lantiq: fix spurious irq storm

Since the MIPS IRQ stack patches, lantiq devices were emitting a storm
of messages like this:

[  567.872172] Spurious IRQ: CAUSE=0x1100c300

Fix this by reworking the IRQ dispatch code

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/lantiq/patches-4.4/0050-MIPS-Lantiq-Fix-cascaded-IRQ-setup.patch

@@ -0,0 +1,87 @@
+From: Felix Fietkau <nbd@nbd.name>
+Date: Thu, 19 Jan 2017 12:14:44 +0100
+Subject: [PATCH] MIPS: Lantiq: Fix cascaded IRQ setup
+
+With the IRQ stack changes integrated, the XRX200 devices started
+emitting a constant stream of kernel messages like this:
+
+[  565.415310] Spurious IRQ: CAUSE=0x1100c300
+
+This appears to be caused by IP0 firing for some reason without being
+handled. Fix this by setting up IP2-6 as a proper chained IRQ handler and
+calling do_IRQ for all MIPS CPU interrupts.
+
+Cc: john@phrozen.org
+Cc: stable@vger.kernel.org
+Signed-off-by: Felix Fietkau <nbd@nbd.name>
+---
+
+--- a/arch/mips/lantiq/irq.c
++++ b/arch/mips/lantiq/irq.c
+@@ -271,6 +271,11 @@ static void ltq_hw5_irqdispatch(void)
+ DEFINE_HWx_IRQDISPATCH(5)
+ #endif
+ 
++static void ltq_hw_irq_handler(struct irq_desc *desc)
++{
++	ltq_hw_irqdispatch(irq_desc_get_irq(desc) - 2);
++}
++
+ #ifdef CONFIG_MIPS_MT_SMP
+ void __init arch_init_ipiirq(int irq, struct irqaction *action)
+ {
+@@ -315,23 +320,19 @@ static struct irqaction irq_call = {
+ asmlinkage void plat_irq_dispatch(void)
+ {
+ 	unsigned int pending = read_c0_status() & read_c0_cause() & ST0_IM;
+-	unsigned int i;
++	int irq;
+ 
+-	if ((MIPS_CPU_TIMER_IRQ == 7) && (pending & CAUSEF_IP7)) {
+-		do_IRQ(MIPS_CPU_TIMER_IRQ);
+-		goto out;
+-	} else {
+-		for (i = 0; i < MAX_IM; i++) {
+-			if (pending & (CAUSEF_IP2 << i)) {
+-				ltq_hw_irqdispatch(i);
+-				goto out;
+-			}
+-		}
++	if (!pending) {
++		spurious_interrupt();
++		return;
+ 	}
+-	pr_alert("Spurious IRQ: CAUSE=0x%08x\n", read_c0_status());
+ 
+-out:
+-	return;
++	pending >>= CAUSEB_IP;
++	while (pending) {
++		irq = fls(pending) - 1;
++		do_IRQ(MIPS_CPU_IRQ_BASE + irq);
++		pending &= ~BIT(irq);
++	}
+ }
+ 
+ static int icu_map(struct irq_domain *d, unsigned int irq, irq_hw_number_t hw)
+@@ -356,11 +357,6 @@ static const struct irq_domain_ops irq_d
+ 	.map = icu_map,
+ };
+ 
+-static struct irqaction cascade = {
+-	.handler = no_action,
+-	.name = "cascade",
+-};
+-
+ int __init icu_of_init(struct device_node *node, struct device_node *parent)
+ {
+ 	struct device_node *eiu_node;
+@@ -392,7 +388,7 @@ int __init icu_of_init(struct device_nod
+ 	mips_cpu_irq_init();
+ 
+ 	for (i = 0; i < MAX_IM; i++)
+-		setup_irq(i + 2, &cascade);
++		irq_set_chained_handler(i + 2, ltq_hw_irq_handler);
+ 
+ 	if (cpu_has_vint) {
+ 		pr_info("Setting up vectored interrupts\n");