Message-Id: <CB269793-D165-4D22-95E5-F978C1ECC79E@collabora.com>
Date: Wed, 27 Aug 2025 15:37:36 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Igor Korotin <igor.korotin.linux@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wolfram Sang <wsa+renesas@...g-engineering.com>,
 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 v4 1/3] rust: i2c: add basic I2C device and driver
 abstractions

Hi Igor,

> On 20 Aug 2025, at 12:19, Igor Korotin <igor.korotin.linux@...il.com> 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              | 396 ++++++++++++++++++++++++++++++++
> rust/kernel/lib.rs              |   2 +
> 4 files changed, 407 insertions(+)
> create mode 100644 rust/kernel/i2c.rs
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index fe168477caa4..c44c7ac317b1 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -11516,6 +11516,14 @@ 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 84d60635e8a9..81796d5e16e8 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -53,6 +53,7 @@
> #include <linux/file.h>
> #include <linux/firmware.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..f5e8c4bc1b7d
> --- /dev/null
> +++ b/rust/kernel/i2c.rs
> @@ -0,0 +1,396 @@
> +// 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::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::acpi_device_id::default()` once stabilized for `const`.
> +        // SAFETY: FFI type is valid to be zero-initialized.
> +        let mut i2c: bindings::i2c_device_id = unsafe { core::mem::zeroed() };
> +        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”.

The generated error is cryptic, so we should add as much information as possible.

> +
> +        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()));

I wonder if these should be private member functions?

> +
> +        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>>`.
> +        drop(unsafe { idev.as_ref().drvdata_obtain::<Pin<KBox<T>>>() });
> +    }
> +
> +    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::Core>>() };
> +
> +        T::shutdown(idev);
> +    }
> +
> +    /// 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> {

Again, perhaps a private member function? I’m trying to simplify the syntax here.

> +        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 `idev.as_raw()`.

I don’t see an “idev” in scope? Perhaps you meant “dev” ?

> +        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 crate::device_id::RawDeviceIdIndex>::index(id)))

Do we really need to use the fully-qualified syntax here?

> +    }
> +}
> +
> +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>,
> +///     ) -> Result<Pin<KBox<Self>>> {
> +///         Err(ENODEV)
> +///     }
> +///
> +///     fn shutdown(_idev: &i2c::I2cClient<Core>) {
> +///     }
> +/// }
> +///```
> +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>,
> +    ) -> Result<Pin<KBox<Self>>>;
> +
> +    /// I2C driver shutdown
> +    ///
> +    /// Called when
> +    fn shutdown(dev: &I2cClient<device::Core>) {
> +        let _ = dev;
> +    }
> +}
> +
> +/// 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 crate::types::AlwaysRefCounted for I2cClient {

I’d import this.

> +    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 ed53169e795c..a5e97a9b1546 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -89,6 +89,8 @@
> pub mod firmware;
> pub mod fmt;
> pub mod fs;
> +#[cfg(CONFIG_I2C = "y")]
> +pub mod i2c;
> pub mod init;
> pub mod io;
> pub mod ioctl;
> -- 
> 2.43.0
> 
> 

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