The patch titled
Subject: ia64: fix clock_getre(CLOCK_MONOTONIC) to report ITC frequency
has been added to the -mm mm-nonmm-unstable branch. Its filename is
ia64-fix-clock_getreclock_monotonic-to-report-itc-frequency.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/ia64-fix-clock_getreclock_monotonic-to-report-itc-frequency.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Sergei Trofimovich <slyich@xxxxxxxxx>
Subject: ia64: fix clock_getre(CLOCK_MONOTONIC) to report ITC frequency
Date: Mon, 15 Aug 2022 06:49:44 +0100
clock_gettime(CLOCK_MONOTONIC, &tp) is very precise on ia64 as it uses
ITC (similar to rdtsc on x86). It's not quite a hrtimer as it is a few
times slower than 1ns. Usually 2-3ns.
clock_getres(CLOCK_MONOTONIC, &res) never reflected that fact and
reported 0.04s precision (1/HZ value).
In https://bugs.gentoo.org/596382 gstreamer's test suite failed loudly
when it noticed precision discrepancy.
Before the change:
clock_getres(CLOCK_MONOTONIC, &res) reported 250Hz precision.
After the change:
clock_getres(CLOCK_MONOTONIC, &res) reports ITC (400Mhz) precision.
The patch is based on matoro's fix. It adds a bit of explanation why we
need to special-case arch-specific clock_getres().
Link: https://lkml.kernel.org/r/20220815054944.4130786-1-slyich@xxxxxxxxx
Signed-off-by: Sergei Trofimovich <slyich@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/ia64/kernel/sys_ia64.c | 26 ++++++++++++++++++++++++
arch/ia64/kernel/syscalls/syscall.tbl | 2 -
2 files changed, 27 insertions(+), 1 deletion(-)
--- a/arch/ia64/kernel/syscalls/syscall.tbl~ia64-fix-clock_getreclock_monotonic-to-report-itc-frequency
+++ a/arch/ia64/kernel/syscalls/syscall.tbl
@@ -240,7 +240,7 @@
228 common timer_delete sys_timer_delete
229 common clock_settime sys_clock_settime
230 common clock_gettime sys_clock_gettime
-231 common clock_getres sys_clock_getres
+231 common clock_getres ia64_clock_getres
232 common clock_nanosleep sys_clock_nanosleep
233 common fstatfs64 sys_fstatfs64
234 common statfs64 sys_statfs64
--- a/arch/ia64/kernel/sys_ia64.c~ia64-fix-clock_getreclock_monotonic-to-report-itc-frequency
+++ a/arch/ia64/kernel/sys_ia64.c
@@ -166,3 +166,29 @@ ia64_mremap (unsigned long addr, unsigne
force_successful_syscall_return();
return addr;
}
+
+asmlinkage long
+ia64_clock_getres(const clockid_t which_clock, struct __kernel_timespec __user *tp)
+{
+ /*
+ * ia64's clock_gettime() syscall is implemented as a vdso call
+ * fsys_clock_gettime(). Currently it handles only
+ * CLOCK_REALTIME and CLOCK_MONOTONIC. Both are based on
+ * 'ar.itc' counter which gets incremented at a constant
+ * frequency. It's usually 400MHz, ~2.5x times slower than CPU
+ * clock frequency. Which is almost a 1ns hrtimer, but not quite.
+ *
+ * Let's special-case these timers to report correct precision
+ * based on ITC frequency and not HZ frequency for supported
+ * clocks.
+ */
+ switch (which_clock) {
+ case CLOCK_REALTIME:
+ case CLOCK_MONOTONIC:
+ s64 tick_ns = DIV_ROUND_UP(NSEC_PER_SEC, local_cpu_data->itc_freq);
+ struct timespec64 rtn_tp = ns_to_timespec64(tick_ns);
+ return put_timespec64(&rtn_tp, tp);
+ }
+
+ return sys_clock_getres(which_clock, tp);
+}
_
Patches currently in -mm which might be from slyich@xxxxxxxxx are
ia64-fix-clock_getreclock_monotonic-to-report-itc-frequency.patch
