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Date:   Thu, 16 Jun 2022 09:24:16 +0100
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Subject: Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command
 handling
To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        Bart Van Assche <bvanassche@acm.org>, <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>
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From:   John Garry <john.garry@huawei.com>
In-Reply-To: <c702f06e-b7da-92be-3c4f-5dd405600235@opensource.wdc.com>
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On 16/06/2022 03:47, Damien Le Moal wrote:
>>> so going backward several years... That internal tag for ATA does not
>>> need to be reserved since this command is always used when the drive is
>>> idle and no other NCQ commands are on-going.
>>
>> So do you mean that ATA_TAG_INTERNAL qc is used for other commands 
>> apart from internal commands?
> 
> No. It is used only for internal commands. What I meant to say is that 
> currently, internal commands are issued only on device scan, device 
> revalidate and error handling. All of these phases are done with the 
> device under EH with the issuing path stopped and all commands 
> completed, 

If I want to allocate a request for an ATA internal command then could I 
use 1x from the regular tags? I didn't think that this was possible as I 
thought that all tags may be outstanding when EH kicks in. I need to 
double check it.

Even if it were true, not using a reserved tag for ATA internal command 
makes things more tricky as this command requires special handling for 
scsi blk_mq_ops and there is no easy way to identify the command as 
reserved (to know special handling is required).

>so no regular commands can be issued. Only internal ones, non 
> NCQ, using the ATA_TAG_INTERNAL. So strictly speaking, we should not 
> need to reserve that internal tag at all.
> 

Thanks,
John