Re: FireWire/SBP2 Target mode

[Chris Boot]

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@xxxxxxxxx>  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 initiatorâs configuration ROM, fetch
> > > > > ORBâs from initiator memory or store status at the initiatorâs
> > > > > status_FIFO. Command block ORBâs 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@xxxxxxxxx
Tel: 01271 414100
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