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>

1 files changed, 8 insertions, 0 deletions

diff --git a/arch/x86/kernel/rtc.c b/arch/x86/kernel/rtc.c
index 0aa29394ed6f..5b9dd445eb89 100644
--- a/arch/x86/kernel/rtc.c
+++ b/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");