genirq: Prevent spurious detection for unconditionally polled interrupts

On a 68k platform a couple of interrupts are demultiplexed and
"polled" from a top level interrupt. Unfortunately there is no way to
determine which of the sub interrupts raised the top level interrupt,
so all of the demultiplexed interrupt handlers need to be
invoked. Given a high enough frequency this can trigger the spurious
interrupt detection mechanism, if one of the demultiplex interrupts
returns IRQ_NONE continuously. But this is a false positive as the
polling causes this behaviour and not buggy hardware/software.

Introduce IRQ_POLLED which can be set at interrupt chip setup time via
irq_set_status_flags(). The flag excludes the interrupt from the
spurious detector and from all core polling activities.

Reported-and-tested-by: Michael Schmitz <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]

include/linux/irq.h

···
  * IRQ_MOVE_PCNTXT		- Interrupt can be migrated from process context
  * IRQ_NESTED_TRHEAD		- Interrupt nests into another thread
  * IRQ_PER_CPU_DEVID		- Dev_id is a per-cpu variable
+ * IRQ_IS_POLLED		- Always polled by another interrupt. Exclude
+ *				  it from the spurious interrupt detection
+ *				  mechanism and from core side polling.
  */
 enum {
 	IRQ_TYPE_NONE		= 0x00000000,
···
 	IRQ_NESTED_THREAD	= (1 << 15),
 	IRQ_NOTHREAD		= (1 << 16),
 	IRQ_PER_CPU_DEVID	= (1 << 17),
+	IRQ_IS_POLLED		= (1 << 18),
 };
 
 #define IRQF_MODIFY_MASK	\
 	(IRQ_TYPE_SENSE_MASK | IRQ_NOPROBE | IRQ_NOREQUEST | \
 	 IRQ_NOAUTOEN | IRQ_MOVE_PCNTXT | IRQ_LEVEL | IRQ_NO_BALANCING | \
-	 IRQ_PER_CPU | IRQ_NESTED_THREAD | IRQ_NOTHREAD | IRQ_PER_CPU_DEVID)
+	 IRQ_PER_CPU | IRQ_NESTED_THREAD | IRQ_NOTHREAD | IRQ_PER_CPU_DEVID | \
+	 IRQ_IS_POLLED)
 
 #define IRQ_NO_BALANCING_MASK	(IRQ_PER_CPU | IRQ_NO_BALANCING)
 

kernel/irq/settings.h

···
 	_IRQ_NO_BALANCING	= IRQ_NO_BALANCING,
 	_IRQ_NESTED_THREAD	= IRQ_NESTED_THREAD,
 	_IRQ_PER_CPU_DEVID	= IRQ_PER_CPU_DEVID,
+	_IRQ_IS_POLLED		= IRQ_IS_POLLED,
 	_IRQF_MODIFY_MASK	= IRQF_MODIFY_MASK,
 };
 
···
 #define IRQ_NOAUTOEN		GOT_YOU_MORON
 #define IRQ_NESTED_THREAD	GOT_YOU_MORON
 #define IRQ_PER_CPU_DEVID	GOT_YOU_MORON
+#define IRQ_IS_POLLED		GOT_YOU_MORON
 #undef IRQF_MODIFY_MASK
 #define IRQF_MODIFY_MASK	GOT_YOU_MORON
 
···
 static inline bool irq_settings_is_nested_thread(struct irq_desc *desc)
 {
 	return desc->status_use_accessors & _IRQ_NESTED_THREAD;
+}
+
+static inline bool irq_settings_is_polled(struct irq_desc *desc)
+{
+	return desc->status_use_accessors & _IRQ_IS_POLLED;
 }

kernel/irq/spurious.c

···
 
 	raw_spin_lock(&desc->lock);
 
-	/* PER_CPU and nested thread interrupts are never polled */
-	if (irq_settings_is_per_cpu(desc) || irq_settings_is_nested_thread(desc))
+	/*
+	 * PER_CPU, nested thread interrupts and interrupts explicitely
+	 * marked polled are excluded from polling.
+	 */
+	if (irq_settings_is_per_cpu(desc) ||
+	    irq_settings_is_nested_thread(desc) ||
+	    irq_settings_is_polled(desc))
 		goto out;
 
 	/*
···
 void note_interrupt(unsigned int irq, struct irq_desc *desc,
 		    irqreturn_t action_ret)
 {
-	if (desc->istate & IRQS_POLL_INPROGRESS)
+	if (desc->istate & IRQS_POLL_INPROGRESS ||
+	    irq_settings_is_polled(desc))
 		return;
 
 	/* we get here again via the threaded handler */