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Message-ID: <113c8918-1d11-b3e0-0435-19405d103e0c@opensource.wdc.com>
Date:   Mon, 20 Jun 2022 16:14:22 +0900
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Subject: Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command
 handling
Content-Language: en-US
To:     Hannes Reinecke <hare@suse.de>, Christoph Hellwig <hch@lst.de>
Cc:     John Garry <john.garry@huawei.com>, axboe@kernel.dk,
        jejb@linux.ibm.com, martin.petersen@oracle.com, brking@us.ibm.com,
        linux-block@vger.kernel.org, linux-ide@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org,
        chenxiang66@hisilicon.com
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 <20220620062828.GA10753@lst.de>
 <2ea41701-8be5-f981-d178-0d8e703962e0@suse.de>
From:   Damien Le Moal <damien.lemoal@opensource.wdc.com>
Organization: Western Digital Research
In-Reply-To: <2ea41701-8be5-f981-d178-0d8e703962e0@suse.de>
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On 6/20/22 16:03, Hannes Reinecke wrote:
> 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.

We cannot easily reuse tags as the CDL case is *not* an error. So no queue
abort come with it. If we had the queue abort, things would be easy as
that would essentially be a regular eh.

Simply using scsi_execute_req() does not work since we have no guarantees
we can get a tag: all 32 commands being executed may complete with needing
sense data, and so scsi_execute_req() would hang waiting for a tag. I
could try hacking scsi_execute_req() to reuse a tag instead of allocating
a new one... Calling scsi_execute_req() from libata is really ugly though.

> 
> But I do agree it's ugly.
> 
> Cheers,
> 
> Hannes


-- 
Damien Le Moal
Western Digital Research