[RFC PATCH v4 03/21] x86/hpet: Calculate ticks-per-second in a separate function

[Ricardo Neri]

It is easier to compute the expiration times of an HPET timer by using
its frequency (i.e., the number of times it ticks in a second) than its
period, as given in the capabilities register.

In addition to the HPET char driver, the HPET-based hardlockup detector
will also need to know the timer's frequency. Thus, create a common
function that both can use.

Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ashok Raj <ashok.raj@xxxxxxxxx>
Cc: Andi Kleen <andi.kleen@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Clemens Ladisch <clemens@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Philippe Ombredanne <pombredanne@xxxxxxxx>
Cc: Kate Stewart <kstewart@xxxxxxxxxxxxxxxxxxx>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Suravee Suthikulpanit <Suravee.Suthikulpanit@xxxxxxx>
Cc: "Ravi V. Shankar" <ravi.v.shankar@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
 drivers/char/hpet.c  | 31 ++++++++++++++++++++++++-------
 include/linux/hpet.h |  1 +
 2 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 3a1e6b3ccd10..747255f552a9 100644
--- a/drivers/char/hpet.c
+++ b/drivers/char/hpet.c
@@ -836,6 +836,29 @@ static unsigned long hpet_calibrate(struct hpets *hpetp)
 	return ret;
 }
 
+u64 hpet_get_ticks_per_sec(u64 hpet_caps)
+{
+	u64 ticks_per_sec, period;
+
+	period = (hpet_caps & HPET_COUNTER_CLK_PERIOD_MASK) >>
+		 HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */
+
+	/*
+	 * The frequency is the reciprocal of the period. The period is given
+	 * in femtoseconds per second. Thus, prepare a dividend to obtain the
+	 * frequency in ticks per second.
+	 */
+
+	/* 10^15 femtoseconds per second */
+	ticks_per_sec = 1000000000000000ULL;
+	ticks_per_sec += period >> 1; /* round */
+
+	/* The quotient is put in the dividend. We drop the remainder. */
+	do_div(ticks_per_sec, period);
+
+	return ticks_per_sec;
+}
+
 int hpet_alloc(struct hpet_data *hdp)
 {
 	u64 cap, mcfg;
@@ -844,7 +867,6 @@ int hpet_alloc(struct hpet_data *hdp)
 	struct hpets *hpetp;
 	struct hpet __iomem *hpet;
 	static struct hpets *last;
-	unsigned long period;
 	unsigned long long temp;
 	u32 remainder;
 
@@ -894,12 +916,7 @@ int hpet_alloc(struct hpet_data *hdp)
 
 	last = hpetp;
 
-	period = (cap & HPET_COUNTER_CLK_PERIOD_MASK) >>
-		HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */
-	temp = 1000000000000000uLL; /* 10^15 femtoseconds per second */
-	temp += period >> 1; /* round */
-	do_div(temp, period);
-	hpetp->hp_tick_freq = temp; /* ticks per second */
+	hpetp->hp_tick_freq = hpet_get_ticks_per_sec(cap);
 
 	printk(KERN_INFO "hpet%d: at MMIO 0x%lx, IRQ%s",
 		hpetp->hp_which, hdp->hd_phys_address,
diff --git a/include/linux/hpet.h b/include/linux/hpet.h
index 8604564b985d..e7b36bcf4699 100644
--- a/include/linux/hpet.h
+++ b/include/linux/hpet.h
@@ -107,5 +107,6 @@ static inline void hpet_reserve_timer(struct hpet_data *hd, int timer)
 }
 
 int hpet_alloc(struct hpet_data *);
+u64 hpet_get_ticks_per_sec(u64 hpet_caps);
 
 #endif				/* !__HPET__ */
-- 
2.17.1