genirq: reenable a nobody cared disabled irq when a new driver arrives

Uwe Kleine-Koenig has some strange hardware where one of the shared
interrupts can be asserted during boot before the appropriate driver
loads. Requesting the shared irq line from another driver result in a
spurious interrupt storm which finally disables the interrupt line.

I have seen similar behaviour on resume before (the hardware does not
work anymore so I can not verify).

Change the spurious disable logic to increment the disable depth and
mark the interrupt with an extra flag which allows us to reenable the
interrupt when a new driver arrives which requests the same irq
line. In the worst case this will disable the irq again via the
spurious trap, but there is a decent chance that the new driver is the
one which can handle the already asserted interrupt and makes the box
usable again.

Eric Biederman said further: This case also happens on a regular basis
in kdump kernels where we deliberately don't shutdown the hardware
before starting the new kernel.  This patch should reduce the need for
using irqpoll in that situation by a small amount.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-and-Acked-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>

include/linux/irq.h

@@ -61,6 +61,7 @@ typedef	void (*irq_flow_handler_t)(unsigned int irq,
 #define IRQ_WAKEUP		0x00100000	/* IRQ triggers system wakeup */
 #define IRQ_MOVE_PENDING	0x00200000	/* need to re-target IRQ destination */
 #define IRQ_NO_BALANCING	0x00400000	/* IRQ is excluded from balancing */
+#define IRQ_SPURIOUS_DISABLED	0x00800000	/* IRQ was disabled by the spurious trap */
 
 #ifdef CONFIG_IRQ_PER_CPU
 # define CHECK_IRQ_PER_CPU(var) ((var) & IRQ_PER_CPU)