linux-kernel - Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command handling

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Date:   Wed, 15 Jun 2022 08:43:39 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Bart Van Assche <bvanassche@....org>,
        John Garry <john.garry@...wei.com>, axboe@...nel.dk,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, brking@...ibm.com,
        hare@...e.de, hch@....de
Cc:     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 03/18] scsi: core: Implement reserved command
 handling

On 6/15/22 03:20, Bart Van Assche wrote:
> On 6/13/22 00:01, Damien Le Moal wrote:
>> On 6/9/22 19:29, John Garry wrote:
>>> +	/*
>>> +	 * This determines how many commands the HBA will set aside
>>> +	 * for internal commands. This number will be added to
>>> +	 * @can_queue to calcumate the maximum number of simultaneous
>>
>> s/calcumate/calculate
>>
>> But this is weird. For SATA, can_queue is 32. Having reserved commands,
>> that number needs to stay the same. We cannot have more than 32 tags.
>> I think keeping can_queue as the max queue depth with at most
>> nr_reserved_cmds tags reserved is better.
>>
>>> +	 * commands sent to the host.
>>> +	 */
>>> +	int nr_reserved_cmds;
> 
> +1 for Damien's request. I also prefer to keep can_queue as the maximum
> queue depth, whether or not nr_reserved_cmds has been set.

For non SATA drives, I still think that is a good idea. However, for SATA,
we always have the internal tag command that is special. With John's
change, it would have to be reserved but that means we are down to 31 max
QD, 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 the solution to all this is a likely a little more complicated if we
want to keep ATA max QD to 32.

> 
> Thanks,
> 
> Bart.


-- 
Damien Le Moal
Western Digital Research