linux-kernel - Re: [PATCH v2 3/3] ibmvscsi: Allow to configure maximum LUN

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Date:	Wed, 04 Nov 2015 14:02:04 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Laurent Vivier <lvivier@...hat.com>, martin.petersen@...cle.com
CC:	brking@...ux.vnet.ibm.com, tyreld@...ux.vnet.ibm.com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] ibmvscsi: Allow to configure maximum LUN

On 11/04/2015 12:46 PM, Laurent Vivier wrote:
> 
> 
> On 04/11/2015 12:16, Hannes Reinecke wrote:
>> On 11/04/2015 11:20 AM, Laurent Vivier wrote:
>>> QEMU allows until 32 LUNs.
>>>
>>> Signed-off-by: Laurent Vivier <lvivier@...hat.com>
>>> ---
>>>  drivers/scsi/ibmvscsi/ibmvscsi.c | 5 ++++-
>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/scsi/ibmvscsi/ibmvscsi.c b/drivers/scsi/ibmvscsi/ibmvscsi.c
>>> index 04de287..4480d3e 100644
>>> --- a/drivers/scsi/ibmvscsi/ibmvscsi.c
>>> +++ b/drivers/scsi/ibmvscsi/ibmvscsi.c
>>> @@ -84,6 +84,7 @@
>>>   */
>>>  static int max_id = 64;
>>>  static int max_channel = 3;
>>> +static int max_lun = 8;
>>>  static int init_timeout = 300;
>>>  static int login_timeout = 60;
>>>  static int info_timeout = 30;
>>> @@ -117,6 +118,8 @@ module_param_named(fast_fail, fast_fail, int, S_IRUGO | S_IWUSR);
>>>  MODULE_PARM_DESC(fast_fail, "Enable fast fail. [Default=1]");
>>>  module_param_named(client_reserve, client_reserve, int, S_IRUGO );
>>>  MODULE_PARM_DESC(client_reserve, "Attempt client managed reserve/release");
>>> +module_param(max_lun, int, S_IRUGO);
>>> +MODULE_PARM_DESC(max_lun, "Maximum LUN value [Default=8]");
>>>  
>>>  static void ibmvscsi_handle_crq(struct viosrp_crq *crq,
>>>  				struct ibmvscsi_host_data *hostdata);
>>> @@ -2289,7 +2292,7 @@ static int ibmvscsi_probe(struct vio_dev *vdev, const struct vio_device_id *id)
>>>  		goto init_pool_failed;
>>>  	}
>>>  
>>> -	host->max_lun = 8;
>>> +	host->max_lun = max_lun;
>>>  	host->max_id = max_id;
>>>  	host->max_channel = max_channel;
>>>  	host->max_cmd_len = 16;
>>>
>> Please, don't do this.
>>
>> 'max_lun' should only be set if the HBA / transport has some hard
>> limitations on the number of bytes it can use.
>> Otherwise the scanning algorithm in scsi_scan.c should do the
>> correct thing, independent on the 'max_lun' setting.
> 
> So you are saying we can remove the line ?
> 
Ho-hum. In principle you could, as ibmvscsi is using SRP internally,
which does support 64 bit LUNs.

However, due to some weird design decisions there is
the function 'lun_from_dev()', which mangles the incoming LUN number
into something ... else.
Which leaves only 4 bits free for the actual LUN number, requiring
you to use max_lun = 16.

Personally I would just do away with that function and use the
incoming LUN numbers as is.

Brian? Any comments?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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