The firewire-ohci driver in drivers/firewire uses filtered physical DMA by default, which is more secure but not suitable for remote debugging. Pass the remote_dma=1 parameter to the driver to get unfiltered physical DMA.
Because the firewire-ohci driver depends on the PCI enumeration to be completed, an initialization routine which runs pretty early has been implemented for x86. This routine runs long before console_init() can be called, i.e. before the printk buffer appears on the console.
To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu: Remote debugging over FireWire early on boot) and pass the parameter “ohci1394_dma=early” to the recompiled kernel on boot.