_data);
> +
> +        $crate::module_device_table!("i2c", $module_table_name, $table_name);
> +    };
> +}
> +
> +/// An adapter for the registration of I2C drivers.
> +pub struct Adapter<T: Driver>(T);
> +
> +// SAFETY: A call to `unregister` for a given instance of `RegType` is guaranteed to be valid if
> +// a preceding call to `register` has been successful.
> +unsafe impl<T: Driver + 'static> driver::RegistrationOps for Adapter<T> {
> +    type RegType = bindings::i2c_driver;
> +
> +    unsafe fn register(
> +        idrv: &Opaque<Self::RegType>,
> +        name: &'static CStr,
> +        module: &'static ThisModule,
> +    ) -> Result {
> +        build_assert!(
> +            T::ACPI_ID_TABLE.is_some() || T::OF_ID_TABLE.is_some() || T::I2C_ID_TABLE.is_some(),
> +            "At least one of ACPI/OF/Legacy tables must be present when registering an i2c driver"
> +        );
> +
> +        let i2c_table = match T::I2C_ID_TABLE {
> +            Some(table) => table.as_ptr(),
> +            None => core::ptr::null(),
> +        };
> +
> +        let of_table = match T::OF_ID_TABLE {
> +            Some(table) => table.as_ptr(),
> +            None => core::ptr::null(),
> +        };
> +
> +        let acpi_table = match T::ACPI_ID_TABLE {
> +            Some(table) => table.as_ptr(),
> +            None => core::ptr::null(),
> +        };
> +
> +        // SAFETY: It's safe to set the fields of `struct i2c_client` on initialization.
> +        unsafe {
> +            (*idrv.get()).driver.name = name.as_char_ptr();
> +            (*idrv.get()).probe = Some(Self::probe_callback);
> +            (*idrv.get()).remove = Some(Self::remove_callback);
> +            (*idrv.get()).shutdown = Some(Self::shutdown_callback);
> +            (*idrv.get()).id_table = i2c_table;
> +            (*idrv.get()).driver.of_match_table = of_table;
> +            (*idrv.get()).driver.acpi_match_table = acpi_table;
> +        }
> +
> +        // SAFETY: `idrv` is guaranteed to be a valid `RegType`.
> +        to_result(unsafe { bindings::i2c_register_driver(module.0, idrv.get()) })
> +    }
> +
> +    unsafe fn unregister(idrv: &Opaque<Self::RegType>) {
> +        // SAFETY: `idrv` is guaranteed to be a valid `RegType`.
> +        unsafe { bindings::i2c_del_driver(idrv.get()) }
> +    }
> +}
> +
> +impl<T: Driver + 'static> Adapter<T> {
> +    extern "C" fn probe_callback(idev: *mut bindings::i2c_client) -> kernel::ffi::c_int {
> +        // SAFETY: The I2C bus only ever calls the probe callback with a valid pointer to a
> +        // `struct i2c_client`.
> +        //
> +        // INVARIANT: `idev` is valid for the duration of `probe_callback()`.
> +        let idev = unsafe { &*idev.cast::<I2cClient<device::CoreInternal>>() };
> +
> +        let info =
> +            Self::i2c_id_info(idev).or_else(|| <Self as driver::Adapter>::id_info(idev.as_ref()));
> +
> +        from_result(|| {
> +            let data = T::probe(idev, info);
> +
> +            idev.as_ref().set_drvdata(data)?;
> +            Ok(0)
> +        })
> +    }
> +
> +    extern "C" fn remove_callback(idev: *mut bindings::i2c_client) {
> +        // SAFETY: `idev` is a valid pointer to a `struct i2c_client`.
> +        let idev = unsafe { &*idev.cast::<I2cClient<device::CoreInternal>>() };
> +
> +        // SAFETY: `remove_callback` is only ever called after a successful call to
> +        // `probe_callback`, hence it's guaranteed that `I2cClient::set_drvdata()` has been called
> +        // and stored a `Pin<KBox<T>>`.
> +        let data = unsafe { idev.as_ref().drvdata_obtain::<T>() };
> +
> +        T::unbind(idev, data.as_ref());
> +    }
> +
> +    extern "C" fn shutdown_callback(idev: *mut bindings::i2c_client) {
> +        // SAFETY: `shutdown_callback` is only ever called for a valid `idev`
> +        let idev = unsafe { &*idev.cast::<I2cClient<device::CoreInternal>>() };
> +
> +        // SAFETY: `shutdown_callback` is only ever called after a successful call to
> +        // `probe_callback`, hence it's guaranteed that `Device::set_drvdata()` has been called
> +        // and stored a `Pin<KBox<T>>`.
> +        let data = unsafe { idev.as_ref().drvdata_obtain::<T>() };
> +
> +        T::shutdown(idev, data.as_ref());
> +    }
> +
> +    /// The [`i2c::IdTable`] of the corresponding driver.
> +    fn i2c_id_table() -> Option<IdTable<<Self as driver::Adapter>::IdInfo>> {
> +        T::I2C_ID_TABLE
> +    }
> +
> +    /// Returns the driver's private data from the matching entry in the [`i2c::IdTable`], if any.
