I did a search for GPIO -- I do not see anyone mentioning using it. I have read the Plan 9 papers, etc, never tried it yet. One of the advantages of Plan 9 as I see it, is to have the Rpi and another computer running Plan 9, connected by a network, and then the second computer can access the Rpi's GPIO, just as easily as the Rpi does. If this works at all, and it works with some timeliness in data transfer, and it can be done with several RPi's and one or more computers then, there is no limit to the multiprocessor / centralized / distributed hardware control possibilities, where the cluster communication details are seamlessly handled by an advanced operating system. I have not read enough about Plan 9 yet, but I am assuming a Plan 9 program can create a synthetic device file for use by other programs, ideally used by a higher level program for hierarchical control ?

The Rpi as part of a Plan 9 network would seem to be an automation / robotics student's dream.

Can the GPIO be handled as a file, and if so, what is the format ? Can anyone give example code to turn a light on and off by writing to a 9P file ?