linux-kernel - Re: [PATCH RFC v2 02/18] scsi: core: Resurrect scsi_{get,free}_host

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Message-ID: <731c26d2-5124-60a6-c8de-d979ee2af467@huawei.com>
Date:   Tue, 14 Jun 2022 10:33:24 +0100
From:   John Garry <john.garry@...wei.com>
To:     Christoph Hellwig <hch@....de>
CC:     <axboe@...nel.dk>, <damien.lemoal@...nsource.wdc.com>,
        <jejb@...ux.ibm.com>, <martin.petersen@...cle.com>,
        <brking@...ibm.com>, <hare@...e.de>, <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 02/18] scsi: core: Resurrect
 scsi_{get,free}_host_dev()

On 14/06/2022 07:44, Christoph Hellwig wrote:
> On Thu, Jun 09, 2022 at 06:29:03PM +0800, John Garry wrote:
>> This reverts commit 6bd49b1a8d43ec118c55f3aaa7577729b52bde15.
> 
> Please add an actual text describing why you are doing this and how
> insteasd of this completely pointless revert line.
> 
> 
> .

OK. And in hindsight it would have been a good opportunity to mention 
something which I am undecided on - that is which scsi_device to use for 
these reserved commands?

In this series I use the scsi shost sdev for all reserved commands, but 
maybe we should use the target sdev.

Pros of using scsi host sdev:
- don't need to worry about request queue freezing
- don't need to worry about running out of request queue budget
- available when scsi host is added - libata adds target sdev after some 
internal commands are sent, and this would be a bit painful to change, 
however adding the sdev earlier would seem to be a good change to make

Cons:
- generally better to use same scsi device as target (or is it?). For 
example, it seems better to have reserved scsi_cmnd.device actually set 
to the target sdev.
- don't need to add stuff like ata_is_scmd_ata_internal() later in this 
series

Prob other reasons which I have forgot about. Please let me know if you 
have any thoughts on this.

Cheers,
John