clocksource: add common i8253 PIT clocksource

This is based upon both arch/arm/mach-footbridge/isa-timer.c and
arch/x86/kernel/i8253.c.

Acked-by: John Stultz <john.stultz@linaro.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

3 files changed, 91 insertions, 0 deletions

diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
new file mode 100644
index 000000000000..110aeeb52f9a
--- /dev/null
+++ b/drivers/clocksource/Kconfig
@@ -0,0 +1,2 @@
+config CLKSRC_I8253
+        bool
diff --git a/drivers/clocksource/Makefile b/drivers/clocksource/Makefile
index be61ece6330b..cfb6383b543a 100644
--- a/drivers/clocksource/Makefile
+++ b/drivers/clocksource/Makefile
@@ -6,3 +6,4 @@ obj-$(CONFIG_CS5535_CLOCK_EVENT_SRC)	+= cs5535-clockevt.o
 obj-$(CONFIG_SH_TIMER_CMT)      += sh_cmt.o
 obj-$(CONFIG_SH_TIMER_MTU2)     += sh_mtu2.o
 obj-$(CONFIG_SH_TIMER_TMU)      += sh_tmu.o
+obj-$(CONFIG_CLKSRC_I8253)      += i8253.o
diff --git a/drivers/clocksource/i8253.c b/drivers/clocksource/i8253.c
new file mode 100644
index 000000000000..225c1761b372
--- /dev/null
+++ b/drivers/clocksource/i8253.c
@@ -0,0 +1,88 @@
+/*
+ * i8253 PIT clocksource
+ */
+#include <linux/clocksource.h>
+#include <linux/init.h>
+#include <linux/io.h>
+#include <linux/spinlock.h>
+#include <linux/timex.h>
+
+#include <asm/i8253.h>
+
+/*
+ * Since the PIT overflows every tick, its not very useful
+ * to just read by itself. So use jiffies to emulate a free
+ * running counter:
+ */
+static cycle_t i8253_read(struct clocksource *cs)
+{
+        static int old_count;
+        static u32 old_jifs;
+        unsigned long flags;
+        int count;
+        u32 jifs;
+
+        raw_spin_lock_irqsave(&i8253_lock, flags);
+        /*
+         * Although our caller may have the read side of xtime_lock,
+         * this is now a seqlock, and we are cheating in this routine
+         * by having side effects on state that we cannot undo if
+         * there is a collision on the seqlock and our caller has to
+         * retry.  (Namely, old_jifs and old_count.)  So we must treat
+         * jiffies as volatile despite the lock.  We read jiffies
+         * before latching the timer count to guarantee that although
+         * the jiffies value might be older than the count (that is,
+         * the counter may underflow between the last point where
+         * jiffies was incremented and the point where we latch the
+         * count), it cannot be newer.
+         */
+        jifs = jiffies;
+        outb_pit(0x00, PIT_MODE);       /* latch the count ASAP */
+        count = inb_pit(PIT_CH0);       /* read the latched count */
+        count |= inb_pit(PIT_CH0) << 8;
+
+        /* VIA686a test code... reset the latch if count > max + 1 */
+        if (count > LATCH) {
+                outb_pit(0x34, PIT_MODE);
+                outb_pit(PIT_LATCH & 0xff, PIT_CH0);
+                outb_pit(PIT_LATCH >> 8, PIT_CH0);
+                count = PIT_LATCH - 1;
+        }
+
+        /*
+         * It's possible for count to appear to go the wrong way for a
+         * couple of reasons:
+         *
+         *  1. The timer counter underflows, but we haven't handled the
+         *     resulting interrupt and incremented jiffies yet.
+         *  2. Hardware problem with the timer, not giving us continuous time,
+         *     the counter does small "jumps" upwards on some Pentium systems,
+         *     (see c't 95/10 page 335 for Neptun bug.)
+         *
+         * Previous attempts to handle these cases intelligently were
+         * buggy, so we just do the simple thing now.
+         */
+        if (count > old_count && jifs == old_jifs)
+                count = old_count;
+
+        old_count = count;
+        old_jifs = jifs;
+
+        raw_spin_unlock_irqrestore(&i8253_lock, flags);
+
+        count = (PIT_LATCH - 1) - count;
+
+        return (cycle_t)(jifs * PIT_LATCH) + count;
+}
+
+static struct clocksource i8253_cs = {
+        .name           = "pit",
+        .rating         = 110,
+        .read           = i8253_read,
+        .mask           = CLOCKSOURCE_MASK(32),
+};
+
+int __init clocksource_i8253_init(void)
+{
+        return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE);
+}