Re: [PATCH v2 3/4] rust: core abstractions for HID drivers

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

On Sun Jul 13, 2025 at 11:12 PM CEST, Rahul Rameshbabu wrote:
> +// SAFETY: Instances of `Device` are always reference-counted.
> +unsafe impl crate::types::AlwaysRefCounted for Device {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
> +        unsafe { bindings::kref_get(&mut ((*self.as_raw()).ref_)) }

I'm confused, what's the lifecycle of a struct hid_device? It embedds a struct
device, so it also inherits its reference count. Additionally it also has a
struct kref. Can you elaborate please?

I don't know what the struct kref is for, but I'm pretty sure you want to manage
the reference count of the embedded struct device here.

> +    }
> +
> +    unsafe fn dec_ref(obj: NonNull<Self>) {
> +        // SAFETY: The safety requirements guarantee that the refcount is non-zero.
> +        unsafe {
> +            bindings::kref_put(
> +                &mut ((*obj.cast::<bindings::hid_device>().as_ptr()).ref_),

I think you want &raw mut instead.

> +                Some(bindings::hiddev_free),
> +            )
> +        }
> +    }
> +}
> +
> +impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
> +    fn as_ref(&self) -> &device::Device<Ctx> {
> +        // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> +        // `struct hid_device`.
> +        let dev = unsafe { addr_of_mut!((*self.as_raw()).dev) };

You also use &raw mut meanwhile.

> +
> +        // SAFETY: `dev` points to a valid `struct device`.
> +        unsafe { device::Device::as_ref(dev) }
> +    }
> +}