diff --git a/plat/arm/css/common/aarch64/css_helpers.S b/plat/arm/css/common/aarch64/css_helpers.S
index d170270..2747618 100644
--- a/plat/arm/css/common/aarch64/css_helpers.S
+++ b/plat/arm/css/common/aarch64/css_helpers.S
@@ -37,19 +37,36 @@
 	.globl	css_calc_core_pos_swap_cluster
 	.weak	plat_is_my_cpu_primary
 
-	/* -----------------------------------------------------
-	 * void plat_secondary_cold_boot_setup (void);
+	/* ---------------------------------------------------------------------
+	 * void plat_secondary_cold_boot_setup(void);
 	 *
-	 * This function performs any platform specific actions
-	 * needed for a secondary cpu after a cold reset e.g
-	 * mark the cpu's presence, mechanism to place it in a
-	 * holding pen etc.
-	 * -----------------------------------------------------
+	 * In the normal boot flow, cold-booting secondary CPUs is not yet
+	 * implemented and they panic.
+	 *
+	 * When booting an EL3 payload, secondary CPUs are placed in a holding
+	 * pen, waiting for their mailbox to be populated. Note that all CPUs
+	 * share the same mailbox ; therefore, populating it will release all
+	 * CPUs from their holding pen. If finer-grained control is needed then
+	 * this should be handled in the code that secondary CPUs jump to.
+	 * ---------------------------------------------------------------------
 	 */
 func plat_secondary_cold_boot_setup
-	/* todo: Implement secondary CPU cold boot setup on CSS platforms */
+#ifndef EL3_PAYLOAD_BASE
+	/* TODO: Implement secondary CPU cold boot setup on CSS platforms */
 cb_panic:
 	b	cb_panic
+#else
+	mov_imm	x0, PLAT_ARM_TRUSTED_MAILBOX_BASE
+
+	/* Wait until the mailbox gets populated */
+poll_mailbox:
+	ldr	x1, [x0]
+	cbz	x1, 1f
+	br	x1
+1:
+	wfe
+	b	poll_mailbox
+#endif /* EL3_PAYLOAD_BASE */
 endfunc plat_secondary_cold_boot_setup
 
 	/* ---------------------------------------------------------------------