[PATCH 1/9] rust: i2c: add basic I2C client abstraction

[Fabien Parent <parent.f@gmail.com>]

From: Fiona Behrens <me@kloenk.dev>

Implement an abstraction to write I2C device drivers. The abstraction
is pretty basic and provides just the infrastructure to probe
a device from I2C/OF device_id and abstract `i2c_client`.
The client will be used by the Regmap abstraction to perform
I/O on the I2C bus.

Signed-off-by: Fiona Behrens <me@kloenk.dev>
Co-developed-by: Fabien Parent <fabien.parent@linaro.org>
Signed-off-by: Fabien Parent <fabien.parent@linaro.org>
---
 MAINTAINERS                     |   1 +
 rust/bindings/bindings_helper.h |   1 +
 rust/helpers/helpers.c          |   1 +
 rust/helpers/i2c.c              |  13 ++
 rust/kernel/i2c.rs              | 288 ++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |   2 +
 6 files changed, 306 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index 6b9e10551392c185b9314c9f94edeaf6e85af58f..961fe4ed39605bf489d1d9e473f47bccb692ff14 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10796,6 +10796,7 @@ F:	include/linux/i2c-smbus.h
 F:	include/linux/i2c.h
 F:	include/uapi/linux/i2c-*.h
 F:	include/uapi/linux/i2c.h
+F:	rust/kernel/i2c.rs
 