> +    ///
> +    /// If this returns `None`, it means there is no match with an entry in the [`i2c::IdTable`].
> +    fn i2c_id_info(dev: &I2cClient) -> Option<&'static <Self as driver::Adapter>::IdInfo> {
> +        let table = Self::i2c_id_table()?;
> +
> +        // SAFETY:
> +        // - `table` has static lifetime, hence it's valid for reads
> +        // - `dev` is guaranteed to be valid while it's alive, and so is `dev.as_raw()`.
> +        let raw_id = unsafe { bindings::i2c_match_id(table.as_ptr(), dev.as_raw()) };
> +
> +        if raw_id.is_null() {
> +            return None;
> +        }
> +
> +        // SAFETY: `DeviceId` is a `#[repr(transparent)` wrapper of `struct i2c_device_id` and
> +        // does not add additional invariants, so it's safe to transmute.
> +        let id = unsafe { &*raw_id.cast::<DeviceId>() };
> +
> +        Some(table.info(<DeviceId as RawDeviceIdIndex>::index(id)))
> +    }
> +}
> +
> +impl<T: Driver + 'static> driver::Adapter for Adapter<T> {
> +    type IdInfo = T::IdInfo;
> +
> +    fn of_id_table() -> Option<of::IdTable<Self::IdInfo>> {
> +        T::OF_ID_TABLE
> +    }
> +
> +    fn acpi_id_table() -> Option<acpi::IdTable<Self::IdInfo>> {
> +        T::ACPI_ID_TABLE
> +    }
> +}
> +
> +/// Declares a kernel module that exposes a single i2c driver.
> +///
> +/// # Examples
> +///
> +/// ```ignore
> +/// kernel::module_i2c_driver! {
> +///     type: MyDriver,
> +///     name: "Module name",
> +///     authors: ["Author name"],
> +///     description: "Description",
> +///     license: "GPL v2",
> +/// }
> +/// ```
> +#[macro_export]
> +macro_rules! module_i2c_driver {
> +    ($($f:tt)*) => {
> +        $crate::module_driver!(<T>, $crate::i2c::Adapter<T>, { $($f)* });
> +    };
> +}
> +
> +/// The i2c driver trait.
> +///
> +/// Drivers must implement this trait in order to get a i2c driver registered.
> +///
> +/// # Example
> +///
> +///```
> +/// # use kernel::{acpi, bindings, c_str, device::Core, i2c, of};
> +///
> +/// struct MyDriver;
> +///
> +/// kernel::acpi_device_table!(
> +///     ACPI_TABLE,
> +///     MODULE_ACPI_TABLE,
> +///     <MyDriver as i2c::Driver>::IdInfo,
> +///     [
> +///         (acpi::DeviceId::new(c_str!("LNUXBEEF")), ())
> +///     ]
> +/// );
> +///
> +/// kernel::i2c_device_table!(
> +///     I2C_TABLE,
> +///     MODULE_I2C_TABLE,
> +///     <MyDriver as i2c::Driver>::IdInfo,
> +///     [
> +///          (i2c::DeviceId::new(c_str!("rust_driver_i2c")), ())
> +///     ]
> +/// );
> +///
> +/// kernel::of_device_table!(
> +///     OF_TABLE,
> +///     MODULE_OF_TABLE,
> +///     <MyDriver as i2c::Driver>::IdInfo,
> +///     [
> +///         (of::DeviceId::new(c_str!("test,device")), ())
> +///     ]
> +/// );
> +///
> +/// impl i2c::Driver for MyDriver {
> +///     type IdInfo = ();
> +///     const I2C_ID_TABLE: Option<i2c::IdTable<Self::IdInfo>> = Some(&I2C_TABLE);
> +///     const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = Some(&OF_TABLE);
> +///     const ACPI_ID_TABLE: Option<acpi::IdTable<Self::IdInfo>> = Some(&ACPI_TABLE);
> +///
> +///     fn probe(
> +///         _idev: &i2c::I2cClient<Core>,
> +///         _id_info: Option<&Self::IdInfo>,
> +///     ) -> impl PinInit<Self, Error> {
> +///         Err(ENODEV)
> +///     }
> +///
> +///     fn shutdown(_idev: &i2c::I2cClient<Core>, this: Pin<&Self>) {
> +///     }
> +/// }
> +///```
> +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 I2C_ID_TABLE: Option<IdTable<Self::IdInfo>> = None;
> +
> +    /// The table of OF device ids supported by the driver.
> +    const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = None;
> +
> +    /// The table of ACPI device ids supported by the driver.
> +    const ACPI_ID_TABLE: Option<acpi::IdTable<Self::IdInfo>> = None;
> +
> +    /// I2C driver probe.
> +    ///
> +    /// Called when a new i2c client is added or discovered.
> +    /// Implementers should attempt to initialize the client here.
> +    fn probe(
> +        dev: &I2cClient<device::Core>,
> +        id_info: Option<&Self::IdInfo>,
> +    ) -> impl PinInit<Self, Error>;
> +
> +    /// I2C driver shutdown.
