ACPI video: Don't export sysfs backlight interface if query _BCL fail

Currently the acpi video module export the backlight interface to sysfs
also if acpi_video_device_lcd_query_levels() fails to read _BLC method
(e.g.  because the method is not available).  In this case the userspace
don't know which brightness level are supported and can't set a brightness
level (echo return with: "write error: Invalid Argument").  This happend
e.g.  on a ASUS RF1 (correct supported by the asus-laptop module).

The video module should not export the backlight interface if query _BLC fail,
because you can't set anything from userspace and this make it useless.

http://bugzilla.kernel.org/show_bug.cgi?id=8375Signed-off-by: Danny Kukawka <dkukawka@suse.de>
Acked-by: Luming Yu <luming.yu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/video.c

@@ -559,7 +559,6 @@ acpi_video_bus_DOS(struct acpi_video_bus *video, int bios_flag, int lcd_flag)
 
 static void acpi_video_device_find_cap(struct acpi_video_device *device)
 {
-	acpi_integer status;
 	acpi_handle h_dummy1;
 	int i;
 	u32 max_level = 0;
@@ -593,9 +592,9 @@ static void acpi_video_device_find_cap(struct acpi_video_device *device)
 		device->cap._DSS = 1;
 	}
 
-	status = acpi_video_device_lcd_query_levels(device, &obj);
+	if (ACPI_SUCCESS(acpi_video_device_lcd_query_levels(device, &obj))) {
 
-	if (obj && obj->type == ACPI_TYPE_PACKAGE && obj->package.count >= 2) {
+		if (obj->package.count >= 2) {
 			int count = 0;
 			union acpi_object *o;
 
@@ -616,6 +615,7 @@ static void acpi_video_device_find_cap(struct acpi_video_device *device)
 						continue;
 					}
 					br->levels[count] = (u32) o->integer.value;
+
 					if (br->levels[count] > max_level)
 						max_level = br->levels[count];
 					count++;
@@ -634,9 +634,13 @@ static void acpi_video_device_find_cap(struct acpi_video_device *device)
 			}
 		}
 
+	} else {
+		ACPI_DEBUG_PRINT((ACPI_DB_INFO, "Could not query available LCD brightness level\n"));
+	}
+
 	kfree(obj);
 
-	if (device->cap._BCL && device->cap._BCM && device->cap._BQC){
+	if (device->cap._BCL && device->cap._BCM && device->cap._BQC && max_level > 0){
 		unsigned long tmp;
 		static int count = 0;
 		char *name;