HACKING

$ nix-shell '<nixpkgs>' -A linux.dev

test versions:
kernelPackages_5_4
kernelPackages_5_10
kernelPackages_5_15

$ make -C $(nix-build -E '(import <nixpkgs> {}).linuxPackages_5_15.kernel.dev' --no-out-link)/lib/modules/*/build M=$(pwd) modules

$ nix-shell '<nixpkgs>' -A linux_5_4.dev

$ sudo insmod ./nshield_solo.ko

$ ls -al /dev/nshield_solo0 
crw------- 1 root root 176, 0 Dec  8 12:29 /dev/nshield_solo0

[17485.297552] nfp_module_init: inserting nshield_solo module
[17485.297554] nfp_init: entered
[17485.297587] nshield_solo 0000:03:00.0: nfp_pci_probe: probing PCI device 0000:03:00.0
[17485.297732] nshield_solo 0000:03:00.0: nfp_pci_probe: devname , slotname 0000:03:00.0, busname PCI Bus 0000:03
[17485.297733] nshield_solo 0000:03:00.0: nfp_setup: Found 'nCipher nShield Solo' at bus 3, slot 0, irq 17.
[17485.297734] nshield_solo 0000:03:00.0: nfp_setup: allocated device structure.
[17485.297750] nshield_solo 0000:03:00.0: i21555_create: entered
[17485.297751] nshield_solo 0000:03:00.0: i21555_create: enable doorbell
[17485.297824] nshield_solo 0000:03:00.0: nfp_setup: nfp_num_devices= 0, ndev = 0000000058d0d525.

References:
 * https://phip1611.de/blog/building-an-out-of-tree-linux-kernel-module-in-nix/
 * https://github.com/phip1611/qemu-debugcon-linux-driver