[PATCH V11 07/15] rust: cpu: Add from_cpu()

[Viresh Kumar <viresh.kumar@linaro.org>]

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              | 30 ++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |  1 +
 4 files changed, 33 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..10c5c3b25873
--- /dev/null
+++ b/rust/kernel/cpu.rs
@@ -0,0 +1,30 @@
+// 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: It is safe to call `get_cpu_device()` for any CPU.
+    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.
+    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