> +    ///
> +    /// Called by the kernel during system reboot or power-off to allow the [`Driver`] to bring the
> +    /// [`Device`] into a safe state. Implementing this callback is optional.
> +    ///
> +    /// Typical actions include stopping transfers, disabling interrupts, or resetting the hardware
> +    /// to prevent undesired behavior during shutdown.
> +    ///
> +    /// This callback is distinct from final resource cleanup, as the driver instance remains valid
> +    /// after it returns. Any deallocation or teardown of driver-owned resources should instead be
> +    /// handled in `Self::drop`.
> +    fn shutdown(dev: &I2cClient<device::Core>, this: Pin<&Self>) {
> +        let _ = (dev, this);
> +    }
> +
> +    /// I2C driver unbind.
> +    ///
> +    /// Called when a [`Device`] is unbound from its bound [`Driver`]. Implementing this callback
> +    /// is optional.
> +    ///
> +    /// This callback serves as a place for drivers to perform teardown operations that require a
> +    /// `&Device<Core>` or `&Device<Bound>` reference. For instance, drivers may try to perform I/O
> +    /// operations to gracefully tear down the device.
> +    ///
> +    /// Otherwise, release operations for driver resources should be performed in `Self::drop`.
> +    fn unbind(dev: &I2cClient<device::Core>, this: Pin<&Self>) {
> +        let _ = (dev, this);
> +    }
> +}
> +
> +/// The i2c client representation.
> +///
> +/// This structure represents the Rust abstraction for a C `struct i2c_client`. The
> +/// implementation abstracts the usage of an existing C `struct i2c_client` that
> +/// gets passed from the C side
> +///
> +/// # Invariants
> +///
> +/// A [`I2cClient`] instance represents a valid `struct i2c_client` created by the C portion of
> +/// the kernel.
> +#[repr(transparent)]
> +pub struct I2cClient<Ctx: device::DeviceContext = device::Normal>(
> +    Opaque<bindings::i2c_client>,
> +    PhantomData<Ctx>,
> +);
> +
> +impl<Ctx: device::DeviceContext> I2cClient<Ctx> {
> +    fn as_raw(&self) -> *mut bindings::i2c_client {
> +        self.0.get()
> +    }
> +}
> +
> +// SAFETY: `I2cClient` is a transparent wrapper of a type that doesn't depend on
> +// `I2cClient`'s generic argument.
> +kernel::impl_device_context_deref!(unsafe { I2cClient });
> +kernel::impl_device_context_into_aref!(I2cClient);
> +
> +// SAFETY: Instances of `I2cClient` are always reference-counted.
> +unsafe impl AlwaysRefCounted for I2cClient {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
> +        unsafe { bindings::get_device(self.as_ref().as_raw()) };
> +    }
> +
> +    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.as_ref().as_raw()).dev) }
> +    }
> +}
> +
> +impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for I2cClient<Ctx> {
> +    fn as_ref(&self) -> &device::Device<Ctx> {
> +        let raw = self.as_raw();
> +        // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> +        // `struct i2c_client`.
> +        let dev = unsafe { &raw mut (*raw).dev };
> +
> +        // SAFETY: `dev` points to a valid `struct device`.
> +        unsafe { device::Device::from_raw(dev) }
> +    }
> +}
> +
> +impl<Ctx: device::DeviceContext> TryFrom<&device::Device<Ctx>> for &I2cClient<Ctx> {
> +    type Error = kernel::error::Error;
> +
> +    fn try_from(dev: &device::Device<Ctx>) -> Result<Self, Self::Error> {
> +        // SAFETY: By the type invariant of `Device`, `dev.as_raw()` is a valid pointer to a
> +        // `struct device`.
> +        if unsafe { bindings::i2c_verify_client(dev.as_raw()).is_null() } {
> +            return Err(EINVAL);
> +        }
> +
> +        // SAFETY: We've just verified that the type of `dev` equals to
> +        // `bindings::i2c_client_type`, hence `dev` must be embedded in a valid
> +        // `struct i2c_client` as guaranteed by the corresponding C code.
> +        let idev = unsafe { container_of!(dev.as_raw(), bindings::i2c_client, dev) };
> +
> +        // SAFETY: `idev` is a valid pointer to a `struct i2c_client`.
> +        Ok(unsafe { &*idev.cast() })
> +    }
> +}
> +
> +// SAFETY: A `I2cClient` is always reference-counted and can be released from any thread.
> +unsafe impl Send for I2cClient {}
> +
> +// SAFETY: `I2cClient` can be shared among threads because all methods of `I2cClient`
> +// (i.e. `I2cClient<Normal>) are thread safe.
> +unsafe impl Sync for I2cClient {}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 3dd7bebe7888..8c0070a8029e 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -94,6 +94,8 @@
>  pub mod firmware;
>  pub mod fmt;
>  pub mod fs;
> +#[cfg(CONFIG_I2C = "y")]
> +pub mod i2c;
>  pub mod id_pool;
>  pub mod init;
>  pub mod io;

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