In the kernel graphics world, there has been a longstanding “line in the sand” that disallows merging
kernel drivers without a corresponding free-software user-space driver. The idea is that
not having a way to test the full functionality means that the kernel
developers cannot verify the proper functioning and security of the
driver; changes to the kernel driver may lead to unforeseen (and
untestable) problems on the user-space side. More recently, though, we
have seen other types of devices with complex drivers, but no useful free
user-space piece, that have been proposed for inclusion into the kernel;
at least one was merged, but the tide has perhaps turned against those types
of drivers at this point—or some of them, anyway.

Source: LWN.net – [$] Free user space for non-graphics drivers