[nrf fromtree] Bluetooth: Controller: Add a DSB in sleep interface

Add a DSB before doing WFE in cpu_sleep(), to ensure the
memory transactions are complete.

Add a note clarifying a dependency for the existing solution
(dependency is satisfied by ARCH code but is good to state
clearly).

Relates to commit f1264b7 ("drivers: entropy: nrf5: add
docs and a DSB in get_entropy_isr").

Signed-off-by: Vinayak Kariappa Chettimada <[email protected]>
(cherry picked from commit 67fa62f)
Signed-off-by: Andrzej Głąbek <[email protected]>

Author: cvinayak

subsys/bluetooth/controller/ll_sw/nordic/hal/nrf5/cpu.h

@@ -11,6 +11,15 @@ static inline void cpu_sleep(void)
 	defined(CONFIG_CPU_CORTEX_M4) || \
 	defined(CONFIG_CPU_CORTEX_M33) || \
 	defined(CONFIG_SOC_SERIES_BSIM_NRFXX)
+	/*
+	 * To guarantee waking up from the event, the
+	 * SEV-On-Pend feature must be enabled (enabled
+	 * during ARCH initialization).
+	 *
+	 * DSB is recommended by spec before WFE (to
+	 * guarantee completion of memory transactions)
+	 */
+	__DSB();
 	__WFE();
 	__SEV();
 	__WFE();