x86: hpet: fix periodic mode programming on AMD 81xx

(See http://bugzilla.kernel.org/show_bug.cgi?id=12961)

It partially reverts commit c23e253e
(x86: hpet: stop HPET_COUNTER when programming periodic mode)

HPET on AMD 81xx chipset needs a second write (with HPET_TN_SETVAL
cleared) to T0_CMP register to set the period in periodic mode.

With this patch HPET_COUNTER is still stopped but not reset when HPET
is programmed in periodic mode. This should help to avoid races when
HPET is programmed in periodic mode and fixes a boot time hang that
I've observed on a machine when using 1000HZ.

[ Impact: fix boot time hang on machines with AMD 81xx chipset ]
Reported-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Tested-by: Jeff Mahoney <jeffm@suse.com>
LKML-Reference: <20090421180037.GA2763@alberich.amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/hpet.c

@@ -236,6 +236,10 @@ static void hpet_stop_counter(void)
 	unsigned long cfg = hpet_readl(HPET_CFG);
 	cfg &= ~HPET_CFG_ENABLE;
 	hpet_writel(cfg, HPET_CFG);
+}
+
+static void hpet_reset_counter(void)
+{
 	hpet_writel(0, HPET_COUNTER);
 	hpet_writel(0, HPET_COUNTER + 4);
 }
@@ -250,6 +254,7 @@ static void hpet_start_counter(void)
 static void hpet_restart_counter(void)
 {
 	hpet_stop_counter();
+	hpet_reset_counter();
 	hpet_start_counter();
 }
 
@@ -309,7 +314,7 @@ static int hpet_setup_msi_irq(unsigned int irq);
 static void hpet_set_mode(enum clock_event_mode mode,
 			  struct clock_event_device *evt, int timer)
 {
-	unsigned long cfg;
+	unsigned long cfg, cmp, now;
 	uint64_t delta;
 
 	switch (mode) {
@@ -317,12 +322,23 @@ static void hpet_set_mode(enum clock_event_mode mode,
 		hpet_stop_counter();
 		delta = ((uint64_t)(NSEC_PER_SEC/HZ)) * evt->mult;
 		delta >>= evt->shift;
+		now = hpet_readl(HPET_COUNTER);
+		cmp = now + (unsigned long) delta;
 		cfg = hpet_readl(HPET_Tn_CFG(timer));
 		/* Make sure we use edge triggered interrupts */
 		cfg &= ~HPET_TN_LEVEL;
 		cfg |= HPET_TN_ENABLE | HPET_TN_PERIODIC |
 		       HPET_TN_SETVAL | HPET_TN_32BIT;
 		hpet_writel(cfg, HPET_Tn_CFG(timer));
+		hpet_writel(cmp, HPET_Tn_CMP(timer));
+		udelay(1);
+		/*
+		 * HPET on AMD 81xx needs a second write (with HPET_TN_SETVAL
+		 * cleared) to T0_CMP to set the period. The HPET_TN_SETVAL
+		 * bit is automatically cleared after the first write.
+		 * (See AMD-8111 HyperTransport I/O Hub Data Sheet,
+		 * Publication # 24674)
+		 */
 		hpet_writel((unsigned long) delta, HPET_Tn_CMP(timer));
 		hpet_start_counter();
 		hpet_print_config();