Apologies for the long silence, I wasn't near my regular desktop setup and therefore could not test the proposed solutions! drm_kms_helper.dp_aux_i2c_transfer_size works for 1, 2 & 4 (i.e. the external monitor/dongle is recognised correctly and works as it should), but not for 8, where the behaviour reverts back to the one that spawned this bugreport in the first place. This MacBook Air has a Haswell CPU as well, by the way, specifically an i7-4650U.