cpufreq: powernv: Fix unsafe notifiers

The PowerNV cpufreq driver registers two notifiers: one to catch
throttle messages from the OCC and one to bump the CPU frequency back
to normal before a reboot. Both require the cpufreq driver to be
registered in order to function since the notifier callbacks use
various cpufreq_*() functions.

Right now we register both notifiers before we've initialised the
driver. This seems to work, but we should head off any protential
problems by registering the notifiers after the driver is initialised.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200206062622.28235-2-oohall@gmail.com

drivers/cpufreq/powernv-cpufreq.c

@@ -1114,9 +1114,6 @@ static int __init powernv_cpufreq_init(void)
 	if (rc)
 		goto out;
 
-	register_reboot_notifier(&powernv_cpufreq_reboot_nb);
-	opal_message_notifier_register(OPAL_MSG_OCC, &powernv_cpufreq_opal_nb);
-
 	if (powernv_pstate_info.wof_enabled)
 		powernv_cpufreq_driver.boost_enabled = true;
 	else
@@ -1125,15 +1122,17 @@ static int __init powernv_cpufreq_init(void)
 	rc = cpufreq_register_driver(&powernv_cpufreq_driver);
 	if (rc) {
 		pr_info("Failed to register the cpufreq driver (%d)\n", rc);
-		goto cleanup_notifiers;
+		goto cleanup;
 	}
 
 	if (powernv_pstate_info.wof_enabled)
 		cpufreq_enable_boost_support();
 
+	register_reboot_notifier(&powernv_cpufreq_reboot_nb);
+	opal_message_notifier_register(OPAL_MSG_OCC, &powernv_cpufreq_opal_nb);
+
 	return 0;
-cleanup_notifiers:
-	unregister_all_notifiers();
+cleanup:
 	clean_chip_info();
 out:
 	pr_info("Platform driver disabled. System does not support PState control\n");