[PATCH v3 2/3] rust: i2c: add manual I2C device creation abstractions

[Igor Korotin <igor.korotin.linux@gmail.com>]

In addition to the basic I2C device support, added rust abstractions
upon `i2c_new_client_device`/`i2c_unregister_device` C functions.

Implement the core abstractions needed for manual creation/deletion
of I2C devices, including:

 * `i2c::Registration` — a NonNull pointer created by the function
                          `i2c_new_client_device`

 * `i2c::I2cAdapterRef` — a safe reference around `struct i2c_adapter`

 * `i2c::I2cBoardInfo` — a safe wrapper around `struct i2c_board_info`

Signed-off-by: Igor Korotin <igor.korotin.linux@gmail.com>
---
 rust/kernel/i2c.rs | 111 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 109 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
index fafcae92cfdb..bbdc41124fd8 100644
--- a/rust/kernel/i2c.rs
+++ b/rust/kernel/i2c.rs
@@ -139,7 +139,6 @@ extern "C" fn probe_callback(pdev: *mut bindings::i2c_client) -> kernel::ffi::c_
         let info =
             Self::i2c_id_info(pdev).or_else(|| <Self as driver::Adapter>::id_info(pdev.as_ref()));
 
-
         from_result(|| {
             let data = T::probe(pdev, info)?;
 
@@ -312,6 +311,77 @@ fn shutdown(dev: &Device<device::Core>) {
     }
 }
 
+/// The i2c adapter reference
+///
+/// This represents the Rust abstraction for a reference to an existing
+/// C `struct i2c_adapter`.
+///
+/// # Invariants
+///
+/// A [`I2cAdapterRef`] instance represents a valid `struct i2c_adapter` created by the C portion of
+/// the kernel.
+#[repr(transparent)]
+pub struct I2cAdapterRef(NonNull<bindings::i2c_adapter>);
+
+impl I2cAdapterRef {
+    fn as_raw(&self) -> *mut bindings::i2c_adapter {
+        self.0.as_ptr()
+    }
+
+    /// Gets pointer to an `i2c_adapter` by index.
+    pub fn get(index: i32) -> Option<Self> {
+        // SAFETY: `index` must refer to a valid I2C adapter; the kernel
+        // guarantees that `i2c_get_adapter(index)` returns either a valid
+        // pointer or NULL. `NonNull::new` guarantees the correct check.
+        let adapter = NonNull::new(unsafe { bindings::i2c_get_adapter(index) })?;
+        Some(Self(adapter))
+    }
+}
+
+impl Drop for I2cAdapterRef {
+    fn drop(&mut self) {
+        // SAFETY: This `I2cAdapterRef` was obtained from `i2c_get_adapter`,
+        // and calling `i2c_put_adapter` exactly once will correctly release
+        // the reference count in the I2C core. It is safe to call from any context
+        unsafe { bindings::i2c_put_adapter(self.as_raw()) }
+    }
+}
+
+/// The i2c board info representation
+///
+/// This structure represents the Rust abstraction for a C `struct i2c_board_info` structure,
+/// which is used for manual I2C client creation.
+#[repr(transparent)]
+pub struct I2cBoardInfo(bindings::i2c_board_info);
+
+impl I2cBoardInfo {
+    const I2C_TYPE_SIZE: usize = 20;
+    /// Create a new board‐info for a kernel driver.
+    #[inline(always)]
+    pub const fn new(type_: &'static CStr, addr: u16) -> Self {
+        build_assert!(
+            type_.len_with_nul() <= Self::I2C_TYPE_SIZE,
+            "Type exceeds 20 bytes"
+        );
+        let src = type_.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_board_info: bindings::i2c_board_info = unsafe { core::mem::zeroed() };
+        let mut i: usize = 0;
+        while i < src.len() {
+            i2c_board_info.type_[i] = src[i];
+            i += 1;
+        }
+
+        i2c_board_info.addr = addr;
+        Self(i2c_board_info)
+    }
+
+    fn as_raw(&self) -> *const bindings::i2c_board_info {
+        &self.0 as *const _
+    }
+}
+
 /// The i2c client representation.
 ///
 /// This structure represents the Rust abstraction for a C `struct i2c_client`. The
@@ -340,7 +410,7 @@ fn as_raw(&self) -> *mut bindings::i2c_client {
 kernel::impl_device_context_into_aref!(Device);
 
 // SAFETY: Instances of `Device` are always reference-counted.
-unsafe impl crate::types::AlwaysRefCounted for Device {
+unsafe impl<Ctx: device::DeviceContext> crate::types::AlwaysRefCounted for Device<Ctx> {
     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()) };
@@ -389,3 +459,40 @@ unsafe impl Send for Device {}
 // SAFETY: `Device` can be shared among threads because all methods of `Device`
 // (i.e. `Device<Normal>) are thread safe.
 unsafe impl Sync for Device {}
+
+/// The registration of an i2c client device.
+///
+/// This type represents the registration of a [`struct i2c_client`]. When an instance of this
+/// type is dropped, its respective i2c client device will be unregistered from the system.
+///
+/// # Invariants
+///
+/// `self.0` always holds a valid pointer to an initialized and registered
+/// [`struct i2c_client`].
+#[repr(transparent)]
+pub struct Registration(NonNull<bindings::i2c_client>);
+
+impl Registration {
+    /// The C `i2c_new_client_device` function wrapper for manual I2C client creation.
+    pub fn new(i2c_adapter: &I2cAdapterRef, i2c_board_info: &I2cBoardInfo) -> Result<Self> {
+        // SAFETY: the kernel guarantees that `i2c_new_client_device()` returns either a valid
+        // pointer or NULL. `NonNull::new` guarantees the correct check.
+        let raw_dev = from_err_ptr(unsafe {
+            bindings::i2c_new_client_device(i2c_adapter.as_raw(), i2c_board_info.as_raw())
+        })?;
+
+        Ok(Self(NonNull::new(raw_dev).unwrap()))
+    }
+}
+
+impl Drop for Registration {
+    fn drop(&mut self) {
+        unsafe { bindings::i2c_unregister_device(self.0.as_ptr()) }
+    }
+}
+
+// SAFETY: A `Device` is always reference-counted and can be released from any thread.
+unsafe impl Send for Registration {}
+
+// SAFETY: A `Device` is always reference-counted and can be released from any thread.
+unsafe impl Sync for Registration {}
-- 
2.43.0