From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hannes Reinecke Subject: Re: Question: request tag usage Date: Fri, 26 Sep 2014 14:41:17 +0200 Message-ID: <54255EED.90005@suse.de> References: <542507C9.2060901@suse.de> <20140926080308.GA21137@infradead.org> <542521DB.1020605@suse.de> <1411726343.2183.9.camel@jarvis> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Received: from cantor2.suse.de ([195.135.220.15]:36959 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751001AbaIZMlV (ORCPT ); Fri, 26 Sep 2014 08:41:21 -0400 In-Reply-To: <1411726343.2183.9.camel@jarvis> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: James Bottomley Cc: Christoph Hellwig , "Elliott, Robert (Server Storage)" , SCSI Mailing List , Jens Axboe On 09/26/2014 12:12 PM, James Bottomley wrote: > On Fri, 2014-09-26 at 10:20 +0200, Hannes Reinecke wrote: >> On 09/26/2014 10:03 AM, Christoph Hellwig wrote: >>> 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 blo= ck >>>> 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. >>> >> Yeah, I've noticed after I've written the mail. >> However, main point still stands: using 'tag' to identify commands >> is pointless if not all of the LLDDs use tagging ... >=20 > Every non parallel LLD uses tagging; they all use the network request > response model, so they can no longer hold the bus until they get an > answer (which was how untagged commands work in SPI), so for most of = the > transports, untagged commands aren't legal. Of course, some drivers > roll their own tags instead of using the block ones. >=20 Hmm, I probably will get shot for this, but we _could_ do something like this: diff --git a/block/blk-core.c b/block/blk-core.c index bf930f4..91570d2 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -2347,7 +2347,8 @@ void blk_start_request(struct request *req) req->resid_len =3D blk_rq_bytes(req); if (unlikely(blk_bidi_rq(req))) req->next_rq->resid_len =3D blk_rq_bytes(req->next_rq); - + if (!blk_queue_tagged(req->q)) + req->tag =3D (++req->q->last_tag) % req->q->nr_requests= ; BUG_ON(test_bit(REQ_ATOM_COMPLETE, &req->atomic_flags)); blk_add_timer(req); } diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 518b465..e8ce575 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -418,6 +418,7 @@ struct request_queue { struct blk_queue_tag *queue_tags; struct list_head tag_busy_list; + unsigned int last_tag; unsigned int nr_sorted; unsigned int in_flight[2]; I know, I know. But it would give us a rather nice command identifier. Cheers, Hannes --=20 Dr. Hannes Reinecke zSeries & Storage hare@suse.de +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 N=C3=BCrnberg GF: J. Hawn, J. Guild, F. Imend=C3=B6rffer, HRB 16746 (AG N=C3=BCrnberg= ) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html