ACPI: reboot: Avoid racing after writing to ACPI RESET_REG

According to the ACPI spec, "The system must reset immediately following
the write to the ACPI RESET_REG register.", but there are cases that the
system does not follow this and results in racing with the subsequetial
reboot mechanism, which brings unexpected behavior.

Fix this by adding a 15ms delay after writing to the ACPI RESET_REG.
Reported-by: Ghorai, Sukumar <sukumar.ghorai@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
[ rjw: Edit comment in the code and subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/reboot.c

@@ -3,6 +3,7 @@
 #include <linux/pci.h>
 #include <linux/acpi.h>
 #include <acpi/reboot.h>
+#include <linux/delay.h>
 
 #ifdef CONFIG_PCI
 static void acpi_pci_reboot(struct acpi_generic_address *rr, u8 reset_value)
@@ -66,4 +67,14 @@ void acpi_reboot(void)
 		acpi_reset();
 		break;
 	}
+
+	/*
+	 * Some platforms do not shut down immediately after writing to the
+	 * ACPI reset register, and this results in racing with the
+	 * subsequent reboot mechanism.
+	 *
+	 * The 15ms delay has been found to be long enough for the system
+	 * to reboot on the affected platforms.
+	 */
+	mdelay(15);
 }