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Date: Mon, 09 May 2011 14:28:41 +0100
From: Russell King - ARM Linux <linux@....linux.org.uk>
To: LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
John Stultz <johnstul@...ibm.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Ralf Baechle <ralf@...ux-mips.org>
Subject: [PATCH 1/4] clocksource: add common i8253 PIT clocksource
This is based upon both arch/arm/mach-footbridge/isa-timer.c and
arch/x86/kernel/i8253.c.
Cc: John Stultz <johnstul@...ibm.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Ralf Baechle <ralf@...ux-mips.org>
Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
---
drivers/clocksource/Kconfig | 3 +
drivers/clocksource/Makefile | 1 +
drivers/clocksource/i8253.c | 88 ++++++++++++++++++++++++++++++++++++++++++
3 files changed, 92 insertions(+), 0 deletions(-)
create mode 100644 drivers/clocksource/Kconfig
create mode 100644 drivers/clocksource/i8253.c
diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
new file mode 100644
index 0000000..6726997
--- /dev/null
+++ b/drivers/clocksource/Kconfig
@@ -0,0 +1,3 @@
+config CLKSRC_I8253
+ bool
+
diff --git a/drivers/clocksource/Makefile b/drivers/clocksource/Makefile
index be61ece..cfb6383 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 0000000..225c176
--- /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);
+}
--
1.7.4.4
--
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