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    • Bernhard Kaindl's avatar
      x86: early boot debugging via FireWire (ohci1394_dma=early) · f212ec4b
      Bernhard Kaindl authored
      This patch adds a new configuration option, which adds support for a new
      early_param which gets checked in arch/x86/kernel/setup_{32,64}.c:setup_arch()
      to decide wether OHCI-1394 FireWire controllers should be initialized and
      enabled for physical DMA access to allow remote debugging of early problems
      like issues ACPI or other subsystems which are executed very early.
      If the config option is not enabled, no code is changed, and if the boot
      paramenter is not given, no new code is executed, and independent of that,
      all new code is freed after boot, so the config option can be even enabled
      in standard, non-debug kernels.
      With specialized tools, it is then possible to get debugging information
      from machines which have no serial ports (notebooks) such as the printk
      buffer contents, or any data which can be referenced from global pointers,
      if it is stored below the 4GB limit and even memory dumps of of the physical
      RAM region below the 4GB limit can be taken without any cooperation from the
      CPU of the host, so the machine can be crashed early, it does not matter.
      In the extreme, even kernel debuggers can be accessed in this way. I wrote
      a small kgdb module and an accompanying gdb stub for FireWire which allows
      to gdb to talk to kgdb using remote remory reads and writes over FireWire.
      An version of the gdb stub fore FireWire is able to read all global data
      from a system which is running a a normal kernel without any kernel debugger,
      without any interruption or support of the system's CPU. That way, e.g. the
      task struct and so on can be read and even manipulated when the physical DMA
      access is granted.
      A HOWTO is included in this patch, in Documentation/debugging-via-ohci1394.txt
      and I've put a copy online at
      It also has links to all the tools which are available to make use of it
      another copy of it is online at:
      ftp://ftp.suse.de/private/bk/firewire/kernel/ohci1394_dma_early-v2.diffSigned-Off-By: default avatarBernhard Kaindl <bk@suse.de>
      Tested-By: default avatarThomas Renninger <trenn@suse.de>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>