linux-kernel - Re: [PATCH RFC v3 2/7] ata: libata-scsi: Add ata_internal

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Date:   Fri, 28 Oct 2022 17:26:24 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     John Garry <john.garry@...wei.com>, Hannes Reinecke <hare@...e.de>,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, bvanassche@....org,
        hch@....de, ming.lei@...hat.com, niklas.cassel@....com
Cc:     axboe@...nel.dk, jinpu.wang@...ud.ionos.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
        linuxarm@...wei.com, john.garry2@...l.dcu.ie
Subject: Re: [PATCH RFC v3 2/7] ata: libata-scsi: Add
 ata_internal_queuecommand()

On 10/28/22 17:14, John Garry wrote:
> On 27/10/2022 23:35, Damien Le Moal wrote:
>>> At what stage do you want to send these commands? The tags for the shost
>>> are not setup until scsi_add_host() -> scsi_mq_setup_tags() is called,
>>> so can't expect blk-mq to manage reserved tags before then.
>>>
>>> If you are required to send commands prior to scsi_add_host(), then I
>>> suppose the low-level driver still needs to manage tags until the shost
>>> is ready. I guess that some very simple scheme can be used, like always
>>> use tag 0, since most probe is done serially per-host. But that's not a
>>> case which I have had to deal with yet.
>> In libata case, ata_dev_configure() will cause a lot of
>> ata_exec_internal_sg() calls for IDENTIFY and various READ LOG commands.
>> That is all done with non-ncq commands, which means that we do not require
>> a hw tag. But given that you are changing ata_exec_internal_sg() to call
>> alloc_request + blk_execute_rq_nowait(), how would these work without a
>> tag, at least a soft one ? Or we would need to keep the current code to
>> use ata_qc_issue() directly for probe time ? That will look very ugly...
>>
> 
> I am not sure if there is really a problem. So libata/libsas allocs the 
> shost quite early, and that is before we try using 
> ata_exec_internal_sg(). Also note that I added patch "ata: libata-scsi: 
> Allocate sdev early in port probe" so that we have ata_device.sdev ready 
> before issuing ata_exec_internal_sg() (sorry if I'm stating the obvious).
> 
> I think Hannes' issue is that some SCSI HBA driver needs to send 
> "internal" commands to probe the HW for info, and this would be before 
> shost is ready. He can tell us more.

OK. Understood.

> 
> Thanks,
> John

-- 
Damien Le Moal
Western Digital Research