On Fri, Sep 26, 2014 at 08:29:29AM +0200, Hannes Reinecke wrote: > Hi Christoph, > > as discussed it would make sense to use the request->tag in eg > scmd_printk() to identify the command. > Which I duly did, only to figure out that the tag is always '-1', ie > tagging is not in use. > (Which is okay from the SCSI side, seeing the TCQ is basically a > SCSI parallel thing). tag are still a live part of SAM for every transport, they've only been renamed to "command identifier" in SAM-4 to confuse everyone. > Looking closer I found plenty of code for handling tags in the block > layer (and the blk-mq stuff, of course), but virtually none of the > non-SPI driver seems to be using them. A quick grep for scsi_activate_tcq disagrees with you. > Which makes the original idea a bit pointless, seeing that we need > to identify the command _always_, and not just if the host happens > to support tagging. > > Which leads me to some questions: > - Is the stuff in blk-mq supposed to work as a superset of SCSI TCQ? Yes. > - If so, should any HBAs with a queue depth > 1 (which does not > support TCQ) set the tag of a command? > (that's what I've initially thought would happen ...) > - If not (and the ->tag field is basically unused), can't we > have the HBA to fill in a value here? blk-mq will always provide, and does rely on a valid request->tag. A LLDD can still use it's own internal tagging or mess with scmd->tag, although in general it should benefit from using the block layer tagging. > Which apparently was too much to hope for ... I guess for now we'll need to stay with the command pointer address. We can revisit this once the old request layer is gone. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
