linux/Documentation/parisc/debugging.rst
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   1=================
   2PA-RISC Debugging
   3=================
   4
   5okay, here are some hints for debugging the lower-level parts of
   6linux/parisc.
   7
   8
   91. Absolute addresses
  10=====================
  11
  12A lot of the assembly code currently runs in real mode, which means
  13absolute addresses are used instead of virtual addresses as in the
  14rest of the kernel.  To translate an absolute address to a virtual
  15address you can lookup in System.map, add __PAGE_OFFSET (0x10000000
  16currently).
  17
  18
  192. HPMCs
  20========
  21
  22When real-mode code tries to access non-existent memory, you'll get
  23an HPMC instead of a kernel oops.  To debug an HPMC, try to find
  24the System Responder/Requestor addresses.  The System Requestor
  25address should match (one of the) processor HPAs (high addresses in
  26the I/O range); the System Responder address is the address real-mode
  27code tried to access.
  28
  29Typical values for the System Responder address are addresses larger
  30than __PAGE_OFFSET (0x10000000) which mean a virtual address didn't
  31get translated to a physical address before real-mode code tried to
  32access it.
  33
  34
  353. Q bit fun
  36============
  37
  38Certain, very critical code has to clear the Q bit in the PSW.  What
  39happens when the Q bit is cleared is the CPU does not update the
  40registers interruption handlers read to find out where the machine
  41was interrupted - so if you get an interruption between the instruction
  42that clears the Q bit and the RFI that sets it again you don't know
  43where exactly it happened.  If you're lucky the IAOQ will point to the
  44instruction that cleared the Q bit, if you're not it points anywhere
  45at all.  Usually Q bit problems will show themselves in unexplainable
  46system hangs or running off the end of physical memory.
  47