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

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Date:	Thu, 5 Nov 2015 09:26:57 +0100
From:	Laurent Vivier <lvivier@...hat.com>
To:	Brian King <brking@...ux.vnet.ibm.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 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) ?

Laurent
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