[mirror_ubuntu-hirsute-kernel.git] / Documentation / parisc / debugging.rst

=================
PA-RISC Debugging
=================

okay, here are some hints for debugging the lower-level parts of
linux/parisc.


1. Absolute addresses
=====================

A lot of the assembly code currently runs in real mode, which means
absolute addresses are used instead of virtual addresses as in the
rest of the kernel.  To translate an absolute address to a virtual
address you can lookup in System.map, add __PAGE_OFFSET (0x10000000
currently).


2. HPMCs
========

When real-mode code tries to access non-existent memory, you'll get
an HPMC instead of a kernel oops.  To debug an HPMC, try to find
the System Responder/Requestor addresses.  The System Requestor
address should match (one of the) processor HPAs (high addresses in
the I/O range); the System Responder address is the address real-mode
code tried to access.

Typical values for the System Responder address are addresses larger
than __PAGE_OFFSET (0x10000000) which mean a virtual address didn't
get translated to a physical address before real-mode code tried to
access it.


3. Q bit fun
============

Certain, very critical code has to clear the Q bit in the PSW.  What
happens when the Q bit is cleared is the CPU does not update the
registers interruption handlers read to find out where the machine
was interrupted - so if you get an interruption between the instruction
that clears the Q bit and the RFI that sets it again you don't know
where exactly it happened.  If you're lucky the IAOQ will point to the
instruction that cleared the Q bit, if you're not it points anywhere
at all.  Usually Q bit problems will show themselves in unexplainable
system hangs or running off the end of physical memory.
Commit	Line	Data
e77e9187 MCC	1	=================
	2	PA-RISC Debugging
	3	=================
	4
1da177e4 LT	5	okay, here are some hints for debugging the lower-level parts of
	6	linux/parisc.
	7
	8
	9	1. Absolute addresses
e77e9187	10	=====================
1da177e4 LT	11
	12	A lot of the assembly code currently runs in real mode, which means
	13	absolute addresses are used instead of virtual addresses as in the
	14	rest of the kernel. To translate an absolute address to a virtual
	15	address you can lookup in System.map, add __PAGE_OFFSET (0x10000000
	16	currently).
	17
	18
	19	2. HPMCs
e77e9187	20	========
1da177e4 LT	21
	22	When real-mode code tries to access non-existent memory, you'll get
	23	an HPMC instead of a kernel oops. To debug an HPMC, try to find
	24	the System Responder/Requestor addresses. The System Requestor
	25	address should match (one of the) processor HPAs (high addresses in
	26	the I/O range); the System Responder address is the address real-mode
	27	code tried to access.
	28
	29	Typical values for the System Responder address are addresses larger
	30	than __PAGE_OFFSET (0x10000000) which mean a virtual address didn't
	31	get translated to a physical address before real-mode code tried to
	32	access it.
	33
	34
	35	3. Q bit fun
e77e9187	36	============
1da177e4 LT	37
	38	Certain, very critical code has to clear the Q bit in the PSW. What
	39	happens when the Q bit is cleared is the CPU does not update the
	40	registers interruption handlers read to find out where the machine
	41	was interrupted - so if you get an interruption between the instruction
	42	that clears the Q bit and the RFI that sets it again you don't know
	43	where exactly it happened. If you're lucky the IAOQ will point to the
c94bed8e	44	instruction that cleared the Q bit, if you're not it points anywhere
1da177e4 LT	45	at all. Usually Q bit problems will show themselves in unexplainable
1da177e4 LT	46	system hangs or running off the end of physical memory.