Re: [PATCH v3 1/2] rust: irq: add support for request_irq()

[Danilo Krummrich <dakr@kernel.org>]

On Wed, May 14, 2025 at 04:20:51PM -0300, Daniel Almeida wrote:
> +/// // This is running in process context.
> +/// fn register_irq(irq: u32, handler: Handler) -> Result<Arc<Registration<Handler>>> {
> +///     let registration = Registration::register(irq, flags::SHARED, c_str!("my-device"), handler);
> +///
> +///     // You can have as many references to the registration as you want, so
> +///     // multiple parts of the driver can access it.
> +///     let registration = Arc::pin_init(registration, GFP_KERNEL)?;

This makes it possible to arbitrarily extend the lifetime of an IRQ
registration. However, we must guarantee that the IRQ is unregistered when the
corresponding device is unbound. We can't allow drivers to hold on to device
resources after the corresponding device has been unbound.

Why does the data need to be part of the IRQ registration itself? Why can't we
pass in an Arc<T> instance already when we register the IRQ?

This way we'd never have a reason to ever access the Registration instance
itself ever again and we can easily wrap it as Devres<irq::Registration> -
analogously to devm_request_irq() on the C side - without any penalties.

> +///     // The handler may be called immediately after the function above
> +///     // returns, possibly in a different CPU.
> +///
> +///     {
> +///         // The data can be accessed from the process context too.
> +///         let mut data = registration.handler().0.lock();
> +///         *data = 42;
> +///     }
> +///
> +///     Ok(registration)
> +/// }