Re: [PATCH] Early boot debugging via FireWire (ohci1394_dma=early)

Stefan Richter
2007-12-06T20:24:18Z

Bernhard Kaindl wrote:
> after summing up the discussion on previous patches, I'm now submitting the
> patch below for formal review and adoption on branches for mainline inclusion.

I will look a bit more into the details later; for now I have just a
quick comment on the documentation.

> --- linux-2.6.24-rc4/Documentation/debugging-via-ohci1394.txt
> +++ linux-2.6.24-rc4/Documentation/debugging-via-ohci1394.txt
> @@ -0,0 +1,179 @@
> +
> + Using physical DMA provided by OHCI-1394 FireWire controllers for debugging
> + ---------------------------------------------------------------------------
> +
> +Introduction
> +------------

There are some really long sentences in it.
Try to use the "." character more often. :-)

> +Basically all FireWire controllers which are in use today are compliant
> +to the OHCI-1394 specification which defines the controller to be a PCI
> +bus master which uses DMA to offload data transfers from the CPU and has
> +a "Physical Response Unit" which executes specific requests by employing
> +PCI-Bus master DMA after applying filters defined by the OHCI-1394 driver.
> +
> +Once properly configured, remote machines can send these requests to
> +ask the OHCI-1394 controller to perform read and write requests on
> +physical system memory and, for read requests, send the result of
> +the physical memory read back to the requester.

In other words, the FireWire controller can be configured to work as a
bus bridge between FireWire bus and local bus (PCI or PCIe).

And yes, there are security implications.

[...]
> +The OHCI-1394 drivers in drivers/firewire and drivers/ieee1394 initialize
> +the OHCI-1394 controllers to a working state and can be used to enable
> +physical DMA. By default you only have to load the driver, and physical
> +DMA access will be granted to all remote nodes, but it can be turned off
> +when using the ohci1394 driver.

This is correct for ohci1394, and it's a bug.
http://bugzilla.kernel.org/show_bug.cgi?id=7794

firewire-ohci however implements filtered physical DMA. The only user
of that is the firewire-sbp2 driver which grants SBP-2 targets access
through the physical response unit. firewire-ohci has at the moment no
option for either unfiltered physical DMA (as needed by firescope at al,
tracked at http://wiki.linux1394.org/ToDo) nor to completely disable
physical DMA.
--
Stefan Richter
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