ARM: uniphier: insert dsb barrier to ensure visibility of store

I noticed secondary CPUs sometimes fail to wake up, and the root
cause is that the sev instruction wakes up slave CPUs before the
preceding the register write is observed by them.

The read-back of the accessed register does not guarantee the order.
In order to ensure the order between the register write and the sev
instruction, a dsb instruction should be executed prior to the sev.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

arch/arm/mach-uniphier/arm64/smp_kick_cpus.c

@@ -21,11 +21,11 @@ void uniphier_smp_kick_all_cpus(void)
 	rom_boot_rsv0 = map_sysmem(UNIPHIER_SMPCTRL_ROM_RSV0, SZ_8);
 
 	writeq((u64)uniphier_secondary_startup, rom_boot_rsv0);
-	readq(rom_boot_rsv0);	/* relax */
 
 	unmap_sysmem(rom_boot_rsv0);
 
 	uniphier_smp_setup();
 
-	asm("sev"); /* Bring up all secondary CPUs from Boot ROM into U-Boot */
+	asm("dsb	ishst\n" /* Ensure the write to ROM_RSV0 is visible */
+	    "sev"); /* Bring up all secondary CPUs from Boot ROM into U-Boot */
 }