1 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 28activation. 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. 30 31 32Usage 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 & 38Start X or whatever. 39$ echo "X is up" > /sys/kernel/debug/tracing/trace_marker 40$ echo nop > /sys/kernel/debug/tracing/current_tracer 41Check for lost events. 42 43 44Usage 45----- 46 47Make sure debugfs is mounted to /sys/kernel/debug. If not, (requires root privileges) 48$ mount -t debugfs debugfs /sys/kernel/debug 49 50Check that the driver you are about to trace is not loaded. 51 52Activate mmiotrace (requires root privileges): 53$ echo mmiotrace > /sys/kernel/debug/tracing/current_tracer 54 55Start storing the trace: 56$ cat /sys/kernel/debug/tracing/trace_pipe > mydump.txt & 57The '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 62During tracing you can place comments (markers) into the trace by 63$ echo "X is up" > /sys/kernel/debug/tracing/trace_marker 64This 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): 69$ echo nop > /sys/kernel/debug/tracing/current_tracer 70The 'cat' process exits. If it does not, kill it by issuing 'fg' command and 71pressing ctrl+c. 72 73Check 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: 81$ cat /sys/kernel/debug/tracing/buffer_size_kb 82gives you a number. Approximately double this number and write it back, for 83instance: 84$ echo 128000 > /sys/kernel/debug/tracing/buffer_size_kb 85Then start again from the top. 86 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 162 163 164