[Nix-dev] Using Nix to build embedded linux firmware

[Felix Lange]

Hi,

I am currently evaluating Nix and it's universe of tools
for a company project and after playing with it for a while,
I'd like to discuss my considerations with the list.

Our company, Travelping GmbH, is a producer of specialized telecommunications
equipment. The firmware for our devices is based on a custom Linux
distribution, TPLINO. In the project I'm working on, we're basically looking
for a new underpinning of TPLINO. We've been working with OpenWRT
for about 5 years now and there are some areas where it doesn't
satisfy our requirements anymore.

Comparing to the alternatives provided by Yocto and Gentoo,
the Nix package manager seemed particularly attractive
for its ability to reliably generate whole system _configurations_
and the advanced upgrade/rollback features.

A closer look at Nix has revealed some questions:

Cross Compilation
	There seems to be some work in the nixpkgs tree that deals with
	cross builds (pkgs/top-level/release-cross.nix). It would be interesing
	to hear from the people who are behind this. Are cross
	builds alive and supported?

Nix's binary size and runtime requirements:
	Our usual size limit for images is 8MB compressed. With an image
	that small, we need to be really picky about what goes in
	and what doesn't. Quick measurement shows that Nix + libraries
	take ~10MB on disk. AFAIK, Nix also requires the C++ STL.

	The size is only a problem for small devices which
	have a very limited amount of flash memory available.
	Those devices might also not be able to execute
	Nix expressions because their CPU power and main memory is
	limited. It seems Nix wasn't really designed to
	run in resource-constrained environments.

	Has anybody run into similar issues before?

Remote installation of packages:
	As a follow-up thought to the last one, would it be possible
	to update an installation remotely, with only a very limited set of
	tools being present? I've looked at Disnix, and the README says
	that all target hosts need to have Nix (and Nixpkgs) installed,
	which is unfortunate but understandable given its intended
	use case (server/cluster deployment).

	The Nix store file structure seems simple
	enough to allow remote management. Is this correct?

Configuration changes:
	System configuration (networking, services...) is not part of Nixpkgs
	but is kept in the NixOS tree. This is good, because we have very
	specific needs in terms of network configuration and implement
	it ourselves anyway.

	How does Nix handle pre/post upgrade scripts? As far as I understand,
	the Nixpkgs tree only contains build instructions, any output is created
	at compile time.

I really enjoyed working with the Nix tools and look forward
to receiving some answers.