Configure USB gadgets with Nix (PoC)
I am not sure this is a good idea, but I am presenting it for feedback.
There are a number of tools to create the USB gadget configfs structure appropriate to a particular application of composite gadgets, and they all seem to accept a data file which is only a light transformation of the config directory they output.
Which leads me to wonder whether we could "just" create the config directory directly from a Nix nested attrset.
What it does
Given an attrset like [usb-gadget.nix], create a USB composite gadget.
make-path-tree.nix is a general-purpose tool which accepts an
attrset and writes a shell script which runs "mkdir" and "echo" to
turn it into filesystem, then default.nix calls the resulting script
and does extra time-dependent configuration:
-
configfs doesn't allow dangling symlinks, so we can't make the links from functions to configs until the functions exist
-
the write to
UDCis apparently the Big Switch that turns the whole thing on, so that must be done last
Demo
$ nix-build .
$ cat result # observe, it is a shell script
$ sudo bash ./result # works only on target hardware
Missing bits
I haven't dealt yet with removing the configuration. The brute force way to do this is
# echo "" > UDC
# find usb_gadget -type l |xargs rm
# find usb_gadget -depth -type d |xargs rmdir