[PATCH] MIPS: Fix broken malta qemu

[Qais Yousef <qsyousef@gmail.com>]

Malta defconfig compiles with GIC on. Hence when compiling for SMP it causes the
new IPI code to be activated. But on qemu malta there's no GIC causing a
BUG_ON(!ipidomain) to be hit in mips_smp_ipi_init().

Since in that configuration one can only run a single core SMP (!), skip IPI
initialisation if we detect that this is the case. It is a sensible behaviour
to introduce and should keep such possible configuration to run rather than die
hard unnecessarily.

Signed-off-by: Qais Yousef <qsyousef@gmail.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
---
 arch/mips/kernel/smp.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/mips/kernel/smp.c b/arch/mips/kernel/smp.c
index 37708d9..27cb638 100644
--- a/arch/mips/kernel/smp.c
+++ b/arch/mips/kernel/smp.c
@@ -243,6 +243,18 @@ static int __init mips_smp_ipi_init(void)
 	struct irq_domain *ipidomain;
 	struct device_node *node;
 
+	/*
+	 * In some cases like qemu-malta, it is desired to try SMP with
+	 * a single core. Qemu-malta has no GIC, so an attempt to set any IPIs
+	 * would cause a BUG_ON() to be triggered since there's no ipidomain.
+	 *
+	 * Since for a single core system IPIs aren't required really, skip the
+	 * initialisation which should generally keep any such configurations
+	 * happy and only fail hard when trying to truely run SMP.
+	 */
+	if (cpumask_weight(cpu_possible_mask) == 1)
+		return 0;
+
 	node = of_irq_find_parent(of_root);
 	ipidomain = irq_find_matching_host(node, DOMAIN_BUS_IPI);
 
-- 
2.7.3