linux-kernel - Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command handling

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Date:   Mon, 20 Jun 2022 08:28:28 +0200
From:   Christoph Hellwig <hch@....de>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        John Garry <john.garry@...wei.com>, axboe@...nel.dk,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, brking@...ibm.com,
        hch@....de, linux-block@...r.kernel.org, linux-ide@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        chenxiang66@...ilicon.com
Subject: Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command
 handling

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.