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 ===================================
 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
 
 

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