ARM: GIC: fix GIC cpumask initialization

Punit Agrawal reports:
> I was trying to boot 3.8-rc5 on Realview EB 11MPCore using
> realview-smp_defconfig as a starting point but the kernel failed to
> progress past the log below (config attached).
>
> Pawel suggested I try reverting 384a290283f - "ARM: gic: use a private
> mapping for CPU target interfaces" that you've authored. With this
> commit reverted the kernel boots.
>
> I am not quite sure why the commit breaks 11MPCore but Pawel (cc'd)
> might be able to shed light on that.

Some early GIC implementations return zero for the first distributor
CPU routing register.  This means we can't rely on that telling us
which CPU interface we're connected to.  We know that these platforms
implement PPIs for IRQs 29-31 - but we shouldn't assume that these
will always be populated.

So, instead, scan for a non-zero CPU routing register in the first
32 IRQs and use that as our CPU mask.

Reported-by: Punit Agrawal <punit.agrawal@arm.com>
Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/common/gic.c

@@ -351,6 +351,25 @@ void __init gic_cascade_irq(unsigned int gic_nr, unsigned int irq)
 	irq_set_chained_handler(irq, gic_handle_cascade_irq);
 }
 
+static u8 gic_get_cpumask(struct gic_chip_data *gic)
+{
+	void __iomem *base = gic_data_dist_base(gic);
+	u32 mask, i;
+
+	for (i = mask = 0; i < 32; i += 4) {
+		mask = readl_relaxed(base + GIC_DIST_TARGET + i);
+		mask |= mask >> 16;
+		mask |= mask >> 8;
+		if (mask)
+			break;
+	}
+
+	if (!mask)
+		pr_crit("GIC CPU mask not found - kernel will fail to boot.\n");
+
+	return mask;
+}
+
 static void __init gic_dist_init(struct gic_chip_data *gic)
 {
 	unsigned int i;
@@ -369,7 +388,9 @@ static void __init gic_dist_init(struct gic_chip_data *gic)
 	/*
 	 * Set all global interrupts to this CPU only.
 	 */
-	cpumask = readl_relaxed(base + GIC_DIST_TARGET + 0);
+	cpumask = gic_get_cpumask(gic);
+	cpumask |= cpumask << 8;
+	cpumask |= cpumask << 16;
 	for (i = 32; i < gic_irqs; i += 4)
 		writel_relaxed(cpumask, base + GIC_DIST_TARGET + i * 4 / 4);
 
@@ -400,7 +421,7 @@ static void __cpuinit gic_cpu_init(struct gic_chip_data *gic)
 	 * Get what the GIC says our CPU mask is.
 	 */
 	BUG_ON(cpu >= NR_GIC_CPU_IF);
-	cpu_mask = readl_relaxed(dist_base + GIC_DIST_TARGET + 0);
+	cpu_mask = gic_get_cpumask(gic);
 	gic_cpu_map[cpu] = cpu_mask;
 
 	/*