[PATCH v2] rust: time: add Ktime

[Alice Ryhl <aliceryhl@google.com>]

Introduce a wrapper around `ktime_t` with a few different useful
methods.

Rust Binder will use these bindings to compute how many milliseconds a
transaction has been active for when dumping the current state of the
Binder driver. This replicates the logic in C Binder [1].

For a usage example in Rust Binder, see [2].

The `ktime_get` method cannot be safely called in NMI context. This
requirement is not checked by these abstractions, but it is intended
that klint [3] or a similar tool will be used to check it in the future.

Link: https://lore.kernel.org/lkml/5ac8c0d09392290be789423f0dd78a520b830fab.1682333709.git.zhangchuang3@xiaomi.com/ [1]
Link: https://r.android.com/3004103 [2]
Link: https://rust-for-linux.com/klint [3]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v2:
- Mention that ktime_get cannot be safely called in NMI context.
- Link to v1: https://lore.kernel.org/r/20240320-rust-ktime_ms_delta-v1-1-ccb8672a0941@google.com
---
 rust/kernel/time.rs | 60 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 25a896eed468..6811d5cadbd4 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -5,6 +5,9 @@
 //! 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 number of nanoseconds per millisecond.
+pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
+
 /// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
 pub type Jiffies = core::ffi::c_ulong;
 
@@ -18,3 +21,60 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
     // matter what the argument is.
     unsafe { bindings::__msecs_to_jiffies(msecs) }
 }
+
+/// A Rust wrapper around a `ktime_t`.
+#[repr(transparent)]
+#[derive(Copy, Clone)]
+pub struct Ktime {
+    inner: bindings::ktime_t,
+}
+
+impl Ktime {
+    /// Create a `Ktime` from a raw `ktime_t`.
+    #[inline]
+    pub fn from_raw(inner: bindings::ktime_t) -> Self {
+        Self { inner }
+    }
+
+    /// Get the current time using `CLOCK_MONOTONIC`.
+    #[inline]
+    pub fn ktime_get() -> Self {
+        // SAFETY: It is always safe to call `ktime_get` outside of NMI context.
+        Self::from_raw(unsafe { bindings::ktime_get() })
+    }
+
+    /// Divide the number of nanoseconds by a compile-time constant.
+    #[inline]
+    fn divns_constant<const DIV: i64>(self) -> i64 {
+        self.to_ns() / DIV
+    }
+
+    /// Returns the number of nanoseconds.
+    #[inline]
+    pub fn to_ns(self) -> i64 {
+        self.inner
+    }
+
+    /// Returns the number of milliseconds.
+    #[inline]
+    pub fn to_ms(self) -> i64 {
+        self.divns_constant::<NSEC_PER_MSEC>()
+    }
+}
+
+/// Returns the number of milliseconds between two ktimes.
+#[inline]
+pub fn ktime_ms_delta(later: Ktime, earlier: Ktime) -> i64 {
+    (later - earlier).to_ms()
+}
+
+impl core::ops::Sub for Ktime {
+    type Output = Ktime;
+
+    #[inline]
+    fn sub(self, other: Ktime) -> Ktime {
+        Self {
+            inner: self.inner - other.inner,
+        }
+    }
+}

---
base-commit: 768409cff6cc89fe1194da880537a09857b6e4db
change-id: 20240320-rust-ktime_ms_delta-74b00c9ab872

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>