ia64: Update fsyscall gettime to use modern vsyscall_update

John Stultz provided the outline for this patch back in May 2014 here:

	http://patches.linaro.org/patch/30501/

but I let this sit on the shelf for too long and in the intervening
years almost every field in "struct timekeeper" was changed. So this
is almost completely different from his original. Though the key change
in arch/ia64/kernel/fsys.S remains the same.

The core logic change with the updated vsyscall method is that we
preserve the base nanosecond value in shifted nanoseconds, which
allows us to avoid truncating and rounding up to the next nanosecond
every tick to avoid inconsistencies.

Thus the logic moved from
nsec = ((cycle_delta * mult)>>shift) + base_nsec;
to
nsec = ((cycle_delta * mult) + base_snsec) >> shift;

Cc: John Stultz <john.stultz@linaro.org>
Cc: linux-ia64@vger.kernel.org
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/Kconfig

@@ -46,7 +46,7 @@ config IA64
 	select ARCH_TASK_STRUCT_ALLOCATOR
 	select ARCH_THREAD_STACK_ALLOCATOR
 	select ARCH_CLOCKSOURCE_DATA
-	select GENERIC_TIME_VSYSCALL_OLD
+	select GENERIC_TIME_VSYSCALL
 	select SYSCTL_ARCH_UNALIGN_NO_WARN
 	select HAVE_MOD_ARCH_SPECIFIC
 	select MODULES_USE_ELF_RELA

arch/ia64/kernel/asm-offsets.c

@@ -211,6 +211,8 @@ void foo(void)
 	BLANK();
 	DEFINE(IA64_TIMESPEC_TV_NSEC_OFFSET,
 	       offsetof (struct timespec, tv_nsec));
+	DEFINE(IA64_TIME_SN_SPEC_SNSEC_OFFSET,
+	       offsetof (struct time_sn_spec, snsec));
 
 	DEFINE(CLONE_SETTLS_BIT, 19);
 #if CLONE_SETTLS != (1<<19)

arch/ia64/kernel/fsys.S

@@ -235,9 +235,9 @@ ENTRY(fsys_gettimeofday)
 	MOV_FROM_ITC(p8, p6, r2, r10)	// CPU_TIMER. 36 clocks latency!!!
 (p9)	ld8 r2 = [r30]		// MMIO_TIMER. Could also have latency issues..
 (p13)	ld8 r25 = [r19]		// get itc_lastcycle value
-	ld8 r9 = [r22],IA64_TIMESPEC_TV_NSEC_OFFSET	// tv_sec
+	ld8 r9 = [r22],IA64_TIME_SN_SPEC_SNSEC_OFFSET	// sec
 	;;
-	ld8 r8 = [r22],-IA64_TIMESPEC_TV_NSEC_OFFSET	// tv_nsec
+	ld8 r8 = [r22],-IA64_TIME_SN_SPEC_SNSEC_OFFSET	// snsec
 (p13)	sub r3 = r25,r2		// Diff needed before comparison (thanks davidm)
 	;;
 (p13)	cmp.gt.unc p6,p7 = r3,r0 // check if it is less than last. p6,p7 cleared
@@ -265,9 +265,9 @@ EX(.fail_efault, probe.w.fault r31, 3)
 	mf
 	;;
 	ld4 r10 = [r20]		// gtod_lock.sequence
-	shr.u r2 = r2,r23	// shift by factor
-	;;
 	add r8 = r8,r2		// Add xtime.nsecs
+	;;
+	shr.u r8 = r8,r23	// shift by factor
 	cmp4.ne p7,p0 = r28,r10
 (p7)	br.cond.dpnt.few .time_redo	// sequence number changed, redo
 	// End critical section.

arch/ia64/kernel/fsyscall_gtod_data.h

@@ -5,10 +5,16 @@
  * fsyscall gettimeofday data
  */
 
+/* like timespec, but includes "shifted nanoseconds" */
+struct time_sn_spec {
+	u64	sec;
+	u64	snsec;
+};
+
 struct fsyscall_gtod_data_t {
 	seqcount_t	seq;
-	struct timespec	wall_time;
-	struct timespec monotonic_time;
+	struct time_sn_spec wall_time;
+	struct time_sn_spec monotonic_time;
 	u64		clk_mask;
 	u32		clk_mult;
 	u32		clk_shift;

arch/ia64/kernel/time.c

@@ -430,30 +430,32 @@ void update_vsyscall_tz(void)
 {
 }
 
-void update_vsyscall_old(struct timespec *wall, struct timespec *wtm,
-			 struct clocksource *c, u32 mult, u64 cycle_last)
+void update_vsyscall(struct timekeeper *tk)
 {
 	write_seqcount_begin(&fsyscall_gtod_data.seq);
 
-        /* copy fsyscall clock data */
-        fsyscall_gtod_data.clk_mask = c->mask;
-        fsyscall_gtod_data.clk_mult = mult;
-        fsyscall_gtod_data.clk_shift = c->shift;
-        fsyscall_gtod_data.clk_fsys_mmio = c->archdata.fsys_mmio;
-        fsyscall_gtod_data.clk_cycle_last = cycle_last;
-
-	/* copy kernel time structures */
-        fsyscall_gtod_data.wall_time.tv_sec = wall->tv_sec;
-        fsyscall_gtod_data.wall_time.tv_nsec = wall->tv_nsec;
-	fsyscall_gtod_data.monotonic_time.tv_sec = wtm->tv_sec
-							+ wall->tv_sec;
-	fsyscall_gtod_data.monotonic_time.tv_nsec = wtm->tv_nsec
-							+ wall->tv_nsec;
+	/* copy vsyscall data */
+	fsyscall_gtod_data.clk_mask = tk->tkr_mono.mask;
+	fsyscall_gtod_data.clk_mult = tk->tkr_mono.mult;
+	fsyscall_gtod_data.clk_shift = tk->tkr_mono.shift;
+	fsyscall_gtod_data.clk_fsys_mmio = tk->tkr_mono.clock->archdata.fsys_mmio;
+	fsyscall_gtod_data.clk_cycle_last = tk->tkr_mono.cycle_last;
+
+	fsyscall_gtod_data.wall_time.sec = tk->xtime_sec;
+	fsyscall_gtod_data.wall_time.snsec = tk->tkr_mono.xtime_nsec;
+
+	fsyscall_gtod_data.monotonic_time.sec = tk->xtime_sec
+					      + tk->wall_to_monotonic.tv_sec;
+	fsyscall_gtod_data.monotonic_time.snsec = tk->tkr_mono.xtime_nsec
+						+ ((u64)tk->wall_to_monotonic.tv_nsec
+							<< tk->tkr_mono.shift);
 
 	/* normalize */
-	while (fsyscall_gtod_data.monotonic_time.tv_nsec >= NSEC_PER_SEC) {
-		fsyscall_gtod_data.monotonic_time.tv_nsec -= NSEC_PER_SEC;
-		fsyscall_gtod_data.monotonic_time.tv_sec++;
+	while (fsyscall_gtod_data.monotonic_time.snsec >=
+					(((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) {
+		fsyscall_gtod_data.monotonic_time.snsec -=
+					((u64)NSEC_PER_SEC) << tk->tkr_mono.shift;
+		fsyscall_gtod_data.monotonic_time.sec++;
 	}
 
 	write_seqcount_end(&fsyscall_gtod_data.seq);