On this device we don't need the openwrt kernel or patches. The
newer kernel also fixes the weird one minute pause at boot when
it was doing something with either mmc or switch.
This changes the practice for building kernel modules: now we expect
that the appropriate Kconfig symbols are set to =m in
config.kernel.config, and then use pkgs.kmodloader to create
a service that loads and unloads all the modules depended on by
a particular requirement.
Note that modules won't be installed on the target device just by
virue of having been built: only the modules that are referenced by a
kmodloader package will be in the closure.
An example may make this clearer: see modules/firewall/default.nix
in this commit.
Why?
If you have a compiled Linux kernel source tree and you change some
symbol from "is not set" to m and then run make modules, you cannot in
general expect that newly compiled module to work. This is because
there are places in the build of the main kernel where it looks to see
which modules _may_ be defined and uses that information to
accommodate them.
For example in an in-kernel build of
https://github.com/torvalds/linux/blob/master/net/netfilter/core.c#L689
some symbols are defined only if CONFIG_NF_CONNTRACK is set, meaning
this code won't work if we have it unset initially then try later to
enable it and build modules only. Or see
https://github.com/torvalds/linux/blob/master/include/linux/netdevice.h#L160
this is step one towards getting rid of rotuer-secrets completely and
turning rotuer into a "profile" module that can be less hackily
customised for other people's networks
imagine: you are using a device that requires
CONFIG_MYDEVICE_FROBOZZ_DRIVER but only if CONFIG_FROBOZZ has been
specified elsewhere. Because we check that every requested config
symbol actually appears in .config then it can't be added
unconditionally or the build will fail if CONFIG_FROBOZZ wasn't asked
for.
I'm not 100% happy about this design but it's the best I've thought of
so far.
