Re: [PATCH V7 03/16] rust: cpu: Add from_cpu()

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jan 13, 2025 at 04:52:58PM +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>
> ---
>  rust/bindings/bindings_helper.h |  1 +
>  rust/kernel/cpu.rs              | 26 ++++++++++++++++++++++++++
>  rust/kernel/lib.rs              |  1 +
>  3 files changed, 28 insertions(+)
>  create mode 100644 rust/kernel/cpu.rs
> 
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index e9fdceb568b8..d63e7f6d10ea 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -10,6 +10,7 @@
>  #include <linux/blk-mq.h>
>  #include <linux/blk_types.h>
>  #include <linux/blkdev.h>
> +#include <linux/cpu.h>
>  #include <linux/cred.h>
>  #include <linux/errname.h>
>  #include <linux/ethtool.h>
> diff --git a/rust/kernel/cpu.rs b/rust/kernel/cpu.rs
> new file mode 100644
> index 000000000000..422e874627d2
> --- /dev/null
> +++ b/rust/kernel/cpu.rs
> @@ -0,0 +1,26 @@
> +// 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.
> +pub 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.

I thought it was pointed out that it could be freed when a cpu was
hot-unplugged?  Or is that a different device in the cpu code?  We seem
to have 2 of them and it's not obvious which is which :(

thanks,

greg k-h