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

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Date:	Thu, 5 Nov 2015 14:00:43 -0600
From:	Brian King <brking@...ux.vnet.ibm.com>
To:	Laurent Vivier <lvivier@...hat.com>,
	Hannes Reinecke <hare@...e.de>, martin.petersen@...cle.com
Cc:	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 11/05/2015 02:26 AM, Laurent Vivier wrote:
> 
> 
> On 05/11/2015 00:06, Brian King wrote:
>> On 11/04/2015 07:02 AM, Hannes Reinecke wrote:
>>> 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.
>>
>> I pulled out my copy of SAM, what lun_from_dev is doing is translating
>> the incoming bus / id / LUN to a LUN in the logical unit addressing format,
>> as defined in 4.6.9 of SAM-4.
>>
>> --------------------------------------
>> |Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
>> --------------------------------------
>> | n  | (10b) |       Target          |
>> --------------------------------------
>> | n+1|   Bus     |        LUN        |
>> --------------------------------------
>>
>> So this means, in the current implementation, we have 6 bits for target (max=63),
>> 3 bits for bus (max=7), and 5 bits for LUN (max=31). 
>>
>> It might not be a bad idea to enforce these limits on the module parameters. Otherwise
>> we'll have a mess when we run through lun_from_dev...
>>
>> As far as eliminating lun_from_dev and just passing the LUN through, I don't think we
>> can do that. The LUN list returned by Report LUNs to a VSCSI disk assumes we are
>> doing the translation above.
> 
> So what I understand is this patch is OK but I have to check the given
> value is less than 32 (the same max value as for QEMU) ?

As far as I'm concerned, yes.

Thanks,

Brian

-- 
Brian King
Power Linux I/O
IBM Linux Technology Center

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