Re: FICON target support

[Hannes Reinecke <hare-l3A5Bk7waGM@public.gmane.org>]

On 5/31/19 1:10 AM, Bart Van Assche wrote:
> On 5/26/19 8:51 AM, Christian Svensson wrote:
>> I am doing some preliminary scouting of how one could add support for
>> FICON (FC-SB-3 specifically). I am mostly interested in Linux being
>> able to act as a FICON target for now.
>>
>> The TL;DR summary for FICON is that it uses Fibre Channel just like
>> FCP/SCSI
>> up to but not including the FC-4 (FCP) layer. At that point it uses
>> its own SBCCS layer.
>>
>> FICON is used by IBM mainframes to interface peripherals like tape, disk,
>> printers, card punchers/readers, etc. It is more like a mainframe
>> USB/Firewire than SCSI in that regard.
>>
>> I would prefer to implement my peripherals in user-space, I don't see any
>> convincing case for having e.g. a virtual tape drive running in kernel
>> space.
>> The I/O heavy disks would probably benefit from being in kernel space,
>> but I am OK limiting the initial scope.
>>
>> I am 100% new to the SCSI and Fibre Channel subsystem in Linux, and I
>> do not even know if the current HBAs support sending frames without FCP
>> but after scouting the code it does seem that it might work out - FCP
>> seems decoupled enough.
>>
>> If you want to learn more the standard document Wireshark has an
>> implementation of the protocol (packet-fcsb3.c/h). If you want all the
>> details
>> you will sadly have to pay $60 and buy it from INCITS at
>> https://webstore.ansi.org/Standards/INCITS/INCITS3742003S2013.
>> Fwiw, FICON uses FC type 0x1B and 0x1C.
>>
>> Any thoughts or ideas where you would start? Do you see any future for
>> this addition to the kernel?
> 
> Hi Chris,
> 
> I'm not sure what the best approach is for the initiator functionality.
> For the target functionality you may want to have a look at tcmu
> (Documentation/target/tcmu-design.txt and
> drivers/target/target_core_user.c). An existing FC target driver is
> available in drivers/scsi/qla2xxx/tcm_qla2xxx.c. A diagram that shows
> the different functional blocks in TCM is available at
> https://en.wikipedia.org/wiki/LIO_(SCSI_target) (LIO is another name for
> TCM).
> 
Hmm. I wonder: do you _know_ what you are asking?
Have you ever worked with a mainframe?
Thing is, from an initiator side you don't even _see_ the FICON frames;
everything is abstracted away to some higher level.
IE you have no idea how the initiator formats frames and what it expects
in return. Which makes debugging pretty much impossible unless you're
having access to a fibrechannel analyser hooked up to a mainframe.

Hmm?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah
HRB 21284 (AG Nürnberg)
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