linux-kernel - Re: FireWire/SBP2 Target mode

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Message-ID: <20120207201730.1c8cf6a3@stein>
Date:	Tue, 7 Feb 2012 20:17:30 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Chris Boot <bootc@...tc.net>
Cc:	Julian Calaby <julian.calaby@...il.com>,
	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 Feb 07 Chris Boot wrote:
> On 06/02/2012 23:09, Chris Boot wrote:
> > On 6 Feb 2012, at 23:00, Julian Calaby wrote:
> >> On Tue, Feb 7, 2012 at 09:28, Chris Boot<bootc@...tc.net>  wrote:
> >>> 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.

An SBP-2 initiator should be prepared to retry its first login attempt if
it sent it shortly after a bus reset.  The target may still hold
reservations for previously loggend in initiators for up to reconnect_hold
+ 2 seconds after bus reset.

> >>> I'm fairly sure my little tweak in my patch is a simple enough
> >>> solution.

Yep.

> >> 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.

Technically there are two things to perform:

 1. Finish the inbound IEEE 1394 transaction to the management agent
    register by means of fw_send_response().  As far as I can tell, you
    don't have to do that in the address_callback().  But there is little
    reason not to.

    fw_send_response() ends the lifetime of an fw_request, so read the
    speed code before you respond.

 2. Finish the inbound SBP-2 transaction; here the login.  This and
    everything that leads up to it is definitely easiest to implement in
    a process context, e.g. workqueue item.
-- 
Stefan Richter
-=====-===-- --=- --===
http://arcgraph.de/sr/
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