acpi: allow usage of acpi_tad on HW-reduced platforms

The specification [1] allows so-called HW-reduced platforms,
which do not implement everything, especially the wakeup related stuff.

In that case, it is still usable as a RTC. This is helpful for [2]
and [3], which is about a device with no other working RTC,
but it does have an HW-reduced TAD, which can be used as a RTC instead.

[1]: https://uefi.org/specs/ACPI/6.5/09_ACPI_Defined_Devices_and_Device_Specific_Objects.html#time-and-alarm-device
[2]: https://bugzilla.kernel.org/show_bug.cgi?id=212313
[3]: linux-surface/linux-surface#415

Signed-off-by: Bart Groeneveld | GPX Solutions B.V. <bart@gpxbv.nl>
Patchset: rtc

drivers/acpi/acpi_tad.c

@@ -432,6 +432,14 @@ static ssize_t caps_show(struct device *dev, struct device_attribute *attr,
 
 static DEVICE_ATTR_RO(caps);
 
+static struct attribute *acpi_tad_attrs[] = {
+	&dev_attr_caps.attr,
+	NULL,
+};
+static const struct attribute_group acpi_tad_attr_group = {
+	.attrs	= acpi_tad_attrs,
+};
+
 static ssize_t ac_alarm_store(struct device *dev, struct device_attribute *attr,
 			      const char *buf, size_t count)
 {
@@ -480,15 +488,14 @@ static ssize_t ac_status_show(struct device *dev, struct device_attribute *attr,
 
 static DEVICE_ATTR_RW(ac_status);
 
-static struct attribute *acpi_tad_attrs[] = {
-	&dev_attr_caps.attr,
+static struct attribute *acpi_tad_ac_attrs[] = {
 	&dev_attr_ac_alarm.attr,
 	&dev_attr_ac_policy.attr,
 	&dev_attr_ac_status.attr,
 	NULL,
 };
-static const struct attribute_group acpi_tad_attr_group = {
-	.attrs	= acpi_tad_attrs,
+static const struct attribute_group acpi_tad_ac_attr_group = {
+	.attrs	= acpi_tad_ac_attrs,
 };
 
 static ssize_t dc_alarm_store(struct device *dev, struct device_attribute *attr,
@@ -564,13 +571,18 @@ static int acpi_tad_remove(struct platform_device *pdev)
 
 	pm_runtime_get_sync(dev);
 
+	if (dd->capabilities & ACPI_TAD_AC_WAKE)
+		sysfs_remove_group(&dev->kobj, &acpi_tad_ac_attr_group);
+
 	if (dd->capabilities & ACPI_TAD_DC_WAKE)
 		sysfs_remove_group(&dev->kobj, &acpi_tad_dc_attr_group);
 
 	sysfs_remove_group(&dev->kobj, &acpi_tad_attr_group);
 
-	acpi_tad_disable_timer(dev, ACPI_TAD_AC_TIMER);
-	acpi_tad_clear_status(dev, ACPI_TAD_AC_TIMER);
+	if (dd->capabilities & ACPI_TAD_AC_WAKE) {
+		acpi_tad_disable_timer(dev, ACPI_TAD_AC_TIMER);
+		acpi_tad_clear_status(dev, ACPI_TAD_AC_TIMER);
+	}
 	if (dd->capabilities & ACPI_TAD_DC_WAKE) {
 		acpi_tad_disable_timer(dev, ACPI_TAD_DC_TIMER);
 		acpi_tad_clear_status(dev, ACPI_TAD_DC_TIMER);
@@ -613,12 +625,6 @@ static int acpi_tad_probe(struct platform_device *pdev)
 		goto remove_handler;
 	}
 
-	if (!acpi_has_method(handle, "_PRW")) {
-		dev_info(dev, "Missing _PRW\n");
-		ret = -ENODEV;
-		goto remove_handler;
-	}
-
 	dd = devm_kzalloc(dev, sizeof(*dd), GFP_KERNEL);
 	if (!dd) {
 		ret = -ENOMEM;
@@ -649,6 +655,12 @@ static int acpi_tad_probe(struct platform_device *pdev)
 	if (ret)
 		goto fail;
 
+	if (caps & ACPI_TAD_AC_WAKE) {
+		ret = sysfs_create_group(&dev->kobj, &acpi_tad_ac_attr_group);
+		if (ret)
+			goto fail;
+	}
+
 	if (caps & ACPI_TAD_DC_WAKE) {
 		ret = sysfs_create_group(&dev->kobj, &acpi_tad_dc_attr_group);
 		if (ret)