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Date:   Mon, 20 Jun 2022 08:28:28 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Hannes Reinecke <hare@suse.de>
Cc:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        John Garry <john.garry@huawei.com>, axboe@kernel.dk,
        jejb@linux.ibm.com, martin.petersen@oracle.com, brking@us.ibm.com,
        hch@lst.de, linux-block@vger.kernel.org, linux-ide@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org,
        chenxiang66@hisilicon.com
Subject: Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command
 handling
Message-ID: <20220620062828.GA10753@lst.de>
References: <1654770559-101375-1-git-send-email-john.garry@huawei.com> <1654770559-101375-4-git-send-email-john.garry@huawei.com> <b4a0ede5-95a3-4388-e808-7627b5484d01@opensource.wdc.com> <88d192b5-741b-7104-7f72-0178aa18bafb@suse.de>
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On Mon, Jun 20, 2022 at 08:24:24AM +0200, Hannes Reinecke wrote:
> So my idea for SATA is simply _not_ to use reserved tags.
> Any TMF functions (or the equivalent thereof) should always be sent as 
> non-NCQ commands. And when doing so we're back to QD=1 on SATA anyway, so 
> there _must_ be tags available. Consequently the main reason for having 
> reserved tags (namely to guarantee that tags are available for TMF) doesn't 
> apply here.

At least in the non-elevator case (which includes all passthrough I/O)
request have tags assigned as soon as they are allocated.  So, we
absolutely can have all tags allocated and then need to do a TMF.