ACPI: scan: Add function to fetch dependent of ACPI device

In some ACPI tables we encounter, devices use the _DEP method to assert
a dependence on other ACPI devices as opposed to the OpRegions that the
specification intends.

We need to be able to find those devices "from" the dependee, so add
a callback and a wrapper to walk over the acpi_dep_list and return
the dependent ACPI device.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/scan.c

@@ -2101,6 +2101,20 @@ static void acpi_bus_attach(struct acpi_device *device, bool first_pass)
 		device->handler->hotplug.notify_online(device);
 }
 
+static int acpi_dev_get_first_consumer_dev_cb(struct acpi_dep_data *dep, void *data)
+{
+	struct acpi_device *adev;
+
+	adev = acpi_bus_get_acpi_device(dep->consumer);
+	if (!adev)
+		/* If we don't find an adev then we want to continue parsing */
+		return 0;
+
+	*(struct acpi_device **)data = adev;
+
+	return 1;
+}
+
 static int acpi_scan_clear_dep(struct acpi_dep_data *dep, void *data)
 {
 	struct acpi_device *adev;
@@ -2164,6 +2178,27 @@ void acpi_dev_clear_dependencies(struct acpi_device *supplier)
 }
 EXPORT_SYMBOL_GPL(acpi_dev_clear_dependencies);
 
+/**
+ * acpi_dev_get_first_consumer_dev - Return ACPI device dependent on @supplier
+ * @supplier: Pointer to the dependee device
+ *
+ * Returns the first &struct acpi_device which declares itself dependent on
+ * @supplier via the _DEP buffer, parsed from the acpi_dep_list.
+ *
+ * The caller is responsible for putting the reference to adev when it is no
+ * longer needed.
+ */
+struct acpi_device *acpi_dev_get_first_consumer_dev(struct acpi_device *supplier)
+{
+	struct acpi_device *adev = NULL;
+
+	acpi_walk_dep_device_list(supplier->handle,
+				  acpi_dev_get_first_consumer_dev_cb, &adev);
+
+	return adev;
+}
+EXPORT_SYMBOL_GPL(acpi_dev_get_first_consumer_dev);
+
 /**
  * acpi_bus_scan - Add ACPI device node objects in a given namespace scope.
  * @handle: Root of the namespace scope to scan.

include/acpi/acpi_bus.h

@@ -692,6 +692,7 @@ static inline bool acpi_device_can_poweroff(struct acpi_device *adev)
 bool acpi_dev_hid_uid_match(struct acpi_device *adev, const char *hid2, const char *uid2);
 
 void acpi_dev_clear_dependencies(struct acpi_device *supplier);
+struct acpi_device *acpi_dev_get_first_consumer_dev(struct acpi_device *supplier);
 struct acpi_device *
 acpi_dev_get_next_match_dev(struct acpi_device *adev, const char *hid, const char *uid, s64 hrv);
 struct acpi_device *