Since #402198 kernel modules are no longer built with wrapped compilers.
Firstly, NIX_CFLAGS_COMPILE is no longer effective, so we change that to
CFLAGS, which is used in the Makefile for evdi.
Secondly, we still need to use the wrapped compiler for userspace stuff,
so separate that out in postBuild.
Fixes#437311
Closes#376820
The issue with providing `moduleMakeFlags` via `passthru` of the kernel
package is that when this package gets overriden[1], the `moduleMakeFlags`
list still references the kernel without the overrides.
This broke e.g. kernel modules of linux-rpi4 which can be reproduced
with
nix-build --argstr system aarch64-linux -A linuxKernel.packages.linux_rpi4.zfs_2_3
This used to break with
Error: modDirVersion 6.6.51 specified in the Nix expression is wrong, it should be: 6.6.51-v8
since KBUILD_OUTPUT referenced the kernel without the changes from
`overrideDerivation` that also changes the `modDirVersion`.
The new approach is to add the build flags right into
`linuxKernel.packages.linux_X_Y`: that way we don't need any hacks to
update `moduleMakeFlags` when the derivation with the passthru gets
overridden.
By using the fixpoint of the package-set, the `kernelModuleMakeFlags`
list is correctly updated. E.g. given
with import ./. {};
linuxKernel.packages.linux_6_6.extend (self: super: {
kernel = super.kernel.overrideAttrs (_: {
name = "linux-snens";
});
})
the `makeFlags` is correctly updated:
$ nix-instantiate snenskek.nix -A zfs_2_3.makeFlags --eval --strict
[ "ARCH=x86_64" "CROSS_COMPILE=" "KBUILD_OUTPUT=/nix/store/gsp68549k1aqbwxwczpgw67w5jjn4shw-linux-snens-dev/lib/modules/6.6.74/build" ]
[1] E.g. `linux-rpi4`.
The build fails with:
```console
module/evdi_fb.c: In function 'evdifb_create':
module/evdi_fb.c:427:21:
error: assignment discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
427 | info->fbops = efbdev->fb_ops;
| ^
````
A resolution is to treat discard-qualifiers warnings as non-error.
See https://hydra.nixos.org/build/239848621
with structuredAttrs lists will be bash arrays which cannot be exported
which will be a issue with some patches and some wrappers like cc-wrapper
this makes it clearer that NIX_CFLAGS_COMPILE must be a string as lists
in env cause a eval failure
