[PATCH v2 0/3] rust: time: Introduce typed clock sources and generalize Instant

[PATCH v2 0/3] rust: time: Introduce typed clock sources and generalize Instant

[FUJITA Tomonori]

This patch series introduces a type-safe abstraction over clock
sources. The goal is to remove the need for runtime clock selection
(via ClockId) and instead leverage Rust's type system to statically
associate the Instant type with a specific clock.

This approach enables compile-time enforcement of clock correctness
across the APIs of Instant type.

This patchset can be applied on top of the division fix for 32-bit
architectures:

https://lore.kernel.org/lkml/20250502004524.230553-1-fujita.tomonori@gmail.com/

Most of the changes to the hrtimer code that were included in v1 have
been removed, as it does not use `Instant` yet. I plan to convert
hrtimer to use `Instant` and `Delta` in a separate patchset.

v2:
- removed most of changes to hrtimer code 
v1: https://lore.kernel.org/rust-for-linux/20250413105629.162349-1-fujita.tomonori@gmail.com/

FUJITA Tomonori (3):
  rust: time: Replace ClockId enum with ClockSource trait
  rust: time: Make Instant generic over ClockSource
  rust: time: Add ktime_get() to ClockSource trait

 rust/helpers/time.c         |  16 +++
 rust/kernel/time.rs         | 201 ++++++++++++++++++++++--------------
 rust/kernel/time/hrtimer.rs |   6 +-
 3 files changed, 145 insertions(+), 78 deletions(-)


base-commit: adf04b7cc66574ec2a43b540046975dbe73e0382
-- 
2.43.0

[PATCH v2 1/3] rust: time: Replace ClockId enum with ClockSource trait

[FUJITA Tomonori]

Replace the ClockId enum with a trait-based abstraction called
ClockSource. This change enables expressing clock sources as types and
leveraging the Rust type system to enforce clock correctness at
compile time.

This also sets the stage for future generic abstractions over Instant
types such as Instant<C>.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/time.rs         | 147 ++++++++++++++++++++----------------
 rust/kernel/time/hrtimer.rs |   6 +-
 2 files changed, 84 insertions(+), 69 deletions(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index b0a8f3c0ba49..1d2600288ed1 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -49,6 +49,87 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
     unsafe { bindings::__msecs_to_jiffies(msecs) }
 }
 
+/// Trait for clock sources.
+///
+/// Selection of the clock source depends on the use case. In some cases the usage of a
+/// particular clock is mandatory, e.g. in network protocols, filesystems. In other
+/// cases the user of the clock has to decide which clock is best suited for the
+/// purpose. In most scenarios clock [`Monotonic`] is the best choice as it
+/// provides a accurate monotonic notion of time (leap second smearing ignored).
+pub trait ClockSource {
+    /// The kernel clock ID associated with this clock source.
+    ///
+    /// This constant corresponds to the C side `clockid_t` value.
+    const ID: bindings::clockid_t;
+}
+
+/// A monotonically increasing clock.
+///
+/// A nonsettable system-wide clock that represents monotonic time since as
+/// described by POSIX, "some unspecified point in the past". On Linux, that
+/// point corresponds to the number of seconds that the system has been
+/// running since it was booted.
+///
+/// The CLOCK_MONOTONIC clock is not affected by discontinuous jumps in the
+/// CLOCK_REAL (e.g., if the system administrator manually changes the
+/// clock), but is affected by frequency adjustments. This clock does not
+/// count time that the system is suspended.
+pub struct Monotonic;
+
+impl ClockSource for Monotonic {
+    const ID: bindings::clockid_t = bindings::CLOCK_MONOTONIC as bindings::clockid_t;
+}
+
+/// A settable system-wide clock that measures real (i.e., wall-clock) time.
+///
+/// Setting this clock requires appropriate privileges. This clock is
+/// affected by discontinuous jumps in the system time (e.g., if the system
+/// administrator manually changes the clock), and by frequency adjustments
+/// performed by NTP and similar applications via adjtime(3), adjtimex(2),
+/// clock_adjtime(2), and ntp_adjtime(3). This clock normally counts the
+/// number of seconds since 1970-01-01 00:00:00 Coordinated Universal Time
+/// (UTC) except that it ignores leap seconds; near a leap second it may be
+/// adjusted by leap second smearing to stay roughly in sync with UTC. Leap
+/// second smearing applies frequency adjustments to the clock to speed up
+/// or slow down the clock to account for the leap second without
+/// discontinuities in the clock. If leap second smearing is not applied,
+/// the clock will experience discontinuity around leap second adjustment.
+pub struct RealTime;
+
+impl ClockSource for RealTime {
+    const ID: bindings::clockid_t = bindings::CLOCK_REALTIME as bindings::clockid_t;
+}
+
+/// A monotonic that ticks while system is suspended.
+///
+/// A nonsettable system-wide clock that is identical to CLOCK_MONOTONIC,
+/// except that it also includes any time that the system is suspended. This
+/// allows applications to get a suspend-aware monotonic clock without
+/// having to deal with the complications of CLOCK_REALTIME, which may have
+/// discontinuities if the time is changed using settimeofday(2) or similar.
+pub struct BootTime;
+
+impl ClockSource for BootTime {
+    const ID: bindings::clockid_t = bindings::CLOCK_BOOTTIME as bindings::clockid_t;
+}
+
+/// International Atomic Time.
+///
+/// A system-wide clock derived from wall-clock time but counting leap seconds.
+///
+/// This clock is coupled to CLOCK_REALTIME and will be set when CLOCK_REALTIME is
+/// set, or when the offset to CLOCK_REALTIME is changed via adjtimex(2). This
+/// usually happens during boot and **should** not happen during normal operations.
+/// However, if NTP or another application adjusts CLOCK_REALTIME by leap second
+/// smearing, this clock will not be precise during leap second smearing.
+///
+/// The acronym TAI refers to International Atomic Time.
+pub struct Tai;
+
+impl ClockSource for Tai {
+    const ID: bindings::clockid_t = bindings::CLOCK_TAI as bindings::clockid_t;
+}
+
 /// A specific point in time.
 ///
 /// # Invariants
@@ -91,72 +172,6 @@ fn sub(self, other: Instant) -> Delta {
     }
 }
 
