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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1			In-kernel memory-mapped I/O tracing
2	
3	
4	Home page and links to optional user space tools:
5	
6		http://nouveau.freedesktop.org/wiki/MmioTrace
7	
8	MMIO tracing was originally developed by Intel around 2003 for their Fault
9	Injection Test Harness. In Dec 2006 - Jan 2007, using the code from Intel,
10	Jeff Muizelaar created a tool for tracing MMIO accesses with the Nouveau
11	project in mind. Since then many people have contributed.
12	
13	Mmiotrace was built for reverse engineering any memory-mapped IO device with
14	the Nouveau project as the first real user. Only x86 and x86_64 architectures
15	are supported.
16	
17	Out-of-tree mmiotrace was originally modified for mainline inclusion and
18	ftrace framework by Pekka Paalanen <pq@iki.fi>.
19	
20	
21	Preparation
22	-----------
23	
24	Mmiotrace feature is compiled in by the CONFIG_MMIOTRACE option. Tracing is
25	disabled by default, so it is safe to have this set to yes. SMP systems are
26	supported, but tracing is unreliable and may miss events if more than one CPU
27	is on-line, therefore mmiotrace takes all but one CPU off-line during run-time
28	activation. You can re-enable CPUs by hand, but you have been warned, there
29	is no way to automatically detect if you are losing events due to CPUs racing.
30	
31	
32	Usage Quick Reference
33	---------------------
34	
35	$ mount -t debugfs debugfs /sys/kernel/debug
36	$ echo mmiotrace > /sys/kernel/debug/tracing/current_tracer
37	$ cat /sys/kernel/debug/tracing/trace_pipe > mydump.txt &
38	Start X or whatever.
39	$ echo "X is up" > /sys/kernel/debug/tracing/trace_marker
40	$ echo nop > /sys/kernel/debug/tracing/current_tracer
41	Check for lost events.
42	
43	
44	Usage
45	-----
46	
47	Make sure debugfs is mounted to /sys/kernel/debug.
48	If not (requires root privileges):
49	$ mount -t debugfs debugfs /sys/kernel/debug
50	
51	Check that the driver you are about to trace is not loaded.
52	
53	Activate mmiotrace (requires root privileges):
54	$ echo mmiotrace > /sys/kernel/debug/tracing/current_tracer
55	
56	Start storing the trace:
57	$ cat /sys/kernel/debug/tracing/trace_pipe > mydump.txt &
58	The 'cat' process should stay running (sleeping) in the background.
59	
60	Load the driver you want to trace and use it. Mmiotrace will only catch MMIO
61	accesses to areas that are ioremapped while mmiotrace is active.
62	
63	During tracing you can place comments (markers) into the trace by
64	$ echo "X is up" > /sys/kernel/debug/tracing/trace_marker
65	This makes it easier to see which part of the (huge) trace corresponds to
66	which action. It is recommended to place descriptive markers about what you
67	do.
68	
69	Shut down mmiotrace (requires root privileges):
70	$ echo nop > /sys/kernel/debug/tracing/current_tracer
71	The 'cat' process exits. If it does not, kill it by issuing 'fg' command and
72	pressing ctrl+c.
73	
74	Check that mmiotrace did not lose events due to a buffer filling up. Either
75	$ grep -i lost mydump.txt
76	which tells you exactly how many events were lost, or use
77	$ dmesg
78	to view your kernel log and look for "mmiotrace has lost events" warning. If
79	events were lost, the trace is incomplete. You should enlarge the buffers and
80	try again. Buffers are enlarged by first seeing how large the current buffers
81	are:
82	$ cat /sys/kernel/debug/tracing/buffer_size_kb
83	gives you a number. Approximately double this number and write it back, for
84	instance:
85	$ echo 128000 > /sys/kernel/debug/tracing/buffer_size_kb
86	Then start again from the top.
87	
88	If you are doing a trace for a driver project, e.g. Nouveau, you should also
89	do the following before sending your results:
90	$ lspci -vvv > lspci.txt
91	$ dmesg > dmesg.txt
92	$ tar zcf pciid-nick-mmiotrace.tar.gz mydump.txt lspci.txt dmesg.txt
93	and then send the .tar.gz file. The trace compresses considerably. Replace
94	"pciid" and "nick" with the PCI ID or model name of your piece of hardware
95	under investigation and your nickname.
96	
97	
98	How Mmiotrace Works
99	-------------------
100	
101	Access to hardware IO-memory is gained by mapping addresses from PCI bus by
102	calling one of the ioremap_*() functions. Mmiotrace is hooked into the
103	__ioremap() function and gets called whenever a mapping is created. Mapping is
104	an event that is recorded into the trace log. Note that ISA range mappings
105	are not caught, since the mapping always exists and is returned directly.
106	
107	MMIO accesses are recorded via page faults. Just before __ioremap() returns,
108	the mapped pages are marked as not present. Any access to the pages causes a
109	fault. The page fault handler calls mmiotrace to handle the fault. Mmiotrace
110	marks the page present, sets TF flag to achieve single stepping and exits the
111	fault handler. The instruction that faulted is executed and debug trap is
112	entered. Here mmiotrace again marks the page as not present. The instruction
113	is decoded to get the type of operation (read/write), data width and the value
114	read or written. These are stored to the trace log.
115	
116	Setting the page present in the page fault handler has a race condition on SMP
117	machines. During the single stepping other CPUs may run freely on that page
118	and events can be missed without a notice. Re-enabling other CPUs during
119	tracing is discouraged.
120	
121	
122	Trace Log Format
123	----------------
124	
125	The raw log is text and easily filtered with e.g. grep and awk. One record is
126	one line in the log. A record starts with a keyword, followed by keyword-
127	dependent arguments. Arguments are separated by a space, or continue until the
128	end of line. The format for version 20070824 is as follows:
129	
130	Explanation	Keyword	Space-separated arguments
131	---------------------------------------------------------------------------
132	
133	read event	R	width, timestamp, map id, physical, value, PC, PID
134	write event	W	width, timestamp, map id, physical, value, PC, PID
135	ioremap event	MAP	timestamp, map id, physical, virtual, length, PC, PID
136	iounmap event	UNMAP	timestamp, map id, PC, PID
137	marker		MARK	timestamp, text
138	version		VERSION	the string "20070824"
139	info for reader	LSPCI	one line from lspci -v
140	PCI address map	PCIDEV	space-separated /proc/bus/pci/devices data
141	unk. opcode	UNKNOWN	timestamp, map id, physical, data, PC, PID
142	
143	Timestamp is in seconds with decimals. Physical is a PCI bus address, virtual
144	is a kernel virtual address. Width is the data width in bytes and value is the
145	data value. Map id is an arbitrary id number identifying the mapping that was
146	used in an operation. PC is the program counter and PID is process id. PC is
147	zero if it is not recorded. PID is always zero as tracing MMIO accesses
148	originating in user space memory is not yet supported.
149	
150	For instance, the following awk filter will pass all 32-bit writes that target
151	physical addresses in the range [0xfb73ce40, 0xfb800000[
152	
153	$ awk '/W 4 / { adr=strtonum($5); if (adr >= 0xfb73ce40 &&
154	adr < 0xfb800000) print; }'
155	
156	
157	Tools for Developers
158	--------------------
159	
160	The user space tools include utilities for:
161	- replacing numeric addresses and values with hardware register names
162	- replaying MMIO logs, i.e., re-executing the recorded writes
163	
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