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().

[[email protected]: coding-style cleanups]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Sergei Trofimovich <[email protected]>
Cc: matoro <[email protected]>
Cc: Émeric Maschino <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

arch/ia64/kernel/sys_ia64.c

@@ -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

@@ -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