linux-kernel - Re: FireWire/SBP2 Target mode

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Date:	Tue, 07 Feb 2012 07:38:30 +0000
From:	Chris Boot <bootc@...tc.net>
To:	Julian Calaby <julian.calaby@...il.com>
CC:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Clemens Ladisch <clemens@...isch.de>,
	target-devel@...r.kernel.org,
	linux1394-devel@...ts.sourceforge.net,
	Boaz Harrosh <bharrosh@...asas.com>,
	Andy Grover <agrover@...hat.com>, linux-scsi@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: FireWire/SBP2 Target mode

On 06/02/2012 23:09, Chris Boot wrote:
>
> On 6 Feb 2012, at 23:00, Julian Calaby wrote:
>
>> Hi,
>>
>> On Tue, Feb 7, 2012 at 09:28, Chris Boot<bootc@...tc.net>  wrote:
>>> On 6 Feb 2012, at 20:26, Stefan Richter wrote:
>>>
>>>> On Feb 06 Chris Boot wrote:
>>>>> On 06/02/2012 14:43, Clemens Ladisch wrote:
>>>>>> Chris Boot wrote:
>>>>>>> You can pull the code from:
>>>>>>> git://github.com/bootc/Linux-SBP-2-Target.git
>>>>>>
>>>>>> The TODO file says:
>>>>>>> * Update Juju so we can get the speed in the fw_address_handler callback
>>>>>>
>>>>>> What is the speed needed for?
>>>>>
>>>>> "The speed at which the block write request to the MANAGEMENT_AGENT
>>>>> register is received shall determine the speed used by the target for
>>>>> all subsequent requests to read the initiator’s configuration ROM, fetch
>>>>> ORB’s from initiator memory or store status at the initiator’s
>>>>> status_FIFO. Command block ORB’s separately specify the speed for
>>>>> requests addressed to the data buffer or page table."
>>>>>
>>>>> (T10/1155D Revision 4 page 53/54)
>>>>
>>>> I guess it is not too hard to add this to the AR-req handler.  On the
>>>> other hand, I see little reason to follow the SBP-2 spec to the letter
>>>> here.  The target driver could just use the maximum speed that the core
>>>> figured out.  On the other hand, this requires of course
>>>>   - the target to wait for core to finish scanning an initiator,
>>>>   - the core to offer an API to look up an fw_device by a
>>>>     card--generation--nodeID tuple.
>>>>
>>>> The intention of the spec is IMO clearly to enable target implementations
>>>> that do not need to implement topology scanning.  I have a hard time to
>>>> think of a valid scenario where an initiator needs to be able to steer a
>>>> target towards a lower wire speed than what the participating links and
>>>> PHYs actually support.
>>>
>>> The only thing stopping me from getting the speed is the fact that struct fw_request is opaque. The value is easily available from request->response.speed and I kind of do that already in a very hackish way. I've sent a separate patch which adds a function that can be used to access that one value.
>>>
>>> Waiting until the bus scan is complete isn't actually that great as I see the first LOGIN requests often before the fw_node is seen at all. I'd have to turn away the requester and hope they try again. I'm fairly sure my little tweak in my patch is a simple enough solution.
>>
>> Stupid question: Could you use a completion queue or something
>> equivalent to wait until you have seen the fw_node, *then* process the
>> LOGIN request?
>
> The fw_address_handler callback is called in interrupt context, and I can't sleep from within there. As far as I'm aware I must call fw_send_response() from within the callback and can't defer that until I've scheduled something on a work queue. Please correct me if I'm wrong though, as that might be useful anyway.

Hmm sorry I've thought about this overnight and clearly I was talking 
rubbish. Yes, I need to reply in the fw_address_handler but all I tend 
to do in there is schedule a task to the the main part of the work 
anyway. As most of the operations require fetching an ORB from the 
initiator I have to do this from user context.

So it's possible I could do this by waiting in my scheduled work 
function until the fw_node is available and get the speed from that - 
but that seems like an inordinate amount of work when I can follow the 
standard and do it really easily by pulling it out of the fw_request.

Chris

-- 
Chris Boot
bootc@...tc.net
Tel: 01271 414100
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