fix(psci): tighten psci_power_down_wfi behaviour

A processing element should never return from a wfi, however, due to a
hardware bug, certain CPUs may wake up because of an external event.
This patch tightens the behaviour of the common power down sequence, it
ensures the routine never returns by entering a wfi loop at its end. It
aligns with the behaviour of the platform implementations.

Change-Id: I36d8b0c64eccb71035bf164b4cd658d66ed7beb4
Signed-off-by: Harrison Mutai <[email protected]>

Author: harrisonmutai-arm

lib/psci/aarch32/psci_helpers.S

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2023, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -143,6 +143,7 @@ endfunc do_stack_maintenance
  */
 func psci_power_down_wfi
 	dsb	sy		// ensure write buffer empty
+1:
 	wfi
-	no_ret	plat_panic_handler
+	b	1b
 endfunc psci_power_down_wfi

lib/psci/aarch64/psci_helpers.S

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2014-2023, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -125,6 +125,7 @@ endfunc psci_do_pwrup_cache_maintenance
  */
 func psci_power_down_wfi
 	dsb	sy		// ensure write buffer empty
+1:
 	wfi
-	no_ret	plat_panic_handler
+	b	1b
 endfunc psci_power_down_wfi