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diff --git a/Documentation/trace/mmiotrace.rst b/Documentation/trace/mmiotrace.rst new file mode 100644 index 000000000..fed13eaea --- /dev/null +++ b/Documentation/trace/mmiotrace.rst @@ -0,0 +1,184 @@ +=================================== +In-kernel memory-mapped I/O tracing +=================================== + + +Home page and links to optional user space tools: + + https://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. You can re-enable CPUs by hand, but you have been warned, there +is no way to automatically detect if you are losing events due to CPUs racing. + + +Usage Quick Reference +--------------------- +:: + + $ mount -t debugfs debugfs /sys/kernel/debug + $ echo mmiotrace > /sys/kernel/debug/tracing/current_tracer + $ cat /sys/kernel/debug/tracing/trace_pipe > mydump.txt & + Start X or whatever. + $ echo "X is up" > /sys/kernel/debug/tracing/trace_marker + $ echo nop > /sys/kernel/debug/tracing/current_tracer + Check for lost events. + + +Usage +----- + +Make sure debugfs is mounted to /sys/kernel/debug. +If not (requires root privileges):: + + $ mount -t debugfs debugfs /sys/kernel/debug + +Check that the driver you are about to trace is not loaded. + +Activate mmiotrace (requires root privileges):: + + $ echo mmiotrace > /sys/kernel/debug/tracing/current_tracer + +Start storing the trace:: + + $ cat /sys/kernel/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. + +During tracing you can place comments (markers) into the trace by +$ echo "X is up" > /sys/kernel/debug/tracing/trace_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 nop > /sys/kernel/debug/tracing/current_tracer + +The 'cat' process exits. If it does not, kill it by issuing 'fg' command and +pressing ctrl+c. + +Check that mmiotrace did not lose events due to a buffer filling up. Either:: + + $ grep -i lost mydump.txt + +which tells you exactly how many events were lost, or use:: + + $ dmesg + +to view your kernel log and look for "mmiotrace has lost events" warning. If +events were lost, the trace is incomplete. You should enlarge the buffers and +try again. Buffers are enlarged by first seeing how large the current buffers +are:: + + $ cat /sys/kernel/debug/tracing/buffer_size_kb + +gives you a number. Approximately double this number and write it back, for +instance:: + + $ echo 128000 > /sys/kernel/debug/tracing/buffer_size_kb + +Then start again from the top. + +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 nickname. + + +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- +dependent 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 + + |