What is an MMIO Trace

MMIO (Memory Mapped I/O) tracing is a technique that can be used to reverse-engineer hardware drivers. It works by intercepting all the reads and writes by a driver to memory, recording how the driver is poking the hardware and what hardware state it reads. This tool was partially written, has been extensively used, by the Nouveau project to reverse-engineer the nVidia binary drivers.

Tracing the binary nVidia drivers

Some bugs in the Nouveau drivers can best be solved by working out what the binary nVidia drivers do and adapting that. This is particularly true for new hardware support and where the nouveau drivers fail to activate a monitor correctly.


  • Kernel 2.6.32-17 or newer. The mmiotrace kernel framework was enabled in the 2.6.32-17-generic kernel in the Lucid development cycle. You'll need to be running a kernel at least as new as this. An appropriate kernel is included in Lucid Beta 1 and later.
  • Working nVidia restricted drivers. We'll be tracing how the binary drivers interact with the graphics card, so we need the nVidia drivers to be working. You can enable the nVidia drivers through the Hardware Drivers configuration applet in System→Administration→Hardware Drivers. Boot at least once normally to check that the nVidia drivers are working correctly.


The initial set up for an MMIO trace needs to be done before the driver is loaded. This means that we need to do it outside of X.

Boot your system in “recovery mode”. You can select this from the GRUB boot menu. If the GRUB menu is not shown by default you can bring it up early in your boot process by pressing the “shift” key. Select “Drop to a root shell prompt” from the recovery menu to bring up a terminal.

From here we will ensure that the nvidia driver is not loaded, enable the mmio tracer, load the nvidia driver, perform the action we want to trace, then collect the log.

Ensure the nvidia driver is not loaded

Run lsmod | grep nvidia. This should not produce any output. If it returns some output like

nvidia               9932176  5 
agpgart                31724  2 nvidia,intel_agp

then you need to remove the nvidia module by running rmmod nvidia. Run lsmod | grep nvidia again to check.

Enable the mmio tracer

First set the buffer size to a reasonable value. If this is too small some data will be lost and you will need to re-run the trace with a larger buffer.

This sets a reasonable default size of 64MiB:

root@Ein:~# echo 64000 > /sys/kernel/debug/tracing/buffer_size_kb

Enable the mmio tracer, and start recording the log:

root@Ein:~# echo mmiotrace > /sys/kernel/debug/tracing/current_tracer
root@Ein:~# cat /sys/kernel/debug/tracing/trace_pipe > mmiotrace.log &

Now load the nvidia driver. If you're using the nvidia-current package you can do that with

root@Ein:~# modprobe nvidia-current

If you are using nvidia-173 or nvidia-96 you can replace “nvidia-current” with the package name.

Run the trace

Now we start X and run a command to exercise the functionality of the nvidia drivers that we need to check. Because the mmio tracer intercepts all access to the graphics card, this makes X much slower - it is not unusual for X to take a couple of minutes to start up while the trace is in progress.

For resolution / display bugs

For bugs where the nouveau drivers aren't able to display anything correctly, we just need the logs of the nvidia drivers bringing up the display. For this, we just need to start X. Running

root@Ein:~# xinit "sleep 10"

will start X, wait for 10 seconds, and then quit X.

For acceleration bugs

For bugs and missing features in nouveau's acceleration code we need to exercise the nvidia driver's 3D engine. To do this, we start X and run a simple OpenGL program, glxgears.


root@Ein:~# xinit -e sh -c "glxgears & sleep 10"

will start X, run glxgears for 10 seconds, then quit X.

For dual-head bugs

For bugs and missing features in nouveau's handling of secondary monitors we want to trace how the nvidia driver sets up the second monitor. For this, we want to run nvidia-settings and enable the secondary monitor. Running

root@Ein:~# xinit nvidia-settings

will start X and nvidia-settings. Quitting nvidia-settings will quit X.

Check the trace

Now we stop tracing and check that the trace is good.

root@Ein:~# echo nop > /sys/kernel/debug/tracing/current_tracer
root@Ein:~# fg
[1]   + running   cat /sys/kernel/debug/tracing/trace_pipe > mmiotrace.log
(Press <Ctrl>+C to finish the log)

Check that there were no lost events. Running grep -i lost mmiotrace.log should return no output, like this:

root@Ein:~# grep -i lost mmiotrace.log

If this returns some output then your trace has lost some events and is incomplete. You need to re-run the trace from step 1 but with a larger buffer size. Generally

X/MMIOTracing (last edited 2010-04-21 00:28:37 by raof)