writing to GATT descriptor produces write not permitted error

Question

I'm using java TinyB to connect to a TimeFlip device with Bluetooth LE.

When trying to write to the Descriptor of the Facet Characteristic to recieve notifications I always get the error:

Exception in thread "main" tinyb.BluetoothException: GDBus.Error:org.bluez.Error.NotPermitted: Write not permitted

But when I write to the same Descriptor using gatttool it works and i get the notifications.

        BluetoothGattService timeFlipService = device.find("f1196f50-71a4-11e6-bdf4-0800200c9a66", Duration.ofSeconds(5));
        BluetoothGattCharacteristic facets = timeFlipService.find("f1196f52-71a4-11e6-bdf4-0800200c9a66", Duration.ofSeconds(5));
        BluetoothGattDescriptor facetsConfig = facets.find("00002902-0000-1000-8000-00805f9b34fb", Duration.ofSeconds(5));  // like this ==> always null, custom method works???

        if(!login(timeFlipService)) {log.error("login to TimeFlip failed");}

        try{
            byte[] enableNotifications = {0x01, 0x00};
            facetsConfig.writeValue(enableNotifications);           //when facesConfig assigned with custom method throws write not permitted error
            facets.enableValueNotifications(new FacetNotification());
        }
        catch(NullPointerException e){
            log.error("NullPointerException in " + (facets == null ? "facet characteristic" : "facet descriptor"));
        }
        catch(BluetoothException b){
            log.error(b.getMessage());
        }
    }

The mentioned "custom method" just gets all Descriptor from a characteristic and returns the one matching a given uuid, as the find() method times out every time.

Answer 1

In Bluez you are supposed to use StartNotify to turn on notifications or indications. Bluez will do the writing of the descriptor for you but if you try to do it yourself it indeed gives an error.