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

["Danilo Krummrich" <dakr@kernel.org>]

On Fri Aug 1, 2025 at 7:59 PM CEST, Daniel Almeida wrote:
>> +/// 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>);
>
> Is there a plan for an owned version? Otherwise I’d personally drop the
> “Ref” nomenclature.
>
>> +
>> +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()) }
>
> Should this implement AlwasyRefCounted?

I don't think it's necessary given the context this is used in, but it's also
not much more code to do so, and ARef<I2cAdapter> turns out much cleaner I
think.

So, yes I agree this should implement AlwasyRefCounted.

>> +    }
>> +}
>> +
>> +/// 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;
>> +        }
>
> Same comment as the last patch.

Still not possible unfortunately. :(