ata_scsi_error() is the current transportt->eh_strategy_handler() for libata. As discussed above, this will be entered in two cases - timeout and ATAPI error completion. This function calls low level libata driver's eng_timeout() callback, the standard callback for which is ata_eng_timeout(). It checks if a qc is active and calls ata_qc_timeout() on the qc if so. Actual error handling occurs in ata_qc_timeout().
If EH is invoked for timeout, ata_qc_timeout() stops BMDMA and completes the qc. Note that as we're currently in EH, we cannot call scsi_done. As described in SCSI EH doc, a recovered scmd should be either retried with scsi_queue_insert() or finished with scsi_finish_command(). Here, we override qc->scsidone with scsi_finish_command() and calls ata_qc_complete().
If EH is invoked due to a failed ATAPI qc, the qc here is completed but not deallocated. The purpose of this half-completion is to use the qc as place holder to make EH code reach this place. This is a bit hackish, but it works.
Once control reaches here, the qc is deallocated by invoking __ata_qc_complete() explicitly. Then, internal qc for REQUEST SENSE is issued. Once sense data is acquired, scmd is finished by directly invoking scsi_finish_command() on the scmd. Note that as we already have completed and deallocated the qc which was associated with the scmd, we don't need to/cannot call ata_qc_complete() again.