Date:	Fri, 19 Feb 2016 10:46:35 +0000
From:	John Garry <john.garry@...wei.com>
To:	Hannes Reinecke <hare@...e.de>, <JBottomley@...n.com>,
	<martin.petersen@...cle.com>
CC:	<linux-scsi@...r.kernel.org>, <john.garry2@...l.dcu.ie>,
	<linux-kernel@...r.kernel.org>, <linuxarm@...wei.com>,
	<zhangfei.gao@...aro.org>
Subject: Re: [PATCH 5/6] hisi_sas: add hisi_sas_slave_configure()

On 18/02/2016 10:57, John Garry wrote:
> On 18/02/2016 10:30, Hannes Reinecke wrote:
>> On 02/18/2016 11:12 AM, John Garry wrote:
>>> On 18/02/2016 07:40, Hannes Reinecke wrote:
>> [ .. ]
>>>> Well, the classical thing would be to associate each request tag
>>>> with a SAS task; or, in your case, associate each slot index with a
>>>> request tag.
>>>> You probably would need to reserve some slots for TMFs, ie you'd
>>>> need to decrease the resulting ->can_queue variable by that.
>>>> But once you've done that you shouldn't hit any QUEUE_FULL issues,
>>>> as the block layer will ensure that no tags will be reused while the
>>>> command is in flight.
>>>> Plus this is something you really need to be doing if you ever
>>>> consider moving to scsi-mq ...
>>>>
>>>> Cheers,
>>>>
>>>> Hannes
>>>>
>>> Hi,
>>>
>>> So would you recommend this method under the assumption that the
>>> can_queue value for the host is similar to the queue depth for the
>>> device?
>>>
>> That depends.
>> Typically the can_queue setting reflects the number of commands the
>> _host_ can queue internally (due to hardware limitations etc).
>> They do not necessarily reflect the queue depth for the device
>> (unless you have a single device, of course).
>> So if the host has a hardware limit on the number of commands it can
>> queue, it should set the 'can_queue' variable to the appropriate
>> number; a host-wide shared tag map is always assumed with recent
>> kernels.
>>
>> The queue_depth of an individual device is controlled by the
>> 'cmd_per_lun' setting, and of course capped by can_queue.
>>
>> But yes, I definitely recommend this method.
>> Is saves one _so much_ time trying to figure out which command slot
>> to use. Drawback is that you have to have some sort of fixed order
>> on them slots to do an efficient lookup.
>>
>> Cheers,
>>
>> Hannes
>>
>
> I would like to make a point on cmd_per_lun before considering tagging
> slots: For our host the can_queue is considerably greater than
> cmd_per_lun (even though we initially set the same in the host template,
> which would be incorrect). Regardless I find the host cmd_per_lun is
> effectively ignored for the slave device queue depth as it is reset in
> sas_slave_configure() to 256 [if this function is used and tagging
> enabled]. So if we we choose a reasonable cmd_per_lun for our host, it
> is ignored, right? Or am I missing something?
>
> Thanks,
> John
>
>

I would like to make another point about why I am making this change in 
case it is not clear. The queue full events are form 
TRANS_TX_CREDIT_TIMEOUT_ERR and TRANS_TX_CLOSE_NORMAL_ERR errors in the 
slot: I want the slot retried when this occurs, so I set status as 
SAS_QUEUE_FULL just so we will report DID_SOFT_ERR to SCSI midlayer so 
we get a retry. I could use SAS_OPEN_REJECT alternatively as the error 
which would have the same affect.
The queue full are not from all slots being consumed in the HBA.

thanks again,
John


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