ModemManager (and latest udev) in OpenWRT

So I wanted to have ModemManager in OpenWRT to handle my mobile broadband connections.

OpenWRT has its own built-in support to handle mobile broadband modems of different types, mainly for AT-controlled modems, but also for QMI-controlled modems through libqmi and qmicli (now available in upstream OpenWRT). But hell, why not package ModemManager for that work, and provide a unified single DBus API to control any kind of mobile broadband modem in the same way?

ModemManager itself isn’t a very complex daemon, so it doesn’t have many dependencies, just: glib/gio, dbus, udev/gudev and libqmi. In the world of OpenWRT, though, none of these are blessed dependencies, as they have a lightweight replacement for everything: libubox (instead of glib), ubus (instead of dbus) and hotplug2 (instead of udev). These replacements target minimal embedded systems, and are perfectly integrated into OpenWRT and provide the building blocks of other core software like the netifd network interface daemon.

Still, having ModemManager available as a OpenWRT package seems like a good idea if you’re not very constrained by the hardware you’re going to use. If you are building a system which should support multiple broadband modems from different vendors, or just building a base system for one specific modem model but leaving the door open to change the model in the future, ModemManager seems a good choice.

 

systemd’s udev in OpenWRT

Since April 2012, udev sources are now integrated within the systemd source code. And that means that udev packages in OpenWRT would need to get updated in order to get built from that new repository. systemd provides some hints on what would be the best way of creating a minimal systemd build which can be used to gather just the udev-specific libraries and binaries. The easiest way involves compiling the whole systemd package with all optional features disabled, and that would get us not only systemd (which we won’t need), but also udevd and libudev. Easy, yes, but then we have some issues in the environment that we would need to handle…

In OpenWRT, the udev package is available in the trunk repository, where only the core OpenWRT packages are given. But in order to get the new udev compiled out of systemd we end up needing additional dependencies (dbus, libcap) which are only available in the packages repository. Moreover, if you also want to compile the glib-based gudev library, glib is also needed, which is also only available in the packages repository. And you cannot require a package from trunk to depend on a package from packages, so… ended up creating a new systemd-udev package within the packages repository.

As I was targeting the uClibc toolchain, I also ended up requiring some additional patches both in uClibc (e.g. to enable utmpx) and systemd to get the thing properly compiled. The patches applied in the systemd sources are just to handle missing features in uClibc (execvpe, secure_getenv…). Whenever uClibc includes those, we’ll be able to remove those patches.

 

ModemManager in OpenWRT

I prepared an easy setup of ModemManager packaging in OpenWRT, where all possible plugins get compiled. It wouldn’t have been difficult to allow selecting via configuration which plugins to get compiled, but didn’t want to spend time on that.

I sent the patches to the OpenWRT mailing list already, but if you want to give it a quick try, you can just clone the Lanedo OpenWRT repositories (trunk and packages) from gