rtc: efi: Fail probing if RTC reads don't work

While the EFI spec mandates an RTC, not every implementation actually adheres
to that rule (or can adhere to it - some systems just don't have an RTC).

For those, we really don't want to probe the EFI RTC driver at all, because if
we do we'd get a non-functional driver that does nothing useful but only spills
our kernel log with warnings.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-efi.c

@@ -259,6 +259,12 @@ static const struct rtc_class_ops efi_rtc_ops = {
 static int __init efi_rtc_probe(struct platform_device *dev)
 {
 	struct rtc_device *rtc;
+	efi_time_t eft;
+	efi_time_cap_t cap;
+
+	/* First check if the RTC is usable */
+	if (efi.get_time(&eft, &cap) != EFI_SUCCESS)
+		return -ENODEV;
 
 	rtc = devm_rtc_device_register(&dev->dev, "rtc-efi", &efi_rtc_ops,
 					THIS_MODULE);