The first step is to ask Marvell. Please do.

You don't have to ask Marvell. The info is widely available. The DreamPlug has a JTAG interface that lets an external debugger stop the processor, access memory, etc. The manufacturer (Global Scale Technologies) sells an easy JTAG interface kit for $39. It attaches to USB on one end, and a few wires attach it to the JTAG connector on the DreamPlug. (The same JTAG interface works with the GuruPlug and the D2Plug.) There's a free software debugger (openocd, Open On-Chip Debugger) that works with JTAG interfaces. I have barely scratched the surface of using it. I haven't found a great reference for what it can do, but see: http://openocd.sourceforge.net/ http://openocd.sourceforge.net/2012/04/kernel-debugging/ (2-hr video tutorial on OpenOCD and kernel debugging) http://www.madore.org/~david/linux/dreamplug.html#u-boot In general JTAG is a way to debug chips without removing them from the system. It uses a few wires that are daisy-chained on the board to each complex chip in the system. Software to drive a JTAG port sends commands serially down the "data in" wire and causes a bitstream to be shifted out of the chips and back to the software on the "data out" wire. The interpretation of the bitstream depends on which chips are attached, and there are some standards for identifying them. All chips with JTAG let you read the pins of the chip ("boundary scan"), and often write them as well. Many chips also make internal values accessible (such as, in a processor, the current program counter, registers, etc). In a system-on-chip, the JTAG interface pins will connect up to many different subsystems inside the chip (like the processor, the ethernet, the memory controller, the serial ports, etc), which can each be addressed from the JTAG interface. For more details, see: https://en.wikipedia.org/wiki/JTAG The JTAG interface doesn't let anybody send an Ethernet or WiFi packet and take over your system. You need to plug a specific hardware connector into a hardware socket designed to fit with it (and on most systems that socket is inside the guts of the system, under the plastic or metal case). But when you have the hardware under your physical control, JTAG is very powerful for accessing and/or reverse-engineering a system. Many free-software folks use JTAG to figure out what a proprietary system's chips are doing, so they can write free software that works with them. Like any tool, it can be used for "good" or "evil"... John _______________________________________________ Freedombox-discuss mailing list Freedombox-discuss@lists.alioth.debian.org http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/freedombox-discuss ----- End forwarded message ----- -- Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org ______________________________________________________________ ICBM: 48.07100, 11.36820 http://www.ativel.com http://postbiota.org 8B29F6BE: 099D 78BA 2FD3 B014 B08A 7779 75B0 2443 8B29 F6BE