Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <5a2047e3-5e71-141a-ec3a-2e22749d3c49@huawei.com>
Date:   Mon, 13 Jun 2022 11:05:33 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.6.1
Subject: Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command
 handling
To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        <axboe@kernel.dk>, <jejb@linux.ibm.com>,
        <martin.petersen@oracle.com>, <brking@us.ibm.com>, <hare@suse.de>,
        <hch@lst.de>
CC:     <linux-block@vger.kernel.org>, <linux-ide@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-scsi@vger.kernel.org>,
        <chenxiang66@hisilicon.com>
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>
 <7f80f3b6-84f6-de48-4e69-4562c96e62c5@huawei.com>
 <e4b108ba-cbc9-5237-f873-2fcea94f2b85@opensource.wdc.com>
 <53fa2856-54f2-c075-2eed-4f05c3459597@huawei.com>
 <d2e452cd-6a91-f25d-748e-94f560deb1bb@opensource.wdc.com>
From:   John Garry <john.garry@huawei.com>
In-Reply-To: <d2e452cd-6a91-f25d-748e-94f560deb1bb@opensource.wdc.com>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 13/06/2022 10:43, Damien Le Moal wrote:
>>> Currently, that is not possible to do cleanly as there are no guarantees
>>> we can get a free tag (there is a race between block layer tag allocation
>>> and libata internal tag counting). So a reserved tag for that would be
>>> nice. We would end up with 31 IO tags at most + 1 reserved tag for NCQ
>>> commands + ATA_TAG_INTERNAL for non-NCQ. That last one would be rendered
>>> rather useless. But that also means that we kind-of go back to the days
>>> when Linux showed ATA drives max QD of 31...
>> So must the ATA_TAG_INTERNAL qc always be available for non-NCQ action
>> like EH, and that is why you cannot reuse for this internal NCQ
>> (queuable) command?
> Currently, ATA_TAG_INTERNAL is always used for non-NCQ commands. Seeing a
> qc with that tag means it is*not*  NCQ.
> 
> I am trying to see if I can reuse the tag from one of the commands that
> completed with that weird good status/sense data available. The problem
> though is that this all needs to be done*before*  calling
> qc->complete_fn() which will free the tag. So we endup with 2 qcs that
> have the same tag, the second one (for the read log command) temporarily
> using the tag but still going through the same completion path without the
> original command fully completed yet. It is a real mess.
> 

Reusing tags seems really messy, but then reserving an NCQ command seems 
wasteful.

Thanks,
John