[ARM SMP] Add hotplug CPU infrastructure

This patch adds the infrastructure to support hotplug CPU on ARM
platforms.

Signed-off-by: Russell King <[e-mail]>

arch/arm/Kconfig

···
 	depends on SMP
 	default "4"
 
+config HOTPLUG_CPU
+	bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
+	depends on SMP && HOTPLUG && EXPERIMENTAL
+	help
+	  Say Y here to experiment with turning CPUs off and on.  CPUs
+	  can be controlled through /sys/devices/system/cpu.
+
 config PREEMPT
 	bool "Preemptible Kernel (EXPERIMENTAL)"
 	depends on EXPERIMENTAL

arch/arm/kernel/irq.c

···
 }
 
 __setup("noirqdebug", noirqdebug_setup);
+
+#ifdef CONFIG_HOTPLUG_CPU
+/*
+ * The CPU has been marked offline.  Migrate IRQs off this CPU.  If
+ * the affinity settings do not allow other CPUs, force them onto any
+ * available CPU.
+ */
+void migrate_irqs(void)
+{
+	unsigned int i, cpu = smp_processor_id();
+
+	for (i = 0; i < NR_IRQS; i++) {
+		struct irqdesc *desc = irq_desc + i;
+
+		if (desc->cpu == cpu) {
+			unsigned int newcpu = any_online_cpu(desc->affinity);
+
+			if (newcpu == NR_CPUS) {
+				if (printk_ratelimit())
+					printk(KERN_INFO "IRQ%u no longer affine to CPU%u\n",
+					       i, cpu);
+
+				cpus_setall(desc->affinity);
+				newcpu = any_online_cpu(desc->affinity);
+			}
+
+			route_irq(desc, i, newcpu);
+		}
+	}
+}
+#endif /* CONFIG_HOTPLUG_CPU */

arch/arm/kernel/process.c

···
 #include <linux/interrupt.h>
 #include <linux/kallsyms.h>
 #include <linux/init.h>
+#include <linux/cpu.h>
 
 #include <asm/system.h>
 #include <asm/io.h>
···
 	/* endless idle loop with no priority at all */
 	while (1) {
 		void (*idle)(void) = pm_idle;
+
+#ifdef CONFIG_HOTPLUG_CPU
+		if (cpu_is_offline(smp_processor_id())) {
+			leds_event(led_idle_start);
+			cpu_die();
+		}
+#endif
+
 		if (!idle)
 			idle = default_idle;
 		preempt_disable();

arch/arm/kernel/smp.c

···
 	return ret;
 }
 
+#ifdef CONFIG_HOTPLUG_CPU
+/*
+ * __cpu_disable runs on the processor to be shutdown.
+ */
+int __cpuexit __cpu_disable(void)
+{
+	unsigned int cpu = smp_processor_id();
+	struct task_struct *p;
+	int ret;
+
+	ret = mach_cpu_disable(cpu);
+	if (ret)
+		return ret;
+
+	/*
+	 * Take this CPU offline.  Once we clear this, we can't return,
+	 * and we must not schedule until we're ready to give up the cpu.
+	 */
+	cpu_clear(cpu, cpu_online_map);
+
+	/*
+	 * OK - migrate IRQs away from this CPU
+	 */
+	migrate_irqs();
+
+	/*
+	 * Flush user cache and TLB mappings, and then remove this CPU
+	 * from the vm mask set of all processes.
+	 */
+	flush_cache_all();
+	local_flush_tlb_all();
+
+	read_lock(&tasklist_lock);
+	for_each_process(p) {
+		if (p->mm)
+			cpu_clear(cpu, p->mm->cpu_vm_mask);
+	}
+	read_unlock(&tasklist_lock);
+
+	return 0;
+}
+
+/*
+ * called on the thread which is asking for a CPU to be shutdown -
+ * waits until shutdown has completed, or it is timed out.
+ */
+void __cpuexit __cpu_die(unsigned int cpu)
+{
+	if (!platform_cpu_kill(cpu))
+		printk("CPU%u: unable to kill\n", cpu);
+}
+
+/*
+ * Called from the idle thread for the CPU which has been shutdown.
+ *
+ * Note that we disable IRQs here, but do not re-enable them
+ * before returning to the caller. This is also the behaviour
+ * of the other hotplug-cpu capable cores, so presumably coming
+ * out of idle fixes this.
+ */
+void __cpuexit cpu_die(void)
+{
+	unsigned int cpu = smp_processor_id();
+
+	local_irq_disable();
+	idle_task_exit();
+
+	/*
+	 * actual CPU shutdown procedure is at least platform (if not
+	 * CPU) specific
+	 */
+	platform_cpu_die(cpu);
+
+	/*
+	 * Do not return to the idle loop - jump back to the secondary
+	 * cpu initialisation.  There's some initialisation which needs
+	 * to be repeated to undo the effects of taking the CPU offline.
+	 */
+	__asm__("mov	sp, %0\n"
+	"	b	secondary_start_kernel"
+		:
+		: "r" ((void *)current->thread_info + THREAD_SIZE - 8));
+}
+#endif /* CONFIG_HOTPLUG_CPU */
+
 /*
  * This is the secondary CPU boot entry.  We're using this CPUs
  * idle thread stack, but a set of temporary page tables.

include/asm-arm/irq.h

···
 struct pt_regs;
 int handle_IRQ_event(unsigned int, struct pt_regs *, struct irqaction *);
 
+extern void migrate_irqs(void);
 #endif
 

include/asm-arm/smp.h

···
 };
 extern struct secondary_data secondary_data;
 
+extern int __cpu_disable(void);
+extern int mach_cpu_disable(unsigned int cpu);
+
+extern void __cpu_die(unsigned int cpu);
+extern void cpu_die(void);
+
+extern void platform_cpu_die(unsigned int cpu);
+extern int platform_cpu_kill(unsigned int cpu);
+extern void platform_cpu_enable(unsigned int cpu);
+
 #endif /* ifndef __ASM_ARM_SMP_H */