-/// An identifier for a clock. Used when specifying clock sources.
-///
-///
-/// Selection of the clock depends on the use case. In some cases the usage of a
-/// particular clock is mandatory, e.g. in network protocols, filesystems.In other
-/// cases the user of the clock has to decide which clock is best suited for the
-/// purpose. In most scenarios clock [`ClockId::Monotonic`] is the best choice as it
-/// provides a accurate monotonic notion of time (leap second smearing ignored).
-#[derive(Clone, Copy, PartialEq, Eq, Debug)]
-#[repr(u32)]
-pub enum ClockId {
-    /// A settable system-wide clock that measures real (i.e., wall-clock) time.
-    ///
-    /// Setting this clock requires appropriate privileges. This clock is
-    /// affected by discontinuous jumps in the system time (e.g., if the system
-    /// administrator manually changes the clock), and by frequency adjustments
-    /// performed by NTP and similar applications via adjtime(3), adjtimex(2),
-    /// clock_adjtime(2), and ntp_adjtime(3). This clock normally counts the
-    /// number of seconds since 1970-01-01 00:00:00 Coordinated Universal Time
-    /// (UTC) except that it ignores leap seconds; near a leap second it may be
-    /// adjusted by leap second smearing to stay roughly in sync with UTC. Leap
-    /// second smearing applies frequency adjustments to the clock to speed up
-    /// or slow down the clock to account for the leap second without
-    /// discontinuities in the clock. If leap second smearing is not applied,
-    /// the clock will experience discontinuity around leap second adjustment.
-    RealTime = bindings::CLOCK_REALTIME,
-    /// A monotonically increasing clock.
-    ///
-    /// A nonsettable system-wide clock that represents monotonic time since—as
-    /// described by POSIX—"some unspecified point in the past". On Linux, that
-    /// point corresponds to the number of seconds that the system has been
-    /// running since it was booted.
-    ///
-    /// The CLOCK_MONOTONIC clock is not affected by discontinuous jumps in the
-    /// CLOCK_REAL (e.g., if the system administrator manually changes the
-    /// clock), but is affected by frequency adjustments. This clock does not
-    /// count time that the system is suspended.
-    Monotonic = bindings::CLOCK_MONOTONIC,
-    /// A monotonic that ticks while system is suspended.
-    ///
-    /// A nonsettable system-wide clock that is identical to CLOCK_MONOTONIC,
-    /// except that it also includes any time that the system is suspended. This
-    /// allows applications to get a suspend-aware monotonic clock without
-    /// having to deal with the complications of CLOCK_REALTIME, which may have
-    /// discontinuities if the time is changed using settimeofday(2) or similar.
-    BootTime = bindings::CLOCK_BOOTTIME,
-    /// International Atomic Time.
-    ///
-    /// A system-wide clock derived from wall-clock time but counting leap seconds.
-    ///
-    /// This clock is coupled to CLOCK_REALTIME and will be set when CLOCK_REALTIME is
-    /// set, or when the offset to CLOCK_REALTIME is changed via adjtimex(2). This
-    /// usually happens during boot and **should** not happen during normal operations.
-    /// However, if NTP or another application adjusts CLOCK_REALTIME by leap second
-    /// smearing, this clock will not be precise during leap second smearing.
-    ///
-    /// The acronym TAI refers to International Atomic Time.
-    TAI = bindings::CLOCK_TAI,
-}
-
-impl ClockId {
-    fn into_c(self) -> bindings::clockid_t {
-        self as bindings::clockid_t
-    }
-}
-
 /// A span of time.
 ///
 /// This struct represents a span of time, with its value stored as nanoseconds.
diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index 17824aa0c0f3..380712d4302a 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -67,7 +67,7 @@
 //! A `restart` operation on a timer in the **stopped** state is equivalent to a
 //! `start` operation.
 
