rust: time: add msecs to jiffies conversion

Defines type aliases and conversions for msecs and jiffies.

This is used by Rust Binder for process freezing. There, we want to
sleep until the freeze operation completes, but we want to be able to
abort the process freezing if it doesn't complete within some timeout.
The freeze timeout is supplied in msecs.

Note that we need to convert to jiffies in Binder. It is not enough to
introduce a variant of `CondVar::wait_timeout` that takes the timeout in
msecs because we need to be able to restart the sleep with the remaining
sleep duration if it is interrupted, and if the API takes msecs rather
than jiffies, then that would require a conversion roundtrip jiffies->
msecs->jiffies that is best avoided.
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Tiago Lam <tiagolam@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240108-rb-new-condvar-methods-v4-2-88e0c871cc05@google.comSigned-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/bindings/bindings_helper.h

@@ -9,6 +9,7 @@
 #include <kunit/test.h>
 #include <linux/errname.h>
 #include <linux/ethtool.h>
+#include <linux/jiffies.h>
 #include <linux/mdio.h>
 #include <linux/phy.h>
 #include <linux/slab.h>

rust/kernel/lib.rs

@@ -48,6 +48,7 @@
 pub mod str;
 pub mod sync;
 pub mod task;
+pub mod time;
 pub mod types;
 pub mod workqueue;
 

rust/kernel/time.rs

+// SPDX-License-Identifier: GPL-2.0
+
+//! Time related primitives.
+//!
+//! This module contains the kernel APIs related to time and timers that
+//! have been ported or wrapped for usage by Rust code in the kernel.
+
+/// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
+pub type Jiffies = core::ffi::c_ulong;
+
+/// The millisecond time unit.
+pub type Msecs = core::ffi::c_uint;
+
+/// Converts milliseconds to jiffies.
+#[inline]
+pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
+    // SAFETY: The `__msecs_to_jiffies` function is always safe to call no
+    // matter what the argument is.
+    unsafe { bindings::__msecs_to_jiffies(msecs) }
+}