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

[Danilo Krummrich <dakr@kernel.org>]

On Thu, Feb 06, 2025 at 04:04:56PM -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@redhat.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Maíra Canal <mairacanal@riseup.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
> 
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 55354e4dec14e..f46cf3bb70695 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -11,6 +11,7 @@
>  #include <linux/blk_types.h>
>  #include <linux/blkdev.h>
>  #include <linux/cred.h>
> +#include <linux/device/faux.h>
>  #include <linux/errname.h>
>  #include <linux/ethtool.h>
>  #include <linux/file.h>
> 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.

"preferred" implies a bit that platform devices are still an option for that
(even if not preferred). Maybe just not mention it at all. But if you want to,
maybe something along the lines of "faux devices are the solution for the
historical abuse of platform devices as virtual devices"?

> +//!
> +//! 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.

Ultimately, this will be used to be passed to C APIs, such as drm_dev_alloc(),
which increment the reference count of the underlying struct device.

Should we consider that in Rust we may have a need to take additional references
in the future too?

Maybe it would be more future proof to call this structure `Registration` and
leave us the option to define faux::Device for reference counting later on.