[PATCH v11 4/8] rust: time: Introduce Instant type

FUJITA Tomonori
2025-02-20T07:10:45Z

Introduce a type representing a specific point in time. We could use
the Ktime type but C's ktime_t is used for both timestamp and
timedelta. To avoid confusion, introduce a new Instant type for
timestamp.

Rename Ktime to Instant and modify their methods for timestamp.

Implement the subtraction operator for Instant:

Delta = Instant A - Instant B

Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
rust/kernel/time.rs | 77 +++++++++++++++++++++++----------------------
1 file changed, 39 insertions(+), 38 deletions(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 622cd01e24d7..d64a05a4f4d1 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -5,6 +5,22 @@
//! 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.
//!
+//! There are two types in this module:
+//!
+//! - The [`Instant`] type represents a specific point in time.
+//! - The [`Delta`] type represents a span of time.
+//!
+//! Note that the C side uses `ktime_t` type to represent both. However, timestamp
+//! and timedelta are different. To avoid confusion, we use two different types.
+//!
+//! A [`Instant`] object can be created by calling the [`Instant::now()`] function.
+//! It represents a point in time at which the object was created.
+//! By calling the [`Instant::elapsed()`] method, a [`Delta`] object representing
+//! the elapsed time can be created. The [`Delta`] object can also be created
+//! by subtracting two [`Instant`] objects.
+//!
+//! A [`Delta`] type supports methods to retrieve the duration in various units.
+//!
//! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
//! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).

@@ -31,59 +47,44 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
unsafe { bindings::__msecs_to_jiffies(msecs) }
}

-/// A Rust wrapper around a `ktime_t`.
+/// A specific point in time.
+///
+/// # Invariants
+///
+/// The `inner` value is in the range from 0 to `KTIME_MAX`.
#[repr(transparent)]
#[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord)]
-pub struct Ktime {
+pub struct Instant {
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 }
- }
-
+impl Instant {
/// 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
+ pub fn now() -> Self {
+ // INVARIANT: The `ktime_get()` function returns a value in the range
+ // from 0 to `KTIME_MAX`.
+ Self {
+ // SAFETY: It is always safe to call `ktime_get()` outside of NMI context.
+ inner: unsafe { bindings::ktime_get() },
+ }
}

- /// Returns the number of milliseconds.
+ /// Return the amount of time elapsed since the [`Instant`].
#[inline]
- pub fn to_ms(self) -> i64 {
- self.divns_constant::<NSEC_PER_MSEC>()
+ pub fn elapsed(&self) -> Delta {
+ Self::now() - *self
}
}

-/// 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;
+impl core::ops::Sub for Instant {
+ type Output = Delta;

+ // By the type invariant, it never overflows.
#[inline]
- fn sub(self, other: Ktime) -> Ktime {
- Self {
- inner: self.inner - other.inner,
+ fn sub(self, other: Instant) -> Delta {
+ Delta {
+ nanos: self.inner - other.inner,
}
}
}
--
2.43.0