Messages in this thread | | | From | John Stultz <> | Date | Thu, 28 Sep 2023 22:15:27 -0700 | Subject | Re: [PATCH 1/4] time: add ktime_get_cycles64() api |
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On Thu, Sep 28, 2023 at 7:37 PM Mahesh Bandewar <maheshb@google.com> wrote: > > add a method to retrieve raw cycles in the same fashion as there are > ktime_get_* methods available for supported time-bases. The method > continues using the 'struct timespec64' since the UAPI uses 'struct > ptp_clock_time'. > > The caller can perform operation equivalent of timespec64_to_ns() to > retrieve raw-cycles value. The precision loss because of this conversion > should be none for 64 bit cycle counters and nominal at 96 bit counters > (considering UAPI of s64 + u32 of 'struct ptp_clock_time). > > Signed-off-by: Mahesh Bandewar <maheshb@google.com> > CC: John Stultz <jstultz@google.com> > CC: Thomas Gleixner <tglx@linutronix.de> > CC: Stephen Boyd <sboyd@kernel.org> > CC: Richard Cochran <richardcochran@gmail.com> > CC: netdev@vger.kernel.org > CC: linux-kernel@vger.kernel.org > --- > include/linux/timekeeping.h | 1 + > kernel/time/timekeeping.c | 24 ++++++++++++++++++++++++ > 2 files changed, 25 insertions(+) > > diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h > index fe1e467ba046..5537700ad113 100644 > --- a/include/linux/timekeeping.h > +++ b/include/linux/timekeeping.h > @@ -43,6 +43,7 @@ extern void ktime_get_ts64(struct timespec64 *ts); > extern void ktime_get_real_ts64(struct timespec64 *tv); > extern void ktime_get_coarse_ts64(struct timespec64 *ts); > extern void ktime_get_coarse_real_ts64(struct timespec64 *ts); > +extern void ktime_get_cycles64(struct timespec64 *ts); > > void getboottime64(struct timespec64 *ts); > > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > index 266d02809dbb..35d603d21bd5 100644 > --- a/kernel/time/timekeeping.c > +++ b/kernel/time/timekeeping.c > @@ -989,6 +989,30 @@ void ktime_get_ts64(struct timespec64 *ts) > } > EXPORT_SYMBOL_GPL(ktime_get_ts64); > > +/** > + * ktime_get_cycles64 - get the raw clock cycles in timespec64 format > + * @ts: pointer to timespec variable > + * > + * This function converts the raw clock cycles into timespce64 format > + * in the varibale pointed to by @ts > + */ > +void ktime_get_cycles64(struct timespec64 *ts) > +{ > + struct timekeeper *tk = &tk_core.timekeeper; > + unsigned int seq; > + u64 now; > + > + WARN_ON_ONCE(timekeeping_suspended); > + > + do { > + seq = read_seqcount_begin(&tk_core.seq); > + now = tk_clock_read(&tk->tkr_mono); > + } while (read_seqcount_retry(&tk_core.seq, seq)); > + > + *ts = ns_to_timespec64(now); > +}
Hey Mahesh, Thanks for sending this out. Unfortunately, I'm a bit confused by this. It might be helpful to further explain what this would be used for in more detail?
Some aspects that are particularly unclear: 1) You seem to be trying to stuff cycle values into a timespec64, which is not very intuitive (and a type error of sorts). It's not clear /why/ that type is useful.
2) Depending on your clocksource, this would have very strange wrapping behavior, so I'm not sure it is generally safe to use.
3) Nit: The interface is called ktime_get_cycles64 (timespec64 returning interfaces usually are postfixed with ts64).
I guess could you clarify why you need this instead of using CLOCK_MONOTONIC_RAW which tries to abstract raw cycles in a way that is safe and avoids wrapping across various clocksources?
thanks -john
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