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. 48If not (requires root privileges): 49$ mount -t debugfs debugfs /sys/kernel/debug 50 51Check that the driver you are about to trace is not loaded. 52 53Activate mmiotrace (requires root privileges): 54$ echo mmiotrace > /sys/kernel/debug/tracing/current_tracer 55 56Start storing the trace: 57$ cat /sys/kernel/debug/tracing/trace_pipe > mydump.txt & 58The 'cat' process should stay running (sleeping) in the background. 59 60Load the driver you want to trace and use it. Mmiotrace will only catch MMIO 61accesses to areas that are ioremapped while mmiotrace is active. 62 63During tracing you can place comments (markers) into the trace by 64$ echo "X is up" > /sys/kernel/debug/tracing/trace_marker 65This makes it easier to see which part of the (huge) trace corresponds to 66which action. It is recommended to place descriptive markers about what you 67do. 68 69Shut down mmiotrace (requires root privileges): 70$ echo nop > /sys/kernel/debug/tracing/current_tracer 71The 'cat' process exits. If it does not, kill it by issuing 'fg' command and 72pressing ctrl+c. 73 74Check that mmiotrace did not lose events due to a buffer filling up. Either 75$ grep -i lost mydump.txt 76which tells you exactly how many events were lost, or use 77$ dmesg 78to view your kernel log and look for "mmiotrace has lost events" warning. If 79events were lost, the trace is incomplete. You should enlarge the buffers and 80try again. Buffers are enlarged by first seeing how large the current buffers 81are: 82$ cat /sys/kernel/debug/tracing/buffer_size_kb 83gives you a number. Approximately double this number and write it back, for 84instance: 85$ echo 128000 > /sys/kernel/debug/tracing/buffer_size_kb 86Then start again from the top. 87 88If you are doing a trace for a driver project, e.g. Nouveau, you should also 89do 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 93and 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 95under investigation and your nickname. 96 97 98How Mmiotrace Works 99------------------- 100 101Access to hardware IO-memory is gained by mapping addresses from PCI bus by 102calling one of the ioremap_*() functions. Mmiotrace is hooked into the 103__ioremap() function and gets called whenever a mapping is created. Mapping is 104an event that is recorded into the trace log. Note that ISA range mappings 105are not caught, since the mapping always exists and is returned directly. 106 107MMIO accesses are recorded via page faults. Just before __ioremap() returns, 108the mapped pages are marked as not present. Any access to the pages causes a 109fault. The page fault handler calls mmiotrace to handle the fault. Mmiotrace 110marks the page present, sets TF flag to achieve single stepping and exits the 111fault handler. The instruction that faulted is executed and debug trap is 112entered. Here mmiotrace again marks the page as not present. The instruction 113is decoded to get the type of operation (read/write), data width and the value 114read or written. These are stored to the trace log. 115 116Setting the page present in the page fault handler has a race condition on SMP 117machines. During the single stepping other CPUs may run freely on that page 118and events can be missed without a notice. Re-enabling other CPUs during 119tracing is discouraged. 120 121 122Trace Log Format 123---------------- 124 125The raw log is text and easily filtered with e.g. grep and awk. One record is 126one line in the log. A record starts with a keyword, followed by keyword- 127dependent arguments. Arguments are separated by a space, or continue until the 128end of line. The format for version 20070824 is as follows: 129 130Explanation Keyword Space-separated arguments 131--------------------------------------------------------------------------- 132 133read event R width, timestamp, map id, physical, value, PC, PID 134write event W width, timestamp, map id, physical, value, PC, PID 135ioremap event MAP timestamp, map id, physical, virtual, length, PC, PID 136iounmap event UNMAP timestamp, map id, PC, PID 137marker MARK timestamp, text 138version VERSION the string "20070824" 139info for reader LSPCI one line from lspci -v 140PCI address map PCIDEV space-separated /proc/bus/pci/devices data 141unk. opcode UNKNOWN timestamp, map id, physical, data, PC, PID 142 143Timestamp is in seconds with decimals. Physical is a PCI bus address, virtual 144is a kernel virtual address. Width is the data width in bytes and value is the 145data value. Map id is an arbitrary id number identifying the mapping that was 146used in an operation. PC is the program counter and PID is process id. PC is 147zero if it is not recorded. PID is always zero as tracing MMIO accesses 148originating in user space memory is not yet supported. 149 150For instance, the following awk filter will pass all 32-bit writes that target 151physical addresses in the range [0xfb73ce40, 0xfb800000[ 152 153$ awk '/W 4 / { adr=strtonum($5); if (adr >= 0xfb73ce40 && 154adr < 0xfb800000) print; }' 155 156 157Tools for Developers 158-------------------- 159 160The 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 163 164 165