rust: hrtimer: add clocksource selection through `ClockId`

Allow selecting a clock source for timers by passing a `ClockId`
variant to `HrTimer::new`.

Acked-by: Frederic Weisbecker <[e-mail]>
Acked-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Lyude Paul <[e-mail]>
Reviewed-by: Benno Lossin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Andreas Hindborg <[e-mail]>

rust/kernel/time.rs

···
         }
     }
 }
+
+/// 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
+    }
+}

rust/kernel/time/hrtimer.rs

···
 //! A `restart` operation on a timer in the **stopped** state is equivalent to a
 //! `start` operation.
 
+use super::ClockId;
 use crate::{init::PinInit, prelude::*, time::Ktime, types::Opaque};
 use core::marker::PhantomData;
 
···
 
 impl<T> HrTimer<T> {
     /// Return an initializer for a new timer instance.
-    pub fn new(mode: HrTimerMode) -> impl PinInit<Self>
+    pub fn new(mode: HrTimerMode, clock: ClockId) -> impl PinInit<Self>
     where
         T: HrTimerCallback,
     {
···
                     bindings::hrtimer_setup(
                         place,
                         Some(T::Pointer::run),
-                        bindings::CLOCK_MONOTONIC as i32,
+                        clock.into_c(),
                         mode.into_c(),
                     );
                 }