MIPS: Fix broken malta qemu

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>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12892/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

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);