linux-kernel - Re: [PATCH 0/3] New firewire stack

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Date:	Wed, 06 Dec 2006 16:21:15 -0500
From:	Kristian Høgsberg <krh@...hat.com>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
CC:	Alexander Neundorf <a.neundorf-work@....net>,
	ben.collins@...ntu.com, linux-kernel@...r.kernel.org,
	adobriyan@...il.com, linux1394-devel@...ts.sourceforge.net
Subject: Re: [PATCH 0/3] New firewire stack

Stefan Richter wrote:
...
> Another question is whether the stack-internal APIs are really fit for
> non-OHCI chips. There is an unfinished low-level driver for GP2Lynx
> which worked to some degree at some point, but other than that I don't
> remember positive or negative reports in this department. Maybe proper
> documentation of the stack-internal APIs would already help embedded
> developers a lot. Furthermore, there may be question marks WRT
> interaction of the FireWire stack with architecture specific kernel code.

I think some of the problems with the current stack come from the fact that it 
was originally written (by Andreas Bombe) for the PCILynx chipset, in other 
words, *not* for the OHCI chipset.  The PCILynx chipset is a much lower level 
chipset, it offloads much more to software.  For example, each self ID is 
received as an individual packet, where the OHCI chipset receives these into a 
special buffer and notifies software when it has received a consistent set of 
packets.  The current stack has a callback for the host controller driver to 
call once the bus reset phase starts, a callback for each received self ID and 
a callback to indicate the end of the bus reset phase.

In the new stack, the controller/core interface is more suited for the OHCI 
controller.  The stack doens't go into a bus reset state, and all self IDs are 
reported as an atomic event.  This makes the upper layers much simpler, suits 
the OHCI controller better, and should only require a few lines extra code in 
a PCILynx driver to buffer up the self IDs.  And it's arguably better to have 
the PCILynx driver do this than have the OHCI controller split up and 
otherwise atomic event.

> But back to the subject matter: Clearly, Kristian concentrates on
> PCI/OHCI-1394 hardware at the moment. If embedded developers have
> specific requirements on the FireWire stack's design, they should IMO
> contribute with a list of requirements or maybe even with patches.

It's true that I'm developing for PCI+OHCI, but I've kept the controller/core 
stack split that the old stack has, nothing outside the OHCI driver depends on 
PCI (I'm using the generic DMA API).  I've shifted the abstraction level up a 
bit for the controller interface, which makes sense in general, but also since 
this is what every desktop or laptop out there has.  That said, I don't think 
anything in the stack design will break for embedded/non-OHCI chipsets.

cheers,
Kristian
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