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Diffstat (limited to 'drivers/clocksource/i8253.c')
-rw-r--r--drivers/clocksource/i8253.c49
1 files changed, 34 insertions, 15 deletions
diff --git a/drivers/clocksource/i8253.c b/drivers/clocksource/i8253.c
index d4350bb10b83..39f7c2d736d1 100644
--- a/drivers/clocksource/i8253.c
+++ b/drivers/clocksource/i8253.c
@@ -20,13 +20,6 @@
DEFINE_RAW_SPINLOCK(i8253_lock);
EXPORT_SYMBOL(i8253_lock);
-/*
- * Handle PIT quirk in pit_shutdown() where zeroing the counter register
- * restarts the PIT, negating the shutdown. On platforms with the quirk,
- * platform specific code can set this to false.
- */
-bool i8253_clear_counter_on_shutdown __ro_after_init = true;
-
#ifdef CONFIG_CLKSRC_I8253
/*
* Since the PIT overflows every tick, its not very useful
@@ -108,21 +101,47 @@ int __init clocksource_i8253_init(void)
#endif
#ifdef CONFIG_CLKEVT_I8253
-static int pit_shutdown(struct clock_event_device *evt)
+void clockevent_i8253_disable(void)
{
- if (!clockevent_state_oneshot(evt) && !clockevent_state_periodic(evt))
- return 0;
-
raw_spin_lock(&i8253_lock);
+ /*
+ * Writing the MODE register should stop the counter, according to
+ * the datasheet. This appears to work on real hardware (well, on
+ * modern Intel and AMD boxes; I didn't dig the Pegasos out of the
+ * shed).
+ *
+ * However, some virtual implementations differ, and the MODE change
+ * doesn't have any effect until either the counter is written (KVM
+ * in-kernel PIT) or the next interrupt (QEMU). And in those cases,
+ * it may not stop the *count*, only the interrupts. Although in
+ * the virt case, that probably doesn't matter, as the value of the
+ * counter will only be calculated on demand if the guest reads it;
+ * it's the interrupts which cause steal time.
+ *
+ * Hyper-V apparently has a bug where even in mode 0, the IRQ keeps
+ * firing repeatedly if the counter is running. But it *does* do the
+ * right thing when the MODE register is written.
+ *
+ * So: write the MODE and then load the counter, which ensures that
+ * the IRQ is stopped on those buggy virt implementations. And then
+ * write the MODE again, which is the right way to stop it.
+ */
outb_p(0x30, PIT_MODE);
+ outb_p(0, PIT_CH0);
+ outb_p(0, PIT_CH0);
- if (i8253_clear_counter_on_shutdown) {
- outb_p(0, PIT_CH0);
- outb_p(0, PIT_CH0);
- }
+ outb_p(0x30, PIT_MODE);
raw_spin_unlock(&i8253_lock);
+}
+
+static int pit_shutdown(struct clock_event_device *evt)
+{
+ if (!clockevent_state_oneshot(evt) && !clockevent_state_periodic(evt))
+ return 0;
+
+ clockevent_i8253_disable();
return 0;
}