Re: [RFC] libata new EH document

[Jeff Garzik <jgarzik@pobox.com>]

Tejun Heo wrote:
>  IMHO, it's a good idea to maintain one qc to one ATA/ATAPI command 
> mapping as long as possible.  And, in the suggested framework, it's 
> guaranteed that no other command can come inbetween CHECK_SENSE and 
> REQUEST_SENSE.
> 
>  Requesting sense from EH, calling scsi_decide_disposition() on the 
> sense and following the verdict should achieve the same effect as 
> emulating autosense.  Is there any compelling reason to break one qc to 
> one command mapping?


Yes, you should have one qc <-> one ATA/ATAPI command.  That's why, in 
the NCQ scenario, I wanted to make sure that one qc was always reserved 
for error handling:  REQUEST SENSE or READ LOG EXT, most importantly.

For SAT layer MODE SELECT translations, that implies multiple calls to 
qc_new/qc_issue/qc_complete before completing the overall SCSI command. 
  The same for handling sata_sil mod15write:  I am beginning to feel 
like the mod15write workaround might be best implemented in a manner 
that caused libata-scsi (not sata_sil) to create/issue/complete multiple 
ATA commands.

The only problem you run into is that a qc may be active during EH, when 
you need another qc.  So avoiding recursive details becomes an issue.

	Jeff