 I2C SUBSYSTEM HOST DRIVERS
 M:	Andi Shyti <andi.shyti@kernel.org>
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index e9fdceb568b8f94e602ee498323e5768a40a6cba..a882efb90bfc27960ef1fd5f2dc8cc40533a1c27 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -16,6 +16,7 @@
 #include <linux/file.h>
 #include <linux/firmware.h>
 #include <linux/fs.h>
+#include <linux/i2c.h>
 #include <linux/jiffies.h>
 #include <linux/jump_label.h>
 #include <linux/mdio.h>
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 0640b7e115be1553549312dcfdf842bcae3bde1b..630e903f516ee14a51f46ff0bcc68e8f9a64021a 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -15,6 +15,7 @@
 #include "device.c"
 #include "err.c"
 #include "fs.c"
+#include "i2c.c"
 #include "io.c"
 #include "jump_label.c"
 #include "kunit.c"
diff --git a/rust/helpers/i2c.c b/rust/helpers/i2c.c
new file mode 100644
index 0000000000000000000000000000000000000000..8ffdc454e7597cc61909da5b3597057aeb5f7299
--- /dev/null
+++ b/rust/helpers/i2c.c
@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/i2c.h>
+
+void *rust_helper_i2c_get_clientdata(const struct i2c_client *client)
+{
+	return i2c_get_clientdata(client);
+}
+
+void rust_helper_i2c_set_clientdata(struct i2c_client *client, void *data)
+{
+	i2c_set_clientdata(client, data);
+}
diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
new file mode 100644
index 0000000000000000000000000000000000000000..efa03335e5b59e72738380e94213976b2464c25b
--- /dev/null
+++ b/rust/kernel/i2c.rs
@@ -0,0 +1,288 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Abstractions for the I2C bus.
+//!
+//! C header: [`include/linux/i2c.h`](srctree/include/linux/i2c.h)
+
+use crate::{
+    bindings, container_of,
+    device::Device,
+    device_id::{self, RawDeviceId},
+    driver,
+    error::{to_result, Result},
+    of,
+    prelude::*,
+    str::CStr,
+    types::{ARef, ForeignOwnable, Opaque},
+    ThisModule,
+};
+
+/// Abstraction for `bindings::i2c_device_id`.
+#[repr(transparent)]
+#[derive(Clone, Copy)]
+pub struct DeviceId(bindings::i2c_device_id);
+
+impl DeviceId {
+    /// Create a new device id from an I2C name.
+    pub const fn new(name: &CStr) -> Self {
+        let src = name.as_bytes_with_nul();
+        // TODO: 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 = unsafe { core::mem::zeroed() };
+
+        let mut i = 0;
+        while i < src.len() {
+            i2c.name[i] = src[i] as _;
+            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`.
+// * `DRIVER_DATA_OFFSET` is the offset to the `data` field.
+unsafe impl RawDeviceId for DeviceId {
+    type RawType = bindings::i2c_device_id;
+
+    const DRIVER_DATA_OFFSET: usize = core::mem::offset_of!(bindings::i2c_device_id, driver_data);
+
+    fn index(&self) -> usize {
+        self.0.driver_data as _
+    }
+}
+
+/// I2C [`DeviceId`] table.
+pub type IdTable<T> = &'static dyn device_id::IdTable<DeviceId, T>;
+
+/// An adapter for the registration of I2C drivers.
+#[doc(hidden)]
+pub struct Adapter<T: Driver + 'static>(T);
+
+impl<T: Driver + 'static> driver::RegistrationOps for Adapter<T> {
+    type RegType = bindings::i2c_driver;
+
+    fn register(
+        i2cdrv: &Opaque<Self::RegType>,
+        name: &'static CStr,
+        module: &'static ThisModule,
+    ) -> Result {
+        // SAFETY: It's safe to set the fields of `struct i2c_driver` on initialization.
+        unsafe {
+            (*i2cdrv.get()).driver.name = name.as_char_ptr();
+            (*i2cdrv.get()).probe = Some(Self::probe_callback);
+            (*i2cdrv.get()).remove = Some(Self::remove_callback);
+            if let Some(t) = T::I2C_ID_TABLE {
+                (*i2cdrv.get()).id_table = t.as_ptr();
+            }
+            if let Some(t) = T::OF_ID_TABLE {
+                (*i2cdrv.get()).driver.of_match_table = t.as_ptr();
+            }
+        }
+
+        // SAFETY: `i2cdrv` is guaranteed to be a valid `RegType`.
+        to_result(unsafe { bindings::i2c_register_driver(module.0, i2cdrv.get()) })
+    }
+
+    fn unregister(i2cdrv: &Opaque<Self::RegType>) {
+        // SAFETY: `i2cdrv` is guaranteed to be a valid `RegType`.
+        unsafe { bindings::i2c_del_driver(i2cdrv.get()) };
+    }
+}
+
+impl<T: Driver> Adapter<T> {
+    /// Get the [`Self::IdInfo`] that matched during probe.
+    fn id_info(client: &mut Client) -> Option<&'static T::IdInfo> {
+        let id = <Self as driver::Adapter>::id_info(client.as_ref());
+        if id.is_some() {
+            return id;
+        }
+
+        // SAFETY: `client` and `client.as_raw()` are guaranteed to be valid.
+        let id = unsafe { bindings::i2c_client_get_device_id(client.as_raw()) };
+        if !id.is_null() {
+            // 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 { &*id.cast::<DeviceId>() };
+            return Some(T::I2C_ID_TABLE?.info(id.index()));
+        }
+
+        None
+    }
+
+    extern "C" fn probe_callback(client: *mut bindings::i2c_client) -> core::ffi::c_int {
+        // SAFETY: The i2c bus only ever calls the probe callback with a valid `client`.
