Commit aa06a9bd authored by Sergei Trofimovich's avatar Sergei Trofimovich Committed by Andrew Morton
Browse files

ia64: fix clock_getres(CLOCK_MONOTONIC) to report ITC frequency 

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. I added a bit of explanation why we
need to special-case arch-specific clock_getres().

[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/20220820181813.2275195-1-slyich@gmail.com


Signed-off-by: default avatarSergei Trofimovich <slyich@gmail.com>
Cc: matoro <matoro_mailinglist_kernel@matoro.tk>
Cc: Émeric Maschino <emeric.maschino@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent cba7543e

arch/ia64/kernel/sys_ia64.c

+26 −0
Original line number Diff line number Diff line
@@ -166,3 +166,29 @@ ia64_mremap (unsigned long addr, unsigned long old_len, unsigned long new_len, u 
		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);

}

arch/ia64/kernel/syscalls/syscall.tbl

+1 −1
Original line number Diff line number Diff line
@@ -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