linux-kernel - Re: remote debugging via FireWire * __fast_

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Date:	Thu, 6 Dec 2007 17:32:19 +0100 (CET)
From:	Bernhard Kaindl <bk@...e.de>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: remote debugging via FireWire * __fast__ firedump!

On Tue, 4 Dec 2007, Stefan Richter wrote:
>
> A quick note to this text from
> http://www.suse.de/~bk/firewire/ohci1394_dma_early.diff :
>> +	  As all changes to the FireWire bus such as enabling and disabling
>> +	  devices cause a bus reset and thereby disable remote DMA for all
>> +	  devices, be sure to have FireWire enabled on the debug host (and
>> +	  the cable plugged) before booting the debug target for debugging.
>
> Bus resets are also caused by bus managing software, which Linux' old
> and new FireWire stacks and the stacks of all other FireWire capable
> desktop OSs are to varying degrees.

Yes, I can also trigger it raw1394_reset_bus() from libraw1394.

>                                      I wonder if the following could
> happen:  The two PCs are directly connected, only the PHY of the
> debugging PC is active, then the PHY of the debugged PC is activated,
> becomes root node, debugging PC examines the bus, then resets the bus to
> force its own PHY to become root node in order to get a working
> isochronous resource manager.  This bus reset would switch remote DMA on
> the debugged PC off.

Yes, that's the case. The PHY of the debugging PC must be active before the
booting a kernel with the early OHCI-1394 initialisation on the debugged
machine.

I tried to to instruct users to do exactly that in that part of the help
text, but changed the wording now in to hope to make it clearer:

+	  [...] be sure to have the cable plugged and FireWire enabled on
+	  the debugging host before booting the debug target for debugging.

and I added new file, installed as Documentation/debugging-via-ohci1394.txt
to give a more detailed HOWTO with links to all available tools and a step
by step guide on how to create a setup for using firescope.

ftp://ftp.suse.de/private/bk/firewire/kernel/ohci1394_dma_early-v2.diff

I'll submit that patch for full review in my next mail.

Bernhard
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