On Wed, Jan 14, 2015 at 09:37:30AM -0800, Dan Williams wrote: > libsas uses the libata allocated tag for NCQ. libsas first asks > libata to allocate a tag/qc for ncq, then it creates a sas_task and > assigns the scsi tag. To use a generic allocator I assume you would > need to flip that ordering to allocate the sas_task tag first and then > use that plus another key to allocate a sub-tag for ata to use for > NCQ. As far as I can tell all libsas drivers as well as ipr use block layer tagging, so they always get requests/scsi_cmnds with a tag already assigned. For libsas drivers currently use per-LUN tags, although with scsi-mq we will get per-host tags. libata on the other hand at the moment assigns it's own tags, which are only stored in struct ata_queued_cmd, but never in the request. libata assigns tags per ata_host. Each ata_host might have multiple ports, which each get their own Scsi_Host allocated. Given that ATA NCQ has a fairly low queue depth I assume we want to stick to assigning ATA tags per-device, but due to the host per device scheme libata that even works using scsi-mq. (Q: how do tags work when port mulipliers are involved?) So for SAS driver using libata we will need a separate tag allocator, but that one might as well be in libsas instead of keeping it in libata. -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
