linux-kernel - Re: FireWire/SBP2 Target mode

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Message-Id: <476E7976-738A-4202-9FC4-FA5B060EA95F@bootc.net>
Date:	Mon, 6 Feb 2012 23:09:49 +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 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.

Chris

-- 
Chris Boot
bootc@...tc.net

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