linux-kernel - Re: [PATCH 03/11] libata: Send internal commands through the block layer

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Date:   Thu, 7 Apr 2022 15:32:07 +0100
From:   John Garry <john.garry@...wei.com>
To:     Christoph Hellwig <hch@....de>
CC:     <axboe@...nel.dk>, <damien.lemoal@...nsource.wdc.com>,
        <bvanassche@....org>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <ming.lei@...hat.com>,
        <hare@...e.de>, <chenxiang66@...ilicon.com>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-ide@...r.kernel.org>, <linux-scsi@...r.kernel.org>,
        <dm-devel@...hat.com>, <beanhuo@...ron.com>
Subject: Re: [PATCH 03/11] libata: Send internal commands through the block
 layer

On 22/03/2022 11:20, Christoph Hellwig wrote:
> On Tue, Mar 22, 2022 at 06:39:37PM +0800, John Garry wrote:
>> When SCSI HBA device drivers are required to process an ATA internal
>> command they still need a tag for the IO. This often requires the driver
>> to set aside a set of tags for these sorts of IOs and manage the tags
>> themselves.
>>
>> If we associate a SCSI command (and request) with an ATA internal command
>> then the tag is already provided, so introduce the change to send ATA
>> internal commands through the block layer with a set of custom blk-mq ops.
>>
>> note: I think that the timeout handling needs to be fixed up.

Hi Christoph,

> Any reason to not just send them through an ATA_16 passthrough CDB and
> just use all the normal SCSI command handling?

I had a go at implementing this but I have come up against a few issues:

- ATA_16 handling translates the passthrough CDB to a ATA TF. However 
ata_exec_internal_sg() is passed a TF already. So what to do? Change the 
callers to generate a ATA_16 CDB? I guess not. Otherwise we could put 
the already-generated TF in the SCSI cmd CDB somehow and use directly.

- We may have no SCSI device (yet) for the target when issuing an 
internal command, but only the ATA port+dev. So need a method to pass 
these pointers to ATA_16 handling

- we would need to change ata_scsi_translate(), ata_scsi_pass_thru() and 
other friends to deal with ATA_TAG_INTERNAL and its peculiarities - 
today it just deals with regular qc's.

It still does seem a reasonable idea to use ATA_16, but it looks like 
significant modifications would be required....

Thanks,
John