+        let dev = unsafe { Device::get_device(core::ptr::addr_of_mut!((*client).dev)) };
+        // SAFETY: `dev` is guaranteed to be embedded in a valid `struct i2c_client` by the
+        // call above.
+        let mut client = unsafe { Client::from_dev(dev) };
+
+        let info = Self::id_info(&mut client);
+        match T::probe(&mut client, info) {
+            Ok(data) => {
+                // Let the `struct i2c_client` own a reference of the driver's private data.
+                // SAFETY: By the type invariant `client.as_raw` returns a valid pointer to a
+                // `struct i2c_client`.
+                unsafe { bindings::i2c_set_clientdata(client.as_raw(), data.into_foreign() as _) };
+            }
+            Err(err) => return Error::to_errno(err),
+        }
+
+        0
+    }
+
+    extern "C" fn remove_callback(client: *mut bindings::i2c_client) {
+        // SAFETY: `client` is a valid pointer to a `struct i2c_client`.
+        let ptr = unsafe { bindings::i2c_get_clientdata(client) };
+
+        // SAFETY: `remove_callback` is only ever called after a successful call to
+        // `probe_callback`, hence it's guaranteed that `ptr` points to a valid and initialized
+        // `KBox<T>` pointer created through `KBox::into_foreign`.
+        let _ = unsafe { KBox::<T>::from_foreign(ptr) };
+    }
+}
+
+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
+    }
+}
+
+/// The I2C driver trait.
+///
+/// Drivers must implement this trait in order to get a i2c driver registered.
+///
+/// # Example
+///
+///```
+/// # use kernel::{bindings, c_str, i2c, of};
+/// #
+/// kernel::of_device_table!(
+///     OF_ID_TABLE,
+///     MODULE_OF_ID_TABLE,
+///     <MyDriver as i2c::Driver>::IdInfo,
+///     [(of::DeviceId::new(c_str!("onnn,ncv6336")), ()),]
+/// );
+///
+/// kernel::i2c_device_table!(
+///     I2C_ID_TABLE,
+///     MODULE_I2C_ID_TABLE,
+///     <MyDriver as i2c::Driver>::IdInfo,
+///     [(i2c::DeviceId::new(c_str!("ncv6336")), ()),]
+/// );
+///
+/// struct MyDriver;
+///
+/// impl i2c::Driver for MyDriver {
+///     type IdInfo = ();
+///     const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = Some(&OF_ID_TABLE);
+///     const I2C_ID_TABLE: Option<i2c::IdTable<Self::IdInfo>> = Some(&I2C_ID_TABLE);
+///
+///     fn probe(_client: &mut i2c::Client,
+///              id_info: Option<&Self::IdInfo>) -> Result<Pin<KBox<Self>>> {
+///         Ok(KBox::new(Self, GFP_KERNEL)?.into())
+///     }
+/// }
+///```
+pub trait Driver {
+    /// 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;
+
+    /// An optional table of I2C device ids supported by the driver.
+    const I2C_ID_TABLE: Option<IdTable<Self::IdInfo>>;
+
+    /// An optional table of OF device ids supported by the driver.
+    const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>>;
+
+    /// I2C driver probe.
+    ///
+    /// Called when a new I2C client is added or discovered.
+    fn probe(client: &mut Client, id_info: Option<&Self::IdInfo>) -> Result<Pin<KBox<Self>>>;
+}
+
+/// An I2C Client.
+///
+/// # Invariants
+///
+/// `Client` holds a valid reference of `ARef<device::Device>` whose underlying `struct device` is a
+/// member of a `struct i2c_client`.
+#[derive(Clone)]
+pub struct Client(ARef<Device>);
+
+impl Client {
+    /// Convert a raw kernel device into a `Client`
+    ///
+    /// # Safety
+    ///
+    /// `dev` must be an `Aref<Device>` whose underlying `bindings::device` is a member of a
+    /// `bindings::i2c_client`.
+    unsafe fn from_dev(dev: ARef<Device>) -> Self {
+        Self(dev)
+    }
+
+    /// Returns the raw `struct i2c_client`.
+    pub fn as_raw(&self) -> *mut bindings::i2c_client {
+        // SAFETY: By the type invariant `self.0.as_raw` is a pointer to the `struct device`
+        // embedded in `struct i2c_client`.
+        unsafe { container_of!(self.0.as_raw(), bindings::i2c_client, dev) }.cast_mut()
+    }
+}
+
+impl AsRef<Device> for Client {
+    fn as_ref(&self) -> &Device {
+        &self.0
+    }
+}
+
+/// Declares a kernel module that exposes a single I2C driver.
+///
+/// # Examples
+///
+/// ```ignore
+/// kernel::module_i2c_driver! {
+///     type: MyDriver,
+///     name: "Module name",
+///     author: "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)* });
+    };
+}
+
+/// Create an I2C `IdTable` with an "alias" for modpost.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::{c_str, i2c};
+///
+/// kernel::i2c_device_table!(
+///     I2C_ID_TABLE,
+///     MODULE_I2C_ID_TABLE,
+///     u32,
+///     [(i2c::DeviceId::new(c_str!("ncv6336")), 0x6336),]
+/// );
+/// ```
+#[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);
+    };
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 7fb9858966e8457611d5868783000844ba640db9..71ef7df94302b689be665676a36bd5c2e6effff3 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -48,6 +48,8 @@
 #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
 pub mod firmware;
 pub mod fs;
+#[cfg(CONFIG_I2C)]
+pub mod i2c;
 pub mod init;
 pub mod ioctl;
 pub mod jump_label;

-- 
2.45.2