[PATCH] clockevents/drivers/arm_global_timer: Fix erratum 740657 workaround

[Rabin Vincent]

From: Rabin Vincent <rabinv at axis.com>

According to the errata document for the Cortex A9 MPCore, the correct
code sequence in the interrupt handler to workaround erratum 740657
"Global Timer can send two interrupts for the same event" is:

 (1) Read the ICCIAR (Interrupt Acknowledge) register
 (2) Modify the comparator value, to set it to a higher value
 (3) Clear the Global Timer flag
 (4) Clear the Pending Status information for Interrupt 27 (Global
     Timer interrupt) in the Distributor of the Interrupt Controller.
 (5) Write the ICCEOIR (End of Interrupt) register

(1) and (5) are done by the GIC driver and (2) and (3) are done by the
Global Timer driver.  However, nobody does (4) and thus the workaround
is inactive and the timer triggers many spurious interrupts:

 <idle>-0 [001] d.h2 99.850527: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] d.h2 99.850538: irq_handler_exit: irq=16 ret=handled
 <idle>-0 [001] d.H2 99.850540: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] d.H2 99.850542: irq_handler_exit: irq=16 ret=unhandled
 <idle>-0 [001] d.h2 99.987832: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] dnh2 99.987845: irq_handler_exit: irq=16 ret=handled
 <idle>-0 [001] dnh2 99.987848: irq_handler_entry: irq=16 name=gt
 <idle>-0 [001] dnh2 99.987850: irq_handler_exit: irq=16 ret=unhandled

Make the Global Timer driver perform step (4) via the GIC driver with
the help of the irq_set_irqchip_state() function, to prevent the
spurious interrupts.

Signed-off-by: Rabin Vincent <rabinv at axis.com>
---
 drivers/clocksource/arm_global_timer.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
index 9df0d16..b9d0f86 100644
--- a/drivers/clocksource/arm_global_timer.c
+++ b/drivers/clocksource/arm_global_timer.c
@@ -140,26 +140,31 @@ static int gt_clockevent_set_next_event(unsigned long evt,
 static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
 {
 	struct clock_event_device *evt = dev_id;
+	bool workaround = clockevent_state_oneshot(evt);
 
 	if (!(readl_relaxed(gt_base + GT_INT_STATUS) &
 				GT_INT_STATUS_EVENT_FLAG))
 		return IRQ_NONE;
 
 	/**
-	 * ERRATA 740657( Global Timer can send 2 interrupts for
+	 * ERRATA 740657 (Global Timer can send 2 interrupts for
 	 * the same event in single-shot mode)
 	 * Workaround:
-	 *	Either disable single-shot mode.
-	 *	Or
-	 *	Modify the Interrupt Handler to avoid the
-	 *	offending sequence. This is achieved by clearing
-	 *	the Global Timer flag _after_ having incremented
-	 *	the Comparator register	value to a higher value.
+	 * - Read the ICCIAR (Interrupt Acknowledge) register
+	 * - Modify the comparator value, to set it to a higher value
+	 * - Clear the Global Timer flag
+	 * - Clear the Pending Status information for Interrupt 27 (Global
+	 *   Timer interrupt) in the Distributor of the Interrupt Controller.
+	 * - Write the ICCEOIR (End of Interrupt) register
 	 */
-	if (clockevent_state_oneshot(evt))
+	if (workaround)
 		gt_compare_set(ULONG_MAX, 0);
 
 	writel_relaxed(GT_INT_STATUS_EVENT_FLAG, gt_base + GT_INT_STATUS);
+
+	if (workaround)
+		irq_set_irqchip_state(irq, IRQCHIP_STATE_PENDING, false);
+
 	evt->event_handler(evt);
 
 	return IRQ_HANDLED;
-- 
2.7.0