On Thu, Jan 15, 2015 at 10:40 AM, Shaohua Li <shli@xxxxxx> wrote: > On Thu, Jan 15, 2015 at 01:28:36AM -0800, Christoph Hellwig wrote: >> 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. > > Yep, the ATA NCQ should work well with block tag as each port is a > scsi host. So looks it's not easy to map scsi tag to ATA tag for libsas > and we eventually will have another tag allocator for libsas. > > Tejun, > I'm afraid I can't work out a patch to delete the old tag allocator for > libata. Can we take this patch series first and later we can delete the > old tag allocator till libsas has its own tag allocator. This isn't > optimal, but I really know nothing about libsas. I still don't understand what we get by adding this new allocator besides complexity, am I missing something? -- 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
