irqchip/gic: Make locking a BL_SWITCHER only feature

The BL switcher code manipulates the logical/physical CPU mapping,
forcing a lock to be taken on the IPI path. With an IPI heavy load,
this single lock becomes contended.

But when CONFIG_BL_SWITCHER is not enabled, there is no reason
to take this lock at all since the CPU mapping is immutable.

This patch allows the lock to be entierely removed when BL_SWITCHER
is not enabled (which is the case in most configurations), leading
to a small improvement of "perf bench sched pipe" (measured on
an 8 core AMD Seattle system):

Before: 101370 ops/sec
After: 103680 ops/sec

Take this opportunity to remove a useless lock being taken when
handling an interrupt on a secondary GIC.

Signed-off-by: Marc Zyngier <[e-mail]>

drivers/irqchip/irq-gic.c

···
 #endif
 };
 
-static DEFINE_RAW_SPINLOCK(irq_controller_lock);
+#ifdef CONFIG_BL_SWITCHER
+
+static DEFINE_RAW_SPINLOCK(cpu_map_lock);
+
+#define gic_lock_irqsave(f)		\
+	raw_spin_lock_irqsave(&cpu_map_lock, (f))
+#define gic_unlock_irqrestore(f)	\
+	raw_spin_unlock_irqrestore(&cpu_map_lock, (f))
+
+#define gic_lock()			raw_spin_lock(&cpu_map_lock)
+#define gic_unlock()			raw_spin_unlock(&cpu_map_lock)
+
+#else
+
+#define gic_lock_irqsave(f)		do { (void)(f); } while(0)
+#define gic_unlock_irqrestore(f)	do { (void)(f); } while(0)
+
+#define gic_lock()			do { } while(0)
+#define gic_unlock()			do { } while(0)
+
+#endif
 
 /*
  * The GIC mapping of CPU interfaces does not necessarily match
···
 	if (cpu >= NR_GIC_CPU_IF || cpu >= nr_cpu_ids)
 		return -EINVAL;
 
-	raw_spin_lock_irqsave(&irq_controller_lock, flags);
+	gic_lock_irqsave(flags);
 	mask = 0xff << shift;
 	bit = gic_cpu_map[cpu] << shift;
 	val = readl_relaxed(reg) & ~mask;
 	writel_relaxed(val | bit, reg);
-	raw_spin_unlock_irqrestore(&irq_controller_lock, flags);
+	gic_unlock_irqrestore(flags);
 
 	return IRQ_SET_MASK_OK_DONE;
 }
···
 
 	chained_irq_enter(chip, desc);
 
-	raw_spin_lock(&irq_controller_lock);
 	status = readl_relaxed(gic_data_cpu_base(chip_data) + GIC_CPU_INTACK);
-	raw_spin_unlock(&irq_controller_lock);
 
 	gic_irq = (status & GICC_IAR_INT_ID_MASK);
 	if (gic_irq == GICC_INT_SPURIOUS)
···
 		return;
 	}
 
-	raw_spin_lock_irqsave(&irq_controller_lock, flags);
+	gic_lock_irqsave(flags);
 
 	/* Convert our logical CPU mask into a physical one. */
 	for_each_cpu(cpu, mask)
···
 	/* this always happens on GIC0 */
 	writel_relaxed(map << 16 | irq, gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
 
-	raw_spin_unlock_irqrestore(&irq_controller_lock, flags);
+	gic_unlock_irqrestore(flags);
 }
 #endif
 
···
 	cur_target_mask = 0x01010101 << cur_cpu_id;
 	ror_val = (cur_cpu_id - new_cpu_id) & 31;
 
-	raw_spin_lock(&irq_controller_lock);
+	gic_lock();
 
 	/* Update the target interface for this logical CPU */
 	gic_cpu_map[cpu] = 1 << new_cpu_id;
···
 		}
 	}
 
-	raw_spin_unlock(&irq_controller_lock);
+	gic_unlock();
 
 	/*
 	 * Now let's migrate and clear any potential SGIs that might be