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

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Date:   Mon, 20 Jun 2022 09:03:47 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Christoph Hellwig <hch@....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,
        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 6/20/22 08:28, Christoph Hellwig wrote:
> 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.

SATA internals may come to the rescue here; if there's an error all NCQ 
commands are aborted. So we'll get at least one command tag back.
As for the command duration limits I'm still waiting for clarification
from Damien if we can reuse tags there.

But I do agree it's ugly.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
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