Re: [PATCH V9 09/17] rust: cpu: Add from_cpu()

[Yury Norov <yury.norov@gmail.com>]

On Fri, Apr 11, 2025 at 04:25:08PM +0530, Viresh Kumar wrote:
> This implements cpu::from_cpu(), which returns a reference to
> Device for a CPU. The C struct is created at initialization time for
> CPUs and is never freed and so ARef isn't returned from this function.
> 
> The new helper will be used by Rust based cpufreq drivers.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  MAINTAINERS                     |  1 +
>  rust/bindings/bindings_helper.h |  1 +
>  rust/kernel/cpu.rs              | 31 +++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs              |  1 +
>  4 files changed, 34 insertions(+)
>  create mode 100644 rust/kernel/cpu.rs
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 12cde55579a0..475abf72869c 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -6155,6 +6155,7 @@ F:	include/linux/cpuhotplug.h
>  F:	include/linux/smpboot.h
>  F:	kernel/cpu.c
>  F:	kernel/smpboot.*
> +F:	rust/kernel/cpu.rs
>  
>  CPU IDLE TIME MANAGEMENT FRAMEWORK
>  M:	"Rafael J. Wysocki" <rafael@kernel.org>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index f53d6e1a21f2..ac92c67d2c38 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/clk.h>
> +#include <linux/cpu.h>
>  #include <linux/cpumask.h>
>  #include <linux/cred.h>
>  #include <linux/device/faux.h>
> diff --git a/rust/kernel/cpu.rs b/rust/kernel/cpu.rs
> new file mode 100644
> index 000000000000..3054165d3818
> --- /dev/null
> +++ b/rust/kernel/cpu.rs
> @@ -0,0 +1,31 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Generic CPU definitions.
> +//!
> +//! C header: [`include/linux/cpu.h`](srctree/include/linux/cpu.h)
> +
> +use crate::{bindings, device::Device, error::Result, prelude::ENODEV};
> +
> +/// Creates a new instance of CPU's device.
> +///
> +/// # Safety
> +///
> +/// Reference counting is not implemented for the CPU device in the C code. When a CPU is
> +/// hot-unplugged, the corresponding CPU device is unregistered, but its associated memory
> +/// is not freed.
> +///
> +/// Callers must ensure that the CPU device is not used after it has been unregistered.
> +/// This can be achieved, for example, by registering a CPU hotplug notifier and removing
> +/// any references to the CPU device within the notifier's callback.
> +pub unsafe fn from_cpu(cpu: u32) -> Result<&'static Device> {
> +    // SAFETY: The pointer returned by `get_cpu_device()`, if not `NULL`, is a valid pointer to
> +    // a `struct device` and is never freed by the C code.
> +    let ptr = unsafe { bindings::get_cpu_device(cpu) };
> +    if ptr.is_null() {
> +        return Err(ENODEV);
> +    }
> +
> +    // SAFETY: The pointer returned by `get_cpu_device()`, if not `NULL`, is a valid pointer to
> +    // a `struct device` and is never freed by the C code.

If you think that writing the same thing twice would help in SAFETY
department, I'd recommend you to write it for a couple more times.

> +    Ok(unsafe { Device::as_ref(ptr) })
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index f4dcfefe94be..db372f806875 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -44,6 +44,7 @@
>  pub mod build_assert;
>  #[cfg(CONFIG_COMMON_CLK)]
>  pub mod clk;
> +pub mod cpu;
>  pub mod cpumask;
>  pub mod cred;
>  pub mod device;
> -- 
> 2.31.1.272.g89b43f80a514