irqchip: gicv3-its: Support safe initialization

It's unsafe to change the configurations of an activated ITS directly
since this will lead to unpredictable results. This patch guarantees
the ITSes being initialized are quiescent.
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NYun Wu <wuyun.wu@huawei.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1425659870-11832-12-git-send-email-marc.zyngier@arm.comSigned-off-by: NJason Cooper <jason@lakedaemon.net>

drivers/irqchip/irq-gic-v3-its.c

@@ -1320,6 +1320,34 @@ static const struct irq_domain_ops its_domain_ops = {
 	.deactivate		= its_irq_domain_deactivate,
 };
 
+static int its_force_quiescent(void __iomem *base)
+{
+	u32 count = 1000000;	/* 1s */
+	u32 val;
+
+	val = readl_relaxed(base + GITS_CTLR);
+	if (val & GITS_CTLR_QUIESCENT)
+		return 0;
+
+	/* Disable the generation of all interrupts to this ITS */
+	val &= ~GITS_CTLR_ENABLE;
+	writel_relaxed(val, base + GITS_CTLR);
+
+	/* Poll GITS_CTLR and wait until ITS becomes quiescent */
+	while (1) {
+		val = readl_relaxed(base + GITS_CTLR);
+		if (val & GITS_CTLR_QUIESCENT)
+			return 0;
+
+		count--;
+		if (!count)
+			return -EBUSY;
+
+		cpu_relax();
+		udelay(1);
+	}
+}
+
 static int its_probe(struct device_node *node, struct irq_domain *parent)
 {
 	struct resource res;
@@ -1348,6 +1376,13 @@ static int its_probe(struct device_node *node, struct irq_domain *parent)
 		goto out_unmap;
 	}
 
+	err = its_force_quiescent(its_base);
+	if (err) {
+		pr_warn("%s: failed to quiesce, giving up\n",
+			node->full_name);
+		goto out_unmap;
+	}
+
 	pr_info("ITS: %s\n", node->full_name);
 
 	its = kzalloc(sizeof(*its), GFP_KERNEL);