Re: [RFC PATCH v6 0/4] iio: position: add Rust driver for ams AS5600

[Igor Korotin <igor.korotin@linux.dev>]

Hi Muchamad,

Reviewed the lifetime/teardown contracts you asked about. read_raw() has
a problem: it takes parent: &Device<Bound> and recovers &I2cClient<Bound>
via TryFrom. That conversion just got removed upstream ("rust: i2c:
remove TryFrom<&Device> implementation for &I2cClient", use-after-free,
unrelated thread), so this won't compile anymore. More importantly, Gary
Guo made the point that needing the conversion at all is the actual
design issue: a class device shouldn't reach back into a generic parent
reference during a callback -- whatever bus-specific resource it needs
should be captured once, at registration time, and handed back via its
own private data.

Concretely, this means dropping devres and letting Data<'bound> carry a
borrowed reference directly, which it's designed for
(Driver::Data<'bound>: Send + 'bound):

  pub struct As5600Priv<'a> {
      client: &'a I2cClient<Bound>,
      #[pin]
      io_lock: Mutex<()>,
  }

  unsafe impl Send for As5600Priv<'_> {}
  unsafe impl Sync for As5600Priv<'_> {}

  unsafe impl IioDriver for As5600Priv<'_> {
      fn read_raw(&self, _chan: *const iio_chan_spec, info: IioChanInfo)
          -> Result<IioVal> {
          let client = self.client;
          ...
      }
  }

In probe(), capture the client once via deref coercion (Core -> Bound)
and split the driver struct so the registered IIO device carries the
same 'bound lifetime:

  struct As5600;

  struct As5600Data<'bound> {
      _iio_dev: Device<As5600Priv<'bound>, Registered>,
  }

  impl Driver for As5600 {
      type Data<'bound> = As5600Data<'bound>;
      fn probe<'bound>(dev: &'bound I2cClient<Core<'_>>, ...) -> ... {
          let client: &'bound I2cClient<Bound> = dev;
          ...
      }
  }

register() then just returns Device<T, Registered> directly instead of
transferring it to parent devres -- that transfer is what forced T:
'static in the first place, which is what made storing the borrowed
client impossible and pushed you toward TryFrom.

I built this end to end against current rust-next: compiles clean, zero
warnings, and zero changes to rust/kernel/i2c.rs. Happy to send the full
diff if it's useful as a starting point for v7.

Cheers
Igor