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

[John Garry <john.garry@huawei.com>]

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?

> 
> So the solution to all this is a likely a little more complicated if we
> want to keep ATA max QD to 32.
> 

thanks,
John