mmiotrace: add user documentation

		In-kernel memory-mapped I/O tracing

Home page and links to optional user space tools:

	http://nouveau.freedesktop.org/wiki/MmioTrace

MMIO tracing was originally developed by Intel around 2003 for their Fault

Injection Test Harness. In Dec 2006 - Jan 2007, using the code from Intel,

Jeff Muizelaar created a tool for tracing MMIO accesses with the Nouveau

project in mind. Since then many people have contributed.

Mmiotrace was built for reverse engineering any memory-mapped IO device with

the Nouveau project as the first real user. Only x86 and x86_64 architectures

are supported.

Out-of-tree mmiotrace was originally modified for mainline inclusion and

ftrace framework by Pekka Paalanen <pq@iki.fi>.

Preparation

-----------

Mmiotrace feature is compiled in by the CONFIG_MMIOTRACE option. Tracing is

disabled by default, so it is safe to have this set to yes. SMP systems are

supported, but tracing is unreliable and may miss events if more than one CPU

is on-line, therefore mmiotrace takes all but one CPU off-line during run-time

activation [not implemented].

Usage Quick Reference

---------------------

$ mount -t debugfs debugfs /debug

$ echo mmiotrace > /debug/tracing/current_tracer

$ cat /debug/tracing/trace_pipe > mydump.txt &

Start X or whatever.

$ echo "X is up" > /debug/tracing/marker

$ echo none > /debug/tracing/current_tracer

Check kernel log.

Usage

-----

Make sure debugfs is mounted to /debug. If not, (requires root privileges)

$ mount -t debugfs debugfs /debug

Check that the driver you are about to trace is not loaded.

Activate mmiotrace (requires root privileges):

$ echo mmiotrace > /debug/tracing/current_tracer

Start storing the trace:

$ cat /debug/tracing/trace_pipe > mydump.txt &

The 'cat' process should stay running (sleeping) in the background.

Load the driver you want to trace and use it. Mmiotrace will only catch MMIO

accesses to areas that are ioremapped while mmiotrace is active.

[Unimplemented feature:]

During tracing you can place comments (markers) into the trace by

$ echo "X is up" > /debug/tracing/marker

This makes it easier to see which part of the (huge) trace corresponds to

which action. It is recommended to place descriptive markers about what you

do.

Shut down mmiotrace (requires root privileges):

$ echo none > /debug/tracing/current_tracer

The 'cat' process exits. If it does not, kill it by 'fg' and pressing ctrl+c.

[This feature is not implemented yet!]

Check your kernel log. If there are messages about mmiotrace losing events,

this is due to buffer overrun, and the trace is incomplete. You should enlarge

the buffers and try again. [How?]

If you are doing a trace for a driver project, e.g. Nouveau, you should also

do the following before sending your results:

$ lspci -vvv > lspci.txt

$ dmesg > dmesg.txt

$ tar zcf pciid-nick-mmiotrace.tar.gz mydump.txt lspci.txt dmesg.txt

and then send the .tar.gz file. The trace compresses considerably. Replace

"pciid" and "nick" with the PCI ID or model name of your piece of hardware

under investigation and your nick name.

How Mmiotrace Works

-------------------

Access to hardware IO-memory is gained by mapping addresses from PCI bus by

calling one of the ioremap_*() functions. Mmiotrace is hooked into the

__ioremap() function and gets called whenever a mapping is created. Mapping is

an event that is recorded into the trace log. Note, that ISA range mappings

are not caught, since the mapping always exists and is returned directly.

MMIO accesses are recorded via page faults. Just before __ioremap() returns,

the mapped pages are marked as not present. Any access to the pages causes a

fault. The page fault handler calls mmiotrace to handle the fault. Mmiotrace

marks the page present, sets TF flag to achieve single stepping and exits the

fault handler. The instruction that faulted is executed and debug trap is

entered. Here mmiotrace again marks the page as not present. The instruction

is decoded to get the type of operation (read/write), data width and the value

read or written. These are stored to the trace log.

Setting the page present in the page fault handler has a race condition on SMP

machines. During the single stepping other CPUs may run freely on that page

and events can be missed without a notice. Re-enabling other CPUs during

tracing is discouraged.

Trace Log Format

----------------

The raw log is text and easily filtered with e.g. grep and awk. One record is

one line in the log. A record starts with a keyword, followed by keyword

dependant arguments. Arguments are separated by a space, or continue until the

end of line. The format for version 20070824 is as follows:

Explanation	Keyword	Space separated arguments

---------------------------------------------------------------------------

read event	R	width, timestamp, map id, physical, value, PC, PID

write event	W	width, timestamp, map id, physical, value, PC, PID

ioremap event	MAP	timestamp, map id, physical, virtual, length, PC, PID

iounmap event	UNMAP	timestamp, map id, PC, PID

marker		MARK	timestamp, text

version		VERSION	the string "20070824"

info for reader	LSPCI	one line from lspci -v

PCI address map	PCIDEV	space separated /proc/bus/pci/devices data

unk. opcode	UNKNOWN	timestamp, map id, physical, data, PC, PID

Timestamp is in seconds with decimals. Physical is a PCI bus address, virtual

is a kernel virtual address. Width is the data width in bytes and value is the

data value. Map id is an arbitrary id number identifying the mapping that was

used in an operation. PC is the program counter and PID is process id. PC is

zero if it is not recorded. PID is always zero as tracing MMIO accesses

originating in user space memory is not yet supported.

For instance, the following awk filter will pass all 32-bit writes that target

physical addresses in the range [0xfb73ce40, 0xfb800000[

$ awk '/W 4 / { adr=strtonum($5); if (adr >= 0xfb73ce40 &&

adr < 0xfb800000) print; }'

Tools for Developers

--------------------

The user space tools include utilities for:

- replacing numeric addresses and values with hardware register names

- replaying MMIO logs, i.e., re-executing the recorded writes

	help

	  Mmiotrace traces Memory Mapped I/O access and is meant for

	  debugging and reverse engineering. It is called from the ioremap

	  implementation and works via page faults. A user space program is

	  required to collect the MMIO data from debugfs files.

	  Tracing is disabled by default and can be enabled from a debugfs

	  file.

	  implementation and works via page faults. Tracing is disabled by

	  default and can be enabled run-time.

	  See http://nouveau.freedesktop.org/wiki/MmioTrace

	  See Documentation/tracers/mmiotrace.txt.

	  If you are not helping to develop drivers, say N.

config MMIOTRACE_TEST