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Pekka Paalanenc6c67c12008-05-12 21:20:59 +02001 In-kernel memory-mapped I/O tracing
2
3
4Home page and links to optional user space tools:
5
6 http://nouveau.freedesktop.org/wiki/MmioTrace
7
8MMIO tracing was originally developed by Intel around 2003 for their Fault
9Injection Test Harness. In Dec 2006 - Jan 2007, using the code from Intel,
10Jeff Muizelaar created a tool for tracing MMIO accesses with the Nouveau
11project in mind. Since then many people have contributed.
12
13Mmiotrace was built for reverse engineering any memory-mapped IO device with
14the Nouveau project as the first real user. Only x86 and x86_64 architectures
15are supported.
16
17Out-of-tree mmiotrace was originally modified for mainline inclusion and
18ftrace framework by Pekka Paalanen <pq@iki.fi>.
19
20
21Preparation
22-----------
23
24Mmiotrace feature is compiled in by the CONFIG_MMIOTRACE option. Tracing is
25disabled by default, so it is safe to have this set to yes. SMP systems are
26supported, but tracing is unreliable and may miss events if more than one CPU
27is on-line, therefore mmiotrace takes all but one CPU off-line during run-time
Pekka Paalanen6f6f3942008-05-12 21:21:03 +020028activation. You can re-enable CPUs by hand, but you have been warned, there
29is no way to automatically detect if you are losing events due to CPUs racing.
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020030
31
32Usage Quick Reference
33---------------------
34
GeunSik Lim156f5a72009-06-02 15:01:37 +090035$ 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 &
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020038Start X or whatever.
GeunSik Lim156f5a72009-06-02 15:01:37 +090039$ echo "X is up" > /sys/kernel/debug/tracing/trace_marker
40$ echo nop > /sys/kernel/debug/tracing/current_tracer
Pekka Paalanen6f6f3942008-05-12 21:21:03 +020041Check for lost events.
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020042
43
44Usage
45-----
46
GeunSik Lim156f5a72009-06-02 15:01:37 +090047Make sure debugfs is mounted to /sys/kernel/debug. If not, (requires root privileges)
48$ mount -t debugfs debugfs /sys/kernel/debug
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020049
50Check that the driver you are about to trace is not loaded.
51
52Activate mmiotrace (requires root privileges):
GeunSik Lim156f5a72009-06-02 15:01:37 +090053$ echo mmiotrace > /sys/kernel/debug/tracing/current_tracer
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020054
55Start storing the trace:
GeunSik Lim156f5a72009-06-02 15:01:37 +090056$ cat /sys/kernel/debug/tracing/trace_pipe > mydump.txt &
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020057The 'cat' process should stay running (sleeping) in the background.
58
59Load the driver you want to trace and use it. Mmiotrace will only catch MMIO
60accesses to areas that are ioremapped while mmiotrace is active.
61
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020062During tracing you can place comments (markers) into the trace by
GeunSik Lim156f5a72009-06-02 15:01:37 +090063$ echo "X is up" > /sys/kernel/debug/tracing/trace_marker
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020064This makes it easier to see which part of the (huge) trace corresponds to
65which action. It is recommended to place descriptive markers about what you
66do.
67
68Shut down mmiotrace (requires root privileges):
GeunSik Lim156f5a72009-06-02 15:01:37 +090069$ echo nop > /sys/kernel/debug/tracing/current_tracer
Pekka Paalanen6f6f3942008-05-12 21:21:03 +020070The 'cat' process exits. If it does not, kill it by issuing 'fg' command and
71pressing ctrl+c.
