[PATCH v2 1/2] i8253: Add support for PIT shutdown quirk

[Michael Kelley <mikelley@microsoft.com>]

Add support for platforms where pit_shutdown() doesn't work because
of a quirk in the PIT emulation. On these platforms setting the
counter register to zero causes the PIT to start running again,
negating the shutdown. Provide a global variable that controls
whether the counter register is zero'ed, which platform specific
code can override.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
---
 drivers/clocksource/i8253.c | 14 ++++++++++++--
 include/linux/i8253.h       |  1 +
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/clocksource/i8253.c b/drivers/clocksource/i8253.c
index 9c38895..2a4202a 100644
--- a/drivers/clocksource/i8253.c
+++ b/drivers/clocksource/i8253.c
@@ -20,6 +20,13 @@
 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 __ro_after_init = true;
+
 #ifdef CONFIG_CLKSRC_I8253
 /*
  * Since the PIT overflows every tick, its not very useful
@@ -109,8 +116,11 @@ static int pit_shutdown(struct clock_event_device *evt)
 	raw_spin_lock(&i8253_lock);
 
 	outb_p(0x30, PIT_MODE);
-	outb_p(0, PIT_CH0);
-	outb_p(0, PIT_CH0);
+
+	if (i8253_clear_counter) {
+		outb_p(0, PIT_CH0);
+		outb_p(0, PIT_CH0);
+	}
 
 	raw_spin_unlock(&i8253_lock);
 	return 0;
diff --git a/include/linux/i8253.h b/include/linux/i8253.h
index e6bb36a..31c4be5 100644
--- a/include/linux/i8253.h
+++ b/include/linux/i8253.h
@@ -21,6 +21,7 @@
 #define PIT_LATCH	((PIT_TICK_RATE + HZ/2) / HZ)
 
 extern raw_spinlock_t i8253_lock;
+extern bool i8253_clear_counter;
 extern struct clock_event_device i8253_clockevent;
 extern void clockevent_i8253_init(bool oneshot);
 
-- 
1.8.3.1