"In use by some other piece of code" is the usual criteria. I.e. a driver using your added shared infrastructure, a userspace program talking to the interface added, etc.
The actual hardware being more than dreams in a simulator also helps, of course.
Posted Feb 14, 2009 0:34 UTC (Sat) by giraffedata (subscriber, #1954)
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I can remember various LWN articles about some proposed feature where kernel developers argued that it needed to be used out of tree and shipped with distributions for a while to prove its worthiness before joining the kernel.org major league. I believe these were major functions, though.