First, qc is allocated and initialized using ata_qc_new_init(). Although ata_qc_new_init() doesn't implement any wait or retry mechanism when qc is not available, internal commands are currently issued only during initialization and error recovery, so no other command is active and allocation is guaranteed to succeed.
Once allocated qc's taskfile is initialized for the command to be executed. qc currently has two mechanisms to notify completion. One is via qc->complete_fn() callback and the other is completion qc->waiting. qc->complete_fn() callback is the asynchronous path used by normal SCSI translated commands and qc->waiting is the synchronous (issuer sleeps in process context) path used by internal commands.
Once initialization is complete, host_set lock is acquired and the qc is issued.
All libata drivers use ata_scsi_queuecmd() as hostt->queuecommand callback. scmds can either be simulated or translated. No qc is involved in processing a simulated scmd. The result is computed right away and the scmd is completed.
For a translated scmd, ata_qc_new_init() is invoked to allocate a qc and the scmd is translated into the qc. SCSI midlayer's completion notification function pointer is stored into qc->scsidone.
qc->complete_fn() callback is used for completion notification. ATA commands use ata_scsi_qc_complete() while ATAPI commands use atapi_qc_complete(). Both functions end up calling qc->scsidone to notify upper layer when the qc is finished. After translation is completed, the qc is issued with ata_qc_issue().
Note that SCSI midlayer invokes hostt->queuecommand while holding host_set lock, so all above occur while holding host_set lock.