[PATCH v2] x86: honor ACPI FADT flag indicating absence of a CMOS RTC

["Jan Beulich" <JBeulich@suse.com>]

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>
---
v2: Emit a log message in this case, as requested by Ingo.
---
 arch/x86/kernel/rtc.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- 3.12-rc6/arch/x86/kernel/rtc.c
+++ 3.12-rc6-x86-ACPI-no-RTC/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");