Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH] rust: time: New module for timekeeping functions | Date | Tue, 21 Feb 2023 13:32:51 +0100 |
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On Tue, Feb 21 2023 at 16:06, Asahi Lina wrote: > + > +use crate::bindings; > +use core::time::Duration; > + > +/// Returns the kernel time elapsed since boot, excluding time spent sleeping, as a [`Duration`]. > +pub fn ktime_get() -> Duration { > + // SAFETY: Function has no side effects and no inputs. > + Duration::from_nanos(unsafe { bindings::ktime_get() }.try_into().unwrap())
Why is this a Duration? From the spec:
Duration
A Duration type to represent a span of time, typically used for system timeouts.
Instant
A measurement of a monotonically nondecreasing clock. Opaque and useful only with Duration.
In my understanding 'Duration' is a time span between two points, while ktime_get() and ktime_get_boottime() return the current time of monotonically nondecreasing clocks, i.e. they fall into the 'Instant' category.
Now the problem is that 'Instant' in it's specification is bound to CLOCK_MONOTONIC and there is no way to express CLOCK_BOOTTIME, but that's a shortcoming of the spec which ignores CLOCK_BOOTTIME completely. IOW, that's also a problem for user space.
This makes sense vs. the other representation:
SystemTime
A measurement of the system clock, useful for talking to external entities like the file system or other processes.
This maps to CLOCK_REALTIME and CLOCK_TAI, i.e. ktime_get_real_ns() and ktime_get_clocktai().
Similar to 'Instant' 'SystemTime' is strictly bound to CLOCK_REALTIME by specification and there is no way to read CLOCK_TAI.
Please fix this in the spec and do not try to work around that by pretending that a clock read is a 'Duration'.
> +} > + > +/// Returns the kernel time elapsed since boot, including time spent sleeping, as a [`Duration`]. > +pub fn ktime_get_boottime() -> Duration { > + Duration::from_nanos( > + // SAFETY: Function has no side effects and no variable inputs. > + unsafe { bindings::ktime_get_with_offset(bindings::tk_offsets_TK_OFFS_BOOT) }
No. Please use ktime_get_boottime() and not the timekeeping internal function.
Thanks,
tglx
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