-use super::ClockId;
+use super::ClockSource;
 use crate::{prelude::*, types::Opaque};
 use core::marker::PhantomData;
 use pin_init::PinInit;
@@ -112,7 +112,7 @@ unsafe impl<T> Sync for HrTimer<T> {}
 
 impl<T> HrTimer<T> {
     /// Return an initializer for a new timer instance.
-    pub fn new(mode: HrTimerMode, clock: ClockId) -> impl PinInit<Self>
+    pub fn new<U: ClockSource>(mode: HrTimerMode) -> impl PinInit<Self>
     where
         T: HrTimerCallback,
     {
@@ -126,7 +126,7 @@ pub fn new(mode: HrTimerMode, clock: ClockId) -> impl PinInit<Self>
                     bindings::hrtimer_setup(
                         place,
                         Some(T::Pointer::run),
-                        clock.into_c(),
+                        U::ID,
                         mode.into_c(),
                     );
                 }
-- 
2.43.0

[PATCH v2 2/3] rust: time: Make Instant generic over ClockSource

[FUJITA Tomonori]

Refactor the Instant type to be generic over a ClockSource type
parameter, enabling static enforcement of clock correctness across
APIs that deal with time. Previously, the clock source was implicitly
fixed (typically CLOCK_MONOTONIC), and developers had to ensure
compatibility manually.

This design eliminates runtime mismatches between clock sources, and
enables stronger type-level guarantees throughout the timer subsystem.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/time.rs | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 1d2600288ed1..3bc76f75bfd0 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -24,6 +24,8 @@
 //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
 //! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
 
+use core::marker::PhantomData;
+
 pub mod hrtimer;
 
 /// The number of nanoseconds per microsecond.
@@ -136,12 +138,21 @@ impl ClockSource for Tai {
 ///
 /// The `inner` value is in the range from 0 to `KTIME_MAX`.
 #[repr(transparent)]
-#[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord)]
-pub struct Instant {
+#[derive(PartialEq, PartialOrd, Eq, Ord)]
+pub struct Instant<C: ClockSource> {
     inner: bindings::ktime_t,
+    _c: PhantomData<C>,
 }
 
-impl Instant {
+impl<C: ClockSource> Clone for Instant<C> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<C: ClockSource> Copy for Instant<C> {}
+
+impl<C: ClockSource> Instant<C> {
     /// Get the current time using `CLOCK_MONOTONIC`.
     #[inline]
     pub fn now() -> Self {
@@ -150,6 +161,7 @@ pub fn now() -> Self {
         Self {
             // SAFETY: It is always safe to call `ktime_get()` outside of NMI context.
             inner: unsafe { bindings::ktime_get() },
+            _c: PhantomData,
         }
     }
 
@@ -160,12 +172,12 @@ pub fn elapsed(&self) -> Delta {
     }
 }
 
-impl core::ops::Sub for Instant {
+impl<C: ClockSource> core::ops::Sub for Instant<C> {
     type Output = Delta;
 
     // By the type invariant, it never overflows.
     #[inline]
-    fn sub(self, other: Instant) -> Delta {
+    fn sub(self, other: Instant<C>) -> Delta {
         Delta {
             nanos: self.inner - other.inner,
         }
-- 
2.43.0

Re: [PATCH v2 2/3] rust: time: Make Instant generic over ClockSource

[Andreas Hindborg]

FUJITA Tomonori <fujita.tomonori@gmail.com> writes:

> Refactor the Instant type to be generic over a ClockSource type
> parameter, enabling static enforcement of clock correctness across
> APIs that deal with time. Previously, the clock source was implicitly
> fixed (typically CLOCK_MONOTONIC), and developers had to ensure
> compatibility manually.
>
> This design eliminates runtime mismatches between clock sources, and
> enables stronger type-level guarantees throughout the timer subsystem.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>


Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>


Best regards,
Andreas Hindborg

[PATCH v2 3/3] rust: time: Add ktime_get() to ClockSource trait

[FUJITA Tomonori]

Introduce the ktime_get() associated function to the ClockSource
trait, allowing each clock source to specify how it retrieves the
current time. This enables Instant::now() to be implemented
generically using the type-level ClockSource abstraction.

