[PATCH] x86_64: fix hpet for systems that don't support legacy replacement

Currently the x86-64 HPET code assumes the entire HPET implementation from
the spec is present.  This breaks on boxes that do not implement the
optional legacy timer replacement functionality portion of the spec.

This patch fixes this issue, allowing x86-64 systems that cannot use the
HPET for the timer interrupt and RTC to still use the HPET as a time
source.  I've tested this patch on a system systems without HPET, with HPET
but without legacy timer replacement, as well as HPET with legacy timer
replacement.

This version adds a minor check to cap the HPET counter value in
gettimeoffset_hpet to avoid possible time inconsistencies.  Please ignore
the A2 version I sent to you earlier.
Acked-by: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/time.c

@@ -64,6 +64,7 @@ static int notsc __initdata = 0;
 unsigned int cpu_khz;					/* TSC clocks / usec, not used here */
 static unsigned long hpet_period;			/* fsecs / HPET clock */
 unsigned long hpet_tick;				/* HPET clocks / interrupt */
+static int hpet_use_timer;
 unsigned long vxtime_hz = PIT_TICK_RATE;
 int report_lost_ticks;				/* command line option */
 unsigned long long monotonic_base;
@@ -105,7 +106,9 @@ static inline unsigned int do_gettimeoffset_tsc(void)
 
 static inline unsigned int do_gettimeoffset_hpet(void)
 {
-	return ((hpet_readl(HPET_COUNTER) - vxtime.last) * vxtime.quot) >> 32;
+	/* cap counter read to one tick to avoid inconsistencies */
+	unsigned long counter = hpet_readl(HPET_COUNTER) - vxtime.last;
+	return (min(counter,hpet_tick) * vxtime.quot) >> 32;
 }
 
 unsigned int (*do_gettimeoffset)(void) = do_gettimeoffset_tsc;
@@ -301,7 +304,7 @@ unsigned long long monotonic_clock(void)
 
 			last_offset = vxtime.last;
 			base = monotonic_base;
-			this_offset = hpet_readl(HPET_T0_CMP) - hpet_tick;
+			this_offset = hpet_readl(HPET_COUNTER);
 
 		} while (read_seqretry(&xtime_lock, seq));
 		offset = (this_offset - last_offset);
@@ -377,7 +380,14 @@ static irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 
 	write_seqlock(&xtime_lock);
 
-	if (vxtime.hpet_address) {
+	if (vxtime.hpet_address)
+		offset = hpet_readl(HPET_COUNTER);
+
+	if (hpet_use_timer) {
+		/* if we're using the hpet timer functionality,
+		 * we can more accurately know the counter value
+		 * when the timer interrupt occured.
+		 */
 		offset = hpet_readl(HPET_T0_CMP) - hpet_tick;
 		delay = hpet_readl(HPET_COUNTER) - offset;
 	} else {
@@ -803,17 +813,18 @@ static int hpet_timer_stop_set_go(unsigned long tick)
  * Set up timer 0, as periodic with first interrupt to happen at hpet_tick,
  * and period also hpet_tick.
  */
-
-	hpet_writel(HPET_TN_ENABLE | HPET_TN_PERIODIC | HPET_TN_SETVAL |
+	if (hpet_use_timer) {
+		hpet_writel(HPET_TN_ENABLE | HPET_TN_PERIODIC | HPET_TN_SETVAL |
 		    HPET_TN_32BIT, HPET_T0_CFG);
-	hpet_writel(hpet_tick, HPET_T0_CMP);
-	hpet_writel(hpet_tick, HPET_T0_CMP); /* AK: why twice? */
-
+		hpet_writel(hpet_tick, HPET_T0_CMP);
+		hpet_writel(hpet_tick, HPET_T0_CMP); /* AK: why twice? */
+		cfg |= HPET_CFG_LEGACY;
+	}
 /*
  * Go!
  */
 
-	cfg |= HPET_CFG_ENABLE | HPET_CFG_LEGACY;
+	cfg |= HPET_CFG_ENABLE;
 	hpet_writel(cfg, HPET_CFG);
 
 	return 0;
@@ -834,8 +845,7 @@ static int hpet_init(void)
 
 	id = hpet_readl(HPET_ID);
 
-	if (!(id & HPET_ID_VENDOR) || !(id & HPET_ID_NUMBER) ||
-	    !(id & HPET_ID_LEGSUP))
+	if (!(id & HPET_ID_VENDOR) || !(id & HPET_ID_NUMBER))
 		return -1;
 
 	hpet_period = hpet_readl(HPET_PERIOD);
@@ -845,6 +855,8 @@ static int hpet_init(void)
 	hpet_tick = (1000000000L * (USEC_PER_SEC / HZ) + hpet_period / 2) /
 		hpet_period;
 
+	hpet_use_timer = (id & HPET_ID_LEGSUP);
+
 	return hpet_timer_stop_set_go(hpet_tick);
 }
 
@@ -901,9 +913,11 @@ void __init time_init(void)
 	set_normalized_timespec(&wall_to_monotonic,
 	                        -xtime.tv_sec, -xtime.tv_nsec);
 
-	if (!hpet_init()) {
+	if (!hpet_init())
                 vxtime_hz = (1000000000000000L + hpet_period / 2) /
 			hpet_period;
+
+	if (hpet_use_timer) {
 		cpu_khz = hpet_calibrate_tsc();
 		timename = "HPET";
 #ifdef CONFIG_X86_PM_TIMER
@@ -968,7 +982,7 @@ void __init time_init_gtod(void)
 	if (unsynchronized_tsc())
 		notsc = 1;
 	if (vxtime.hpet_address && notsc) {
-		timetype = "HPET";
+		timetype = hpet_use_timer ? "HPET" : "PIT/HPET";
 		vxtime.last = hpet_readl(HPET_T0_CMP) - hpet_tick;
 		vxtime.mode = VXTIME_HPET;
 		do_gettimeoffset = do_gettimeoffset_hpet;
@@ -983,7 +997,7 @@ void __init time_init_gtod(void)
 		printk(KERN_INFO "Disabling vsyscall due to use of PM timer\n");
 #endif
 	} else {
-		timetype = vxtime.hpet_address ? "HPET/TSC" : "PIT/TSC";
+		timetype = hpet_use_timer ? "HPET/TSC" : "PIT/TSC";
 		vxtime.mode = VXTIME_TSC;
 	}