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;
IronPort-SDR: OcS1xaDXjWQ/Con6wTG75OCI7Y9r2X7gdj6XC3V4zdW4jZWlsJxOUNH1mY6EESy1yxvHPeWAyq
 n5FiPrZcbSlAHoFUzsZLNhFxnnCz0ZXhHHBAo4GMYncDReRWs8aNwvUkqiWqfjTOktMcRig8PI
 IrNDKrqKiqHHwb+U020JM7ohP5+dmta2Sq7fizRjVXbqWd/B3M+i/iywoBByi1Af69U41ThFXh
 RLlvqGWiagTgCiMWfI/1Nj4sks5cz7/Y6dU0JHrjM8EPan6X4rQ9U/QFGt3VNHXOqi5L5DKV9R
 eEvdkSaKN1udX4iCb9I2t4zd
IronPort-SDR: eb83THqGHhzoJL0lFk9Rj/iRSelM968nbN8V5Tt3Bqd4Pnmdv/MiDFErCK7k1PZBWIPPzN7mNd
 3BBiydGxSpFk5zgK3y+hFKE9nrpUAA/+zkX13x4dWpGzqvUkaFZ/jnHti3aV6y67J+Mzpi/N4k
 R1FD1VD2WWEzOjXSTtwENdACBSO7DdQKp5YvxrDPOWBrNabdB8jVFIatLUn8Iwtx5+qC4toQjO
 qoEKbf/FF5o3cbwHkXrU/Wg6oMXSbOtshTwrwgRoKxtxg75cMtihxWv9fH0GKltH+iICxXFzgo
 llc=
WDCIronportException: Internal
Message-ID: <41fa0f12-cdcf-f2c4-7366-1abd04312f1f@opensource.wdc.com>
Date:   Mon, 20 Jun 2022 20:24:48 +0900
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.10.0
Subject: Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command
 handling
Content-Language: en-US
To:     Christoph Hellwig <hch@lst.de>
Cc:     Hannes Reinecke <hare@suse.de>, John Garry <john.garry@huawei.com>,
        Bart Van Assche <bvanassche@acm.org>, 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
References: <1654770559-101375-4-git-send-email-john.garry@huawei.com>
 <b4a0ede5-95a3-4388-e808-7627b5484d01@opensource.wdc.com>
 <9e89360d-3325-92af-0436-b34df748f3e2@acm.org>
 <e36bba7e-d78d-27b4-a0e2-9d921bc82f5d@opensource.wdc.com>
 <3a27b6ff-e495-8f11-6925-1487c9d14fa9@huawei.com>
 <c702f06e-b7da-92be-3c4f-5dd405600235@opensource.wdc.com>
 <ecfb0694-21b8-55b4-c9b8-5e738f59ce8d@huawei.com>
 <98fa010d-3555-a82b-e960-f47aeeb38151@opensource.wdc.com>
 <7b046321-fdb3-33f0-94a0-78a25cbbe02e@suse.de>
 <9de5ed1b-e874-28ac-0532-cd5420892064@opensource.wdc.com>
 <20220620090543.GA13643@lst.de>
From:   Damien Le Moal <damien.lemoal@opensource.wdc.com>
Organization: Western Digital Research
In-Reply-To: <20220620090543.GA13643@lst.de>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 6/20/22 18:05, Christoph Hellwig wrote:
> On Mon, Jun 20, 2022 at 06:02:30PM +0900, Damien Le Moal wrote:
>> So reserving a tag/req to be able to do NCQ at the cost of max qd being 31
>> works for that. We could keep max qd at 32 by creating one more "fake" tag
>> and having a request for it, that is, having the fake tag visible to the
>> block layer as a reserved tag, as John's series is doing, but for the
>> reserved tags, we actually need to use an effective tag (qc->hw_tag) when
>> issuing the commands. And for that, we can reuse the tag of one of the
>> failed commands.
> 
> Take a look at the magic flush request in blk-flush.c, which is
> preallocated but borrows a tag from the request that wants a pre- or
> post-flush.  The logic is rather ugly, but maybe it might actually
> become cleaner by generalizing it a bit.

Thanks. Will check.
I am also looking at scsi_unjam_host() and scsi_eh_get_sense(). These
reuse a scsi command to do eh operations. So I could use that too, modulo
making it work outside of eh context to keep the command flow intact.

-- 
Damien Le Moal
Western Digital Research