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Date:   Mon, 20 Jun 2022 10:02:30 +0200
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To:     John Garry <john.garry@huawei.com>,
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From:   Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command
 handling
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On 6/15/22 09:35, John Garry wrote:
> On 15/06/2022 00:43, Damien Le Moal wrote:
>> 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,
> 
> My intention is to keep regular tag depth at 32 for SATA. We add an 
> extra tag as a reserved tag. Indeed, this is called a 'tag', but it's 
> just really the placeholder for what will be the ATA_TAG_INTERNAL request.
> 
> About how we set scsi_host.can_queue, in this series we set .can_queue 
> as max regular tags, and the handling is as follows:
> 
> scsi_mq_setup_tags():
> tag_set->queue_depth = shost->can_queue + shost->nr_reserved_cmds
> tag_set->reserved_tags = shost->nr_reserved_cmds
> 
> So we honour the rule that blk_mq_tag_set.queue_depth is the total tag 
> depth, including reserved.
> 
> Incidentally I think Christoph prefers to keep .can_queue at total max 
> tags including reserved:
> https://lore.kernel.org/linux-scsi/337339b7-6f4a-a25c-f11c-7f701b42d6a8@suse.de/ 
> 
> 
>> 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?
> 
Well.

The problem is that 'ATA_TAG_INTERNAL' currently is overloaded to
a) signal internal commands
b) 'magic' tag when looking up commands

My proposal would be to separate these use-cases, and use a flag (eg 
ATA_QCFLAG_INTERNAL) to determine internal commands.

The we'll be needing an internal tag-lookup map
(NCQ tag -> blk-mq tag) for ata_qc_from_tag() to retrieve the command 
corresponding to a driver tag.

I guess we'll need that anyway with libsas, as there we're working with 
a budget tag which has no relationship with the NCQ tag whatsoever ...

But with that we can kill 'ATA_TAG_INTERNAL', and let the driver figure 
out how to allocate internal tags.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Felix Imendörffer