metag: smp: don't spin waiting for CPU to start

Use a completion to block until a secondary CPU has started up, like ARM
do, instead of a loop of udelays.

On Meta, SMP is really SMT, with each "CPU" being a different hardware
thread on the same Meta processor core, so as well as being more
efficient and latency friendly, using a completion prevents the bogomips
of the secondary CPU from being drastically skewed every time by the
execution of the tight in-cache udelay loop on the other CPU.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>

arch/metag/kernel/smp.c

@@ -8,6 +8,7 @@
  * published by the Free Software Foundation.
  */
 #include <linux/atomic.h>
+#include <linux/completion.h>
 #include <linux/delay.h>
 #include <linux/init.h>
 #include <linux/spinlock.h>
@@ -62,6 +63,8 @@ static DEFINE_PER_CPU(struct ipi_data, ipi_data) = {
 
 static DEFINE_SPINLOCK(boot_lock);
 
+static DECLARE_COMPLETION(cpu_running);
+
 /*
  * "thread" is assumed to be a valid Meta hardware thread ID.
  */
@@ -235,20 +238,12 @@ int __cpuinit __cpu_up(unsigned int cpu, struct task_struct *idle)
 	 */
 	ret = boot_secondary(thread, idle);
 	if (ret == 0) {
-		unsigned long timeout;
-
 		/*
 		 * CPU was successfully started, wait for it
 		 * to come online or time out.
 		 */
-		timeout = jiffies + HZ;
-		while (time_before(jiffies, timeout)) {
-			if (cpu_online(cpu))
-				break;
-
-			udelay(10);
-			barrier();
-		}
+		wait_for_completion_timeout(&cpu_running,
+					    msecs_to_jiffies(1000));
 
 		if (!cpu_online(cpu))
 			ret = -EIO;
@@ -391,6 +386,7 @@ asmlinkage void secondary_start_kernel(void)
 	 * OK, now it's safe to let the boot CPU continue
 	 */
 	set_cpu_online(cpu, true);
+	complete(&cpu_running);
 
 	/*
 	 * Enable local interrupts.