When a cable becomes unplugged or training is lost for any reason, the TRAINING_LOST interrupt will occur. At this point, video data and MSA values are unreliable.
When the cable is plugged in again, no action is required from you; the core properly resets itself and applies HPD. In a scenario, where the cable is plugged in but the training is lost, the software is expected to assert a HPD upon the occurrence of a TRAINING_LOST interrupt, so that the source can retrain the link.