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020072
Pekka Paalanen6f6f3942008-05-12 21:21:03 +020073Check that mmiotrace did not lose events due to a buffer filling up. Either
74$ grep -i lost mydump.txt
75which tells you exactly how many events were lost, or use
76$ dmesg
77to view your kernel log and look for "mmiotrace has lost events" warning. If
78events were lost, the trace is incomplete. You should enlarge the buffers and
79try again. Buffers are enlarged by first seeing how large the current buffers
80are:
GeunSik Lim156f5a72009-06-02 15:01:37 +090081$ cat /sys/kernel/debug/tracing/buffer_size_kb
Pekka Paalanen6f6f3942008-05-12 21:21:03 +020082gives you a number. Approximately double this number and write it back, for
83instance:
GeunSik Lim156f5a72009-06-02 15:01:37 +090084$ echo 128000 > /sys/kernel/debug/tracing/buffer_size_kb
Pekka Paalanen6f6f3942008-05-12 21:21:03 +020085Then start again from the top.
Pekka Paalanenc6c67c12008-05-12 21:20:59 +020086
87If you are doing a trace for a driver project, e.g. Nouveau, you should also
88do the following before sending your results:
89$ lspci -vvv > lspci.txt
90$ dmesg > dmesg.txt
91$ tar zcf pciid-nick-mmiotrace.tar.gz mydump.txt lspci.txt dmesg.txt
92and then send the .tar.gz file. The trace compresses considerably. Replace
93"pciid" and "nick" with the PCI ID or model name of your piece of hardware
94under investigation and your nick name.
95
96
97How Mmiotrace Works
98-------------------
99
100Access to hardware IO-memory is gained by mapping addresses from PCI bus by
101calling one of the ioremap_*() functions. Mmiotrace is hooked into the
102__ioremap() function and gets called whenever a mapping is created. Mapping is
103an event that is recorded into the trace log. Note, that ISA range mappings
104are not caught, since the mapping always exists and is returned directly.
105
106MMIO accesses are recorded via page faults. Just before __ioremap() returns,
107the mapped pages are marked as not present. Any access to the pages causes a
108fault. The page fault handler calls mmiotrace to handle the fault. Mmiotrace
109marks the page present, sets TF flag to achieve single stepping and exits the
110fault handler. The instruction that faulted is executed and debug trap is
111entered. Here mmiotrace again marks the page as not present. The instruction
112is decoded to get the type of operation (read/write), data width and the value
113read or written. These are stored to the trace log.
114
115Setting the page present in the page fault handler has a race condition on SMP
116machines. During the single stepping other CPUs may run freely on that page
117and events can be missed without a notice. Re-enabling other CPUs during
118tracing is discouraged.
119
120
121Trace Log Format
122----------------
123
124The raw log is text and easily filtered with e.g. grep and awk. One record is
125one line in the log. A record starts with a keyword, followed by keyword
126dependant arguments. Arguments are separated by a space, or continue until the
127end of line. The format for version 20070824 is as follows:
128
129Explanation Keyword Space separated arguments
130---------------------------------------------------------------------------
131
132read event R width, timestamp, map id, physical, value, PC, PID
133write event W width, timestamp, map id, physical, value, PC, PID
134ioremap event MAP timestamp, map id, physical, virtual, length, PC, PID
135iounmap event UNMAP timestamp, map id, PC, PID
136marker MARK timestamp, text
137version VERSION the string "20070824"
138info for reader LSPCI one line from lspci -v
139PCI address map PCIDEV space separated /proc/bus/pci/devices data
140unk. opcode UNKNOWN timestamp, map id, physical, data, PC, PID
141
142Timestamp is in seconds with decimals. Physical is a PCI bus address, virtual
143is a kernel virtual address. Width is the data width in bytes and value is the
144data value. Map id is an arbitrary id number identifying the mapping that was
145used in an operation. PC is the program counter and PID is process id. PC is
146zero if it is not recorded. PID is always zero as tracing MMIO accesses
147originating in user space memory is not yet supported.
148
149For instance, the following awk filter will pass all 32-bit writes that target
150physical addresses in the range [0xfb73ce40, 0xfb800000[
151
152$ awk '/W 4 / { adr=strtonum($5); if (adr >= 0xfb73ce40 &&
153adr < 0xfb800000) print; }'
154
155
156Tools for Developers
157--------------------
158
159The user space tools include utilities for:
160- replacing numeric addresses and values with hardware register names
161- replaying MMIO logs, i.e., re-executing the recorded writes
162
163