clocksource: add common i8253 PIT clocksource
Russell King [Sun, 8 May 2011 17:47:58 +0000 (18:47 +0100)]
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>

drivers/Kconfig
drivers/clocksource/Kconfig [new file with mode: 0644]
drivers/clocksource/Makefile
drivers/clocksource/i8253.c [new file with mode: 0644]

index 177c7d1..557a469 100644 (file)
@@ -119,4 +119,7 @@ source "drivers/platform/Kconfig"
 source "drivers/clk/Kconfig"
 
 source "drivers/hwspinlock/Kconfig"
+
+source "drivers/clocksource/Kconfig"
+
 endmenu
diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
new file mode 100644 (file)
index 0000000..110aeeb
--- /dev/null
@@ -0,0 +1,2 @@
+config CLKSRC_I8253
+       bool
index be61ece..cfb6383 100644 (file)
@@ -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 (file)
index 0000000..225c176
--- /dev/null
@@ -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);
+}