linux-kernel - Re: [PATCH v2 1/4] rust: device: implement Device::parent()

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Message-ID: <D8LGHC6DMPIW.29Y3XD1X6Q1L3@proton.me>
Date: Thu, 20 Mar 2025 22:44:38 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>, bhelgaas@...gle.com, gregkh@...uxfoundation.org, rafael@...nel.org, ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu
Cc: linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] rust: device: implement Device::parent()

On Thu Mar 20, 2025 at 11:27 PM CET, Danilo Krummrich wrote:
> Device::parent() returns a reference to the device' parent device, if
> any.
>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> ---
>  rust/kernel/device.rs | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index 21b343a1dc4d..f6bdc2646028 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -65,6 +65,21 @@ pub(crate) fn as_raw(&self) -> *mut bindings::device {
>          self.0.get()
>      }
>  
> +    /// Returns a reference to the parent device, if any.
> +    pub fn parent<'a>(&self) -> Option<&'a Self> {
> +        // SAFETY:
> +        // - By the type invariant `self.as_raw()` is always valid.
> +        // - The parent device is only ever set at device creation.
> +        let parent = unsafe { (*self.as_raw()).parent };
> +
> +        if parent.is_null() {
> +            None
> +        } else {
> +            // SAFETY: Since `parent` is not NULL, it must be a valid pointer to a `struct device`.
> +            Some(unsafe { Self::as_ref(parent) })

Why is this valid for `'static`? Since you declare the lifetime `'a`
independently from the elided one on `&self`, the user can set it to
`'static`.

---
Cheers,
Benno

> +        }
> +    }
> +
>      /// Convert a raw C `struct device` pointer to a `&'a Device`.
>      ///
>      /// # Safety