x86/time: Honor ACPI FADT flag indicating absence of a CMOS RTC

Even though the omission was found only during code review
(originally in the Xen hypervisor, looking through ACPI v5 flags
and their meanings and uses), we shouldn't be creating a
corresponding platform device in that case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/5265029D02000078000FC4D2@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/rtc.c

@@ -192,6 +192,14 @@ static __init int add_rtc_cmos(void)
 	if (mrst_identify_cpu())
 		return -ENODEV;
 
+#ifdef CONFIG_ACPI
+	if (acpi_gbl_FADT.boot_flags & ACPI_FADT_NO_CMOS_RTC) {
+		/* This warning can likely go away again in a year or two. */
+		pr_info("ACPI: not registering RTC platform device\n");
+		return -ENODEV;
+	}
+#endif
+
 	platform_device_register(&rtc_device);
 	dev_info(&rtc_device.dev,
 		 "registered platform RTC device (no PNP device found)\n");