linux-kernel - Re: [PATCH 0/7] hisi_sas: Misc bugfixes and an optimisation patch

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Date:   Thu, 11 Oct 2018 03:15:34 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     John Garry <john.garry@...wei.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        jejb@...ux.vnet.ibm.com, linuxarm@...wei.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>,
        chenxiang <chenxiang66@...ilicon.com>
Subject: Re: [PATCH 0/7] hisi_sas: Misc bugfixes and an optimisation patch

On Thu, Oct 11, 2018 at 10:59:11AM +0100, John Garry wrote:
> 
> > blk-mq tags are always per-host (which has actually caused problems for
> > ATA, which is now using its own per-device tags).
> > 
> 
> So, for example, if Scsi_host.can_queue = 2048 and Scsi_host.nr_hw_queues =
> 16, then rq tags are still in range [0, 2048) for that HBA, i.e. invariant
> on queue count?

Yes, if can_queue is 2048 you will gets tags from 0..2047.

IFF you device needs different tags for different queues it can use
the blk_mq_unique_tag heper to generate unique global tag.

But unless you actuall have multiple hardware queues that latter part
is rather irrelevant to start with.