Wednesday, November 11, 2009

evtest-capture and replaying events

Many of you are probably familiar with evtest, a small debugging utility written by Vojtek Pavlik to check what input device data is leaving the kernel. I use it a lot and one of the standard requests I make in bugreports is run evtest against the device file. The information it prints tells us the capabilities a device has and the events being sent when a certain action is performed.

I've used this information to add uinput device to my uinput test devices collection to debug various server and/or driver failures.
That worked for some cases but suffered from a major caveat: it was incredibly hard to reproduce issues that resulted from complex interactions. For months, I've been meaning to fix this and last weekend I finally had time to sit down and hack something up. That work is now in the evtest repository.

It works quite simple:
The user runs evtest-capture against the device and performs the action that reproduces the bug. evtest-capture saves the event stream into an xml file, this xml file can then be converted into a standalone uinput-based C program that resembles both the device and the interaction. I can re-create and run that program on my test box and reproduce and hopefully debug the issue.

The full set of steps is (as root where necessary):

$> evtest-capture /dev/input/event0 pointer-crashes.xml
$> xsltproc evtest-create-device.xsl pointer-crashes.xml > pointer-crash-device.c
$> gcc -o pointer-crash-device pointer-crash-device.c
$> ./pointer-crash-device
#verify the issue happens with this test device.

Like so many things, it's not perfect but it will hopefully help us reproduce a lot more bugs and thus make it simpler to find and address these bugs.

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