Currently, kernel config options whose value is "yes" always override
options whose value is "no".
This is not always desired.
Generally speaking, if someone defines an option to have the value
"no", presumably they are disabling the option for a reason, so it's
not always OK to silently enable it due to another, probably unrelated
reason.
For example, a user may want to reduce the kernel attack surface and
therefore may want to disable features that are being enabled in
common-config.nix.
In fact, common-config.nix was already silently enabling options that
were intended to be disabled in hardened/config.nix for security
reasons, such as INET_DIAG.
By eliminating the custom merge function, these config options will
now use the default module option merge functions which make sure
that all options with the highest priority have the same value.
A user that wishes to override an option defined in common-config.nix
can currently use mkForce or mkOverride to do so, e.g.:
BINFMT_MISC = mkForce (option no);
That said, this is not going to be necessary in the future, because
the plan is for kernel config options defined in nixpkgs to use a
lower priority by default, like it currently happens for other module
options.
This should make the composability of kernel configurations more straigthforward.
- now distinguish freeform options from tristate ones
- will look for a structured config in kernelPatches too
one can now access the structuredConfig from a kernel via linux_test.configfile.structuredConfig
in order to reinject it into another kernel, no need to rewrite the config from scratch
The following merge strategies are used in case of conflict:
-- freeform items must be equal or they conflict (mergeEqualOption)
-- for tristate (y/m/n) entries, I use the mergeAnswer strategy which takes the best available value, "best" being defined by the user (by default "y" > "m" > "n", e.g. if one entry is both marked "y" and "n", "y" wins)
-- if one item is both marked optional/mandatory, mandatory wins (mergeFalseByDefault)
