Re: [PATCH v4 3/3] samples: rust: add Rust I2C sample driver

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

On Tue Sep 9, 2025 at 7:55 PM CEST, Igor Korotin wrote:
> On 8/27/2025 8:38 PM, Daniel Almeida wrote:
>>> +#[pin_data]
>>> +struct DriverModule {
>>> +    #[pin]
>>> +    _driver: kernel::driver::Registration<Ops<SampleDriver>>,
>>> +    _reg: i2c::Registration,
>>> +}
>> 
>> I was expecting this to be ARef of something, most likely I2cClient?

This is the Registration of an I2C device, just like auxiliary::Registration or
faux::Registration. There is no point in reference counting the registration
object.

You can't have an ARef<I2cClient> here either because you want to manage the
lifetime of the I2cClient to be registered in the system, i.e.
i2c_new_client_device() and i2c_unregister_device().

>> This is where my knowledge of i2c drivers start to fall short, but others will
>> probably chime in :)
>
> You're right to mention this. This rust_driver_i2c is not the standard 
> way of handling I2C devices.

It is indeed the normal way of handling this. This Registration object
represents the lifetime of calling i2c_new_client_device() e.g. from a platform
device probe (as Igor also mentions below) and calling i2c_unregister_device()
e.g. from platform device remove.

The ARef<I2cClient> can be obtained from the subsequent bus callback from the
I2C bus.

In fact, you could add a method to i2c::Registration to provide you with an
ARef<I2cClient> if that's ever needed.

But the i2c::Registration itself simply manages the lifetime for the I2C device
being registered in the system.

> The idea was suggested by Danilo in the 
> review of Patch v2: this driver merges an I2C driver sample with manual 
> I2C device creation. The module creates a new I2cClient in its init 
> function, and this new I2cClient is then probed by the SampleDriver.
>
> In a normal driver it should be different. For example, let’s say 
> there’s a platform device — the probe function for this platform device 
> would create an I2cClient, and an appropriate I2C driver would then 
> probe it.
> Thanks for the review.
>
> Best Regards
> Igor