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

[Viresh Kumar]

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@xxxxxxxxxx>
---
 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.
+    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 2d5c3d7d2e21..e9106b29c359 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -39,6 +39,7 @@
 pub mod block;
 #[doc(hidden)]
 pub mod build_assert;
+pub mod cpu;
 pub mod cred;
 pub mod device;
 pub mod device_id;
-- 
2.31.1.272.g89b43f80a514