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>

2 files changed, 11 insertions, 0 deletions

diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
index 32db16d2e7ad..2f8e36f91acd 100644
--- a/arch/powerpc/include/asm/smp.h
+++ b/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);
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 46f89e66a273..b12f5f0a408f 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/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
          */