linux-kernel - Re: FireWire/SBP2 Target mode

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Message-ID: <CAGRGNgWf0epcam-jpCzMCSqefCKDSm892mw-X49FrcoRO0QYrQ@mail.gmail.com>
Date:	Tue, 7 Feb 2012 10:00:38 +1100
From:	Julian Calaby <julian.calaby@...il.com>
To:	Chris Boot <bootc@...tc.net>
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

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?

Thanks,

-- 
Julian Calaby

Email: julian.calaby@...il.com
Profile: http://www.google.com/profiles/julian.calaby/
.Plan: http://sites.google.com/site/juliancalaby/
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