This change enhances the type safety and extensibility of timekeeping
by statically associating time retrieval mechanisms with their
respective clock types. It also reduces the reliance on hardcoded
clock logic within Instant.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/helpers/time.c | 16 ++++++++++++++++
 rust/kernel/time.rs | 32 ++++++++++++++++++++++++++++----
 2 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/rust/helpers/time.c b/rust/helpers/time.c
index 3d31473bce08..31208f1042a1 100644
--- a/rust/helpers/time.c
+++ b/rust/helpers/time.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
 #include <linux/ktime.h>
+#include <linux/timekeeping.h>
 
 s64 rust_helper_ktime_to_us(const ktime_t kt)
 {
@@ -11,3 +12,18 @@ s64 rust_helper_ktime_to_ms(const ktime_t kt)
 {
 	return ktime_to_ms(kt);
 }
+
+ktime_t rust_helper_ktime_get_real(void)
+{
+	return ktime_get_with_offset(TK_OFFS_REAL);
+}
+
+ktime_t rust_helper_ktime_get_boottime(void)
+{
+	return ktime_get_with_offset(TK_OFFS_BOOT);
+}
+
+ktime_t rust_helper_ktime_get_clocktai(void)
+{
+	return ktime_get_with_offset(TK_OFFS_TAI);
+}
diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 3bc76f75bfd0..1be5ecd814d3 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -63,6 +63,11 @@ pub trait ClockSource {
     ///
     /// This constant corresponds to the C side `clockid_t` value.
     const ID: bindings::clockid_t;
+
+    /// Get the current time from the clock source.
+    ///
+    /// The function must return a value in the range from 0 to `KTIME_MAX`.
+    fn ktime_get() -> bindings::ktime_t;
 }
 
 /// A monotonically increasing clock.
@@ -80,6 +85,11 @@ pub trait ClockSource {
 
 impl ClockSource for Monotonic {
     const ID: bindings::clockid_t = bindings::CLOCK_MONOTONIC as bindings::clockid_t;
+
+    fn ktime_get() -> bindings::ktime_t {
+        // SAFETY: It is always safe to call `ktime_get()` outside of NMI context.
+        unsafe { bindings::ktime_get() }
+    }
 }
 
 /// A settable system-wide clock that measures real (i.e., wall-clock) time.
@@ -100,6 +110,11 @@ impl ClockSource for Monotonic {
 
 impl ClockSource for RealTime {
     const ID: bindings::clockid_t = bindings::CLOCK_REALTIME as bindings::clockid_t;
+
+    fn ktime_get() -> bindings::ktime_t {
+        // SAFETY: It is always safe to call `ktime_get_real()` outside of NMI context.
+        unsafe { bindings::ktime_get_real() }
+    }
 }
 
 /// A monotonic that ticks while system is suspended.
@@ -113,6 +128,11 @@ impl ClockSource for RealTime {
 
 impl ClockSource for BootTime {
     const ID: bindings::clockid_t = bindings::CLOCK_BOOTTIME as bindings::clockid_t;
+
+    fn ktime_get() -> bindings::ktime_t {
+        // SAFETY: It is always safe to call `ktime_get_boottime()` outside of NMI context.
+        unsafe { bindings::ktime_get_boottime() }
+    }
 }
 
 /// International Atomic Time.
@@ -130,6 +150,11 @@ impl ClockSource for BootTime {
 
 impl ClockSource for Tai {
     const ID: bindings::clockid_t = bindings::CLOCK_TAI as bindings::clockid_t;
+
+    fn ktime_get() -> bindings::ktime_t {
+        // SAFETY: It is always safe to call `ktime_get_tai()` outside of NMI context.
+        unsafe { bindings::ktime_get_clocktai() }
+    }
 }
 
 /// A specific point in time.
@@ -153,14 +178,13 @@ fn clone(&self) -> Self {
 impl<C: ClockSource> Copy for Instant<C> {}
 
 impl<C: ClockSource> Instant<C> {
-    /// Get the current time using `CLOCK_MONOTONIC`.
+    /// Get the current time from the clock source.
     #[inline]
     pub fn now() -> Self {
-        // INVARIANT: The `ktime_get()` function returns a value in the range
+        // INVARIANT: The `ClockSource::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() },
+            inner: C::ktime_get(),
             _c: PhantomData,
         }
     }
-- 
2.43.0