Chapter 3. Error handling
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## Chapter 3. Error handling

This chapter describes how errors are handled under libata. Readers are advised to read SCSI EH (Documentation/scsi/scsi_eh.txt) and ATA exceptions doc first.

## Origins of commands

In libata, a command is represented with struct ata_queued_cmd or qc. qc's are preallocated during port initialization and repetitively used for command executions. Currently only one qc is allocated per port but yet-to-be-merged NCQ branch allocates one for each tag and maps each qc to NCQ tag 1-to-1.

libata commands can originate from two sources - libata itself and SCSI midlayer. libata internal commands are used for initialization and error handling. All normal blk requests and commands for SCSI emulation are passed as SCSI commands through queuecommand callback of SCSI host template.

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Chapter 2. libata Driver API | Home | How commands are issued