linux-kernel - Re: [PATCH v6 1/2] rust: Add trait to convert a device reference to a bus device reference

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Message-ID: <c2becbdf-1258-4a33-9fa9-3b63b53ad099@kernel.org>
Date: Fri, 24 Oct 2025 16:48:23 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Markus Probst <markus.probst@...teo.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Lee Jones <lee@...nel.org>,
 Pavel Machek <pavel@...nel.org>, Dave Ertman <david.m.ertman@...el.com>,
 Ira Weiny <ira.weiny@...el.com>, Leon Romanovsky <leon@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 bjorn3_gh@...tonmail.com, Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Bjorn Helgaas <bhelgaas@...gle.com>,
 Krzysztof Wilczyński <kwilczynski@...nel.org>,
 rust-for-linux@...r.kernel.org, linux-leds@...r.kernel.org,
 linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/2] rust: Add trait to convert a device reference to a
 bus device reference

On 10/23/25 7:28 PM, Markus Probst wrote:
> +// SAFETY: `auxilary::Device` is a transparent wrapper of `struct auxiliary_device`.
> +// The offset is guaranteed to point to a valid device field inside `auxilary::Device`.

s/auxilary/auxiliary/

> +unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for Device<Ctx> {
> +    const OFFSET: usize = offset_of!(bindings::auxiliary_device, dev);
> +}

<snip>

> +/// Bus devices can implement this trait to allow abstractions to provide the bus device in
> +/// class device callbacks.

In the first line, please briefly mention what the trait represents.
Subsequently, you can mention the use-case below as a more general description.

> +///
> +/// This must not be used by drivers and is intended for bus and class device abstractions only.
> +///
> +/// # Safety
> +///
> +/// `AsBusDevice::OFFSET` must be a offset to a device field in the implemented struct.

"... the offset of the embedded base `struct device` field within a bus device
structure"

> +pub(crate) unsafe trait AsBusDevice<Ctx: DeviceContext>: AsRef<Device<Ctx>> {
> +    /// The relative offset to the device field.
> +    ///
> +    /// Use `offset_of!(bindings, field)` macro to avoid breakage.
> +    const OFFSET: usize;
> +
> +    /// Convert a reference to [`Device`] into `Self`.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `dev` must be contained in `Self`.
> +    unsafe fn from_device(dev: &Device<Ctx>) -> &Self
> +    where
> +        Self: Sized,
> +    {
> +        let raw = dev.as_raw();
> +        // SAFETY: `raw - Self::OFFSET` is guaranteed by the safety requirements
> +        // to be a valid pointer to `Self`.
> +        unsafe { &*raw.byte_sub(Self::OFFSET).cast::<Self>() }
> +    }
> +}