I don't believe all of the drivers for the rPi use closed source code. I believe it is just the graphics unit. The documentation is closed for competitive reasons. Not that you would ever notice when running the rPi.
But, I believe this is out of date. In 2014 the sources for the Broadcom VideoCore GPU were released. It's not totally clear to me how much this opens the core, but it appears that significant progress has been made to developing open source drivers now.
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Anyone know how much of the binary "blob" is still in use and how much the graphics unit is driven by FOSS?
Any other non-FOSS required to run the rPi?