powerpc: Add optional smp_ops->prepare_cpu SMP callback

Some platforms (will) need to perform allocations before bringing
a new CPU online. Doing it from smp_ops->setup_cpu is the wrong
thing to do:

 - It has no useful failure path (too late)
 - Calling any allocator will enable interrupts prematurely
   causing problems with large decrementer among others

Instead, add a new callback that is called from __cpu_up (so from
the context trying to online the new CPU) at a point where we
can safely allocate and handle failures.

This will be used by XIVE support.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/smp.h

@@ -44,6 +44,7 @@ struct smp_ops_t {
 #endif
 	void  (*probe)(void);
 	int   (*kick_cpu)(int nr);
+	int   (*prepare_cpu)(int nr);
 	void  (*setup_cpu)(int nr);
 	void  (*bringup_done)(void);
 	void  (*take_timebase)(void);

arch/powerpc/kernel/smp.c

@@ -521,6 +521,16 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
 
 	cpu_idle_thread_init(cpu, tidle);
 
+	/*
+	 * The platform might need to allocate resources prior to bringing
+	 * up the CPU
+	 */
+	if (smp_ops->prepare_cpu) {
+		rc = smp_ops->prepare_cpu(cpu);
+		if (rc)
+			return rc;
+	}
+
 	/* Make sure callin-map entry is 0 (can be leftover a CPU
 	 * hotplug
 	 */