linux-kernel - Re: [PATCH] rust/kernel: Add faux device bindings

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Message-ID: <7c2b7173-6979-497e-840c-7b82b0c69e01@t-8ch.de>
Date: Thu, 6 Feb 2025 22:44:39 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Lyude Paul <lyude@...hat.com>
Cc: rust-for-linux@...r.kernel.org, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Maíra Canal <mairacanal@...eup.net>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Mika Westerberg <mika.westerberg@...ux.intel.com>, 
	Xiangfei Ding <dingxiangfei2009@...il.com>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rust/kernel: Add faux device bindings

On 2025-02-06 16:04:56-0500, Lyude Paul wrote:
> This introduces a crate for working with faux devices in rust, along with
> adding sample code to show how the API is used. Unlike other types of
> devices, we don't provide any hooks for device probe/removal - since these
> are optional for the faux API and are unnecessary in rust.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Maíra Canal <mairacanal@...eup.net>
> ---
>  rust/bindings/bindings_helper.h  |  1 +
>  rust/kernel/faux.rs              | 60 ++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs               |  1 +
>  samples/rust/Kconfig             | 10 ++++++
>  samples/rust/Makefile            |  1 +
>  samples/rust/rust_driver_faux.rs | 29 +++++++++++++++
>  6 files changed, 102 insertions(+)
>  create mode 100644 rust/kernel/faux.rs
>  create mode 100644 samples/rust/rust_driver_faux.rs

<snip>

> diff --git a/rust/kernel/faux.rs b/rust/kernel/faux.rs
> new file mode 100644
> index 0000000000000..5e58f397b747a
> --- /dev/null
> +++ b/rust/kernel/faux.rs
> @@ -0,0 +1,60 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +//! Abstractions for the faux bus.
> +//!
> +//! This crate provides bindings for working with faux devices in kernel modules. It should be
> +//! preferred for creating virtual devices over the platform API.
> +//!
> +//! C header: [`include/linux/device/faux.h`]
> +use crate::{bindings, device, error::from_err_ptr, prelude::*};
> +use core::ptr::{addr_of_mut, null, NonNull};
> +
> +/// The faux device representation.
> +///
> +/// This type represents the registration of a [`struct faux_device`]. When an instance of this type
> +/// is dropped, its respective faux device will be unregistered from the system.
> +///
> +/// # Invariants
> +///
> +/// `self.0` always holds a valid pointer to an initialized and registered [`struct faux_device`].
> +///
> +/// [`struct faux_device`]: srctree/include/linux/device/faux.h
> +#[repr(transparent)]
> +pub struct Device(NonNull<bindings::faux_device>);
> +
> +impl Device {
> +    /// Create and register a new faux device with the given name.
> +    pub fn new(name: &CStr) -> Result<Self> {
> +        // SAFETY:
> +        // - `name` is copied by this function into its own storage
> +        // - `faux_ops` is safe to leave NULL according to the C API
> +        // - `faux_device_create` returns a valid non-null pointer to a `faux_device`, or error

In the v3 patch the C API returns NULL on error.

> +        from_err_ptr(unsafe { bindings::faux_device_create(name.as_char_ptr(), null()) })
> +            .map(|d| Self(unsafe { NonNull::new_unchecked(d) }))
> +    }
> +
> +    fn as_raw(&self) -> *mut bindings::faux_device {
> +        self.0.as_ptr()
> +    }
> +}
> +
> +impl AsRef<device::Device> for Device {
> +    fn as_ref(&self) -> &device::Device {
> +        // SAFETY: The underlying `device` in `faux_device` is guaranteed by the C API to be
> +        // a valid initialized `device`.
> +        unsafe { device::Device::as_ref(addr_of_mut!((*self.as_raw()).dev)) }
> +    }
> +}
> +
> +impl Drop for Device {
> +    fn drop(&mut self) {
> +        // SAFETY: self.0 is a valid registered faux_device via our type invariants.
> +        unsafe { bindings::faux_device_destroy(self.as_raw()) }
> +    }
> +}
> +
> +// SAFETY: The faux device API is thread-safe
> +unsafe impl Send for Device {}
> +
> +// SAFETY: The faux device API is thread-safe
> +unsafe impl Sync for Device {}

<snip>