irq_bcm2836: Send event when onlining sleeping cores

In order to reduce power consumption and bus traffic, it is sensible
for secondary cores to enter a low-power idle state when waiting to
be started. The wfe instruction causes a core to wait until an event
or interrupt arrives before continuing to the next instruction.
The sev instruction sends a wakeup event to the other cores, so call
it from bcm2836_smp_boot_secondary, the function that wakes up the
waiting cores during booting.

It is harmless to use this patch without the corresponding change
adding wfe to the ARMv7/ARMv8-32 stubs, but if the stubs are updated
and this patch is not applied then the other cores will sleep forever.

See: #1989

Signed-off-by: Phil Elwell <phil@raspberrypi.org>

drivers/irqchip/irq-bcm2836.c

@@ -248,6 +248,9 @@ static int __init bcm2836_smp_boot_secondary(unsigned int cpu,
 	writel(secondary_startup_phys,
 	       intc.base + LOCAL_MAILBOX3_SET0 + 16 * cpu);
 
+	dsb(sy); /* Ensure write has completed before waking the other CPUs */
+	sev();
+
 	return 0;
 }