irqchip/gic: Atomically update affinity

The GIC driver uses a RMW sequence to update the affinity, and
relies on the gic_lock_irqsave/gic_unlock_irqrestore sequences
to update it atomically.

But these sequences only expand into anything meaningful if
the BL_SWITCHER option is selected, which almost never happens.

It also turns out that using a RMW and locks is just as silly,
as the GIC distributor supports byte accesses for the GICD_TARGETRn
registers, which when used make the update atomic by definition.

Drop the terminally broken code and replace it by a byte write.

Fixes: 04c8b0f8 ("irqchip/gic: Make locking a BL_SWITCHER only feature")
Cc: stable@vger.kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

drivers/irqchip/irq-gic.c

@@ -329,10 +329,8 @@ static int gic_irq_set_vcpu_affinity(struct irq_data *d, void *vcpu)
 static int gic_set_affinity(struct irq_data *d, const struct cpumask *mask_val,
 			    bool force)
 {
-	void __iomem *reg = gic_dist_base(d) + GIC_DIST_TARGET + (gic_irq(d) & ~3);
-	unsigned int cpu, shift = (gic_irq(d) % 4) * 8;
-	u32 val, mask, bit;
-	unsigned long flags;
+	void __iomem *reg = gic_dist_base(d) + GIC_DIST_TARGET + gic_irq(d);
+	unsigned int cpu;
 
 	if (!force)
 		cpu = cpumask_any_and(mask_val, cpu_online_mask);
@@ -342,13 +340,7 @@ static int gic_set_affinity(struct irq_data *d, const struct cpumask *mask_val,
 	if (cpu >= NR_GIC_CPU_IF || cpu >= nr_cpu_ids)
 		return -EINVAL;
 
-	gic_lock_irqsave(flags);
-	mask = 0xff << shift;
-	bit = gic_cpu_map[cpu] << shift;
-	val = readl_relaxed(reg) & ~mask;
-	writel_relaxed(val | bit, reg);
-	gic_unlock_irqrestore(flags);
-
+	writeb_relaxed(gic_cpu_map[cpu], reg);
 	irq_data_update_effective_affinity(d, cpumask_of(cpu));
 
 	return IRQ_SET_MASK_OK_DONE;