linux-stable/Documentation/debugging-modules.txt

Debugging Modules after 2.6.3
-----------------------------

In almost all distributions, the kernel asks for modules which don't
exist, such as "net-pf-10" or whatever.  Changing "modprobe -q" to
"succeed" in this case is hacky and breaks some setups, and also we
want to know if it failed for the fallback code for old aliases in
fs/char_dev.c, for example.

In the past a debugging message which would fill people's logs was
emitted.  This debugging message has been removed.  The correct way
of debugging module problems is something like this:

echo '#! /bin/sh' > /tmp/modprobe
echo 'echo "$@" >> /tmp/modprobe.log' >> /tmp/modprobe
echo 'exec /sbin/modprobe "$@"' >> /tmp/modprobe
chmod a+x /tmp/modprobe
echo /tmp/modprobe > /proc/sys/kernel/modprobe

Note that the above applies only when the *kernel* is requesting
that the module be loaded -- it won't have any effect if that module
is being loaded explicitly using "modprobe" from userspace.