[RFC PATCH] irqchip/gic, gic-v3: Ensure data visibility in peripheral

[Leo Yan <leo.yan@linaro.org>]

When an interrupt line is assered, GIC handles interrupt with the flow
(with EOImode == 1):

  gic_handle_irq()
   `> do_read_iar()                          => Change int state to active
   `> gic_write_eoir()                       => Drop int priority
   `> handle_domain_irq()
       `> generic_handle_irq_desc()
       `> handle_fasteoi_irq()
           `> handle_irq_event()             => Peripheral handler and
	                                        de-assert int line
           `> cond_unmask_eoi_irq()
	       `> chip->irq_eoi()
	           `> gic_eoimode1_eoi_irq() => Change int state to inactive

In this flow, it has no explicit memory barrier between the functions
handle_irq_event() and chip->irq_eoi(), it's possible that the
outstanding data has not reached device in handle_irq_event() but the
callback chip->irq_eoi() is invoked, this can lead to state transition
for level triggered interrupt:

  Flow                             |  Interrupt state in GIC
  ---------------------------------+-------------------------------------
  Interrupt line is asserted       |  'inactive' -> 'pending'
  do_read_iar()                    |  'pending'  -> 'pending & active'
  handle_irq_event()               |  Write peripheral register but it's
                                   |    not visible for device, so the
				   |    interrupt line is still asserted
  chip->irq_eoi()                  |  'pending & active' -> 'pending'
                                  ...
  Produce spurious interrupt       |
      with interrupt ID: 1024      |
                                   |  Finally the peripheral reigster is
				   |  updated and the interrupt line is
				   |  deasserted: 'pending' -> 'inactive'

To avoid this potential issue, this patch adds wmb() barrier prior to
invoke EOI operation, this can make sure the interrupt line is
de-asserted in peripheral before deactivating interrupt in GIC.  At the
end, this can avoid spurious interrupt.

Reported-by: Orito Takao <orito.takao@socionext.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
---
 drivers/irqchip/irq-gic-v3.c | 4 ++++
 drivers/irqchip/irq-gic.c    | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
index e0f4debe64e1..fcd1477e2274 100644
--- a/drivers/irqchip/irq-gic-v3.c
+++ b/drivers/irqchip/irq-gic-v3.c
@@ -530,6 +530,8 @@ static void gic_irq_nmi_teardown(struct irq_data *d)
 
 static void gic_eoi_irq(struct irq_data *d)
 {
+	/* Ensure the data is written to peripheral */
+	wmb();
 	gic_write_eoir(gic_irq(d));
 }
 
@@ -541,6 +543,8 @@ static void gic_eoimode1_eoi_irq(struct irq_data *d)
 	 */
 	if (gic_irq(d) >= 8192 || irqd_is_forwarded_to_vcpu(d))
 		return;
+	/* Ensure the data is written to peripheral */
+	wmb();
 	gic_write_dir(gic_irq(d));
 }
 
diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
index d329ec3d64d8..3e9d4b6dc31b 100644
--- a/drivers/irqchip/irq-gic.c
+++ b/drivers/irqchip/irq-gic.c
@@ -222,6 +222,8 @@ static void gic_eoi_irq(struct irq_data *d)
 	if (hwirq < 16)
 		hwirq = this_cpu_read(sgi_intid);
 
+	/* Ensure the data is written to peripheral */
+	wmb();
 	writel_relaxed(hwirq, gic_cpu_base(d) + GIC_CPU_EOI);
 }
 
@@ -236,6 +238,8 @@ static void gic_eoimode1_eoi_irq(struct irq_data *d)
 	if (hwirq < 16)
 		hwirq = this_cpu_read(sgi_intid);
 
+	/* Ensure the data is written to peripheral */
+	wmb();
 	writel_relaxed(hwirq, gic_cpu_base(d) + GIC_CPU_DEACTIVATE);
 }
 
-- 
2.25.1