[PATCH v6 2/3] rust: i2c: Add basic I2C driver abstractions

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

Hello Danilo

> On 10/5/25 12:23 PM, Igor Korotin wrote:
> > +impl Registration {
> > +    /// The C `i2c_new_client_device` function wrapper for manual I2C client creation.
> > +    pub fn new(i2c_adapter: &I2cAdapter, i2c_board_info: &I2cBoardInfo) -> Result<Self> {
> > +        // SAFETY: the kernel guarantees that `i2c_new_client_device()` returns either a valid
> > +        // pointer or NULL. `from_err_ptr` separates errors. Following `NonNull::new` checks for NULL.
> > +        let raw_dev = from_err_ptr(unsafe {
> > +            bindings::i2c_new_client_device(i2c_adapter.as_raw(), i2c_board_info.as_raw())
> > +        })?;
> > +
> > +        let dev_ptr = NonNull::new(raw_dev).ok_or(ENODEV)?;
> > +
> > +        Ok(Self(dev_ptr))
> > +    }
> > +}
> 
> I wonder if we want to ensure that a Registration can't out-live the driver that
> registers the I2C client device.
> 
> This should only ever be called by drivers bound to more complex devices, so if
> the parent driver is unbound I don't think I2C client device registered by this
> driver should be able to survive.
> 
> Hence, I think Registration::new() should return
> impl PinInit<Devres<Self>, Error> instead.

Maybe I'm missing something here, but as far as I understand, Devres is bound to 
an existing device. However `Registration::new` creates new device and registers 
new i2c_client using function `i2c_new_client_device`. Created i2c_client uses 
i2c_adapter as its parent.

The driver that declares Registration doesn't own that i2c_adapter. `Registration` 
itself is not part of the new client’s managed resources, so returning 
`impl PinInit<Devres<Self>, Error>` wouldn’t make sense here.

Drop for Registration calls `i2c_unregister_client()`, which gracefully unregisters
and deallocates the i2c_client.

Cheers 
Igor