rtc: pm8xxx: fix uefi offset lookup

On many Qualcomm platforms the PMIC RTC control and time registers are
read-only so that the RTC time can not be updated. Instead an offset
needs be stored in some machine-specific non-volatile memory, which a
driver can take into account.

On platforms where the offset is stored in a Qualcomm specific UEFI
variable the variables are also accessed in a non-standard way, which
means that the OS cannot assume that the variable service is available
by the time the driver probes.

Use the new 'qcom,uefi-rtc-info' property to determine whether to probe
defer until the UEFI offset becomes available so that the offset can be
used also when the RTC driver is built in or when a dependency of the
UEFI variable driver is built as a module (e.g. the driver for the SCM
interconnects).

Fixes: bba38b8 ("rtc: pm8xxx: add support for uefi offset")
Reported-by: Rob Clark <robdclark@gmail.com>
Link: https://lore.kernel.org/lkml/CAF6AEGsfke=x0p1b2-uNX6DuQfRyEjVbJaxTbVLDT2YvSkGJbg@mail.gmail.com/
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20250423075143.11157-3-johan+linaro@kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: jhovold

drivers/rtc/rtc-pm8xxx.c

@@ -576,13 +576,20 @@ static int pm8xxx_rtc_probe_offset(struct pm8xxx_rtc *rtc_dd)
 	}
 
 	/* Use UEFI storage as fallback if available */
-	if (efivar_is_available()) {
-		rc = pm8xxx_rtc_read_uefi_offset(rtc_dd);
-		if (rc == 0)
-			rtc_dd->use_uefi = true;
+	rtc_dd->use_uefi = of_property_read_bool(rtc_dd->dev->of_node,
+						 "qcom,uefi-rtc-info");
+	if (!rtc_dd->use_uefi)
+		return 0;
+
+	if (!efivar_is_available()) {
+		if (IS_ENABLED(CONFIG_EFI))
+			return -EPROBE_DEFER;
+
+		dev_warn(rtc_dd->dev, "efivars not available\n");
+		rtc_dd->use_uefi = false;
 	}
 
-	return 0;
+	return pm8xxx_rtc_read_uefi_offset(rtc_dd);
 }
 
 static int pm8xxx_rtc_probe(struct platform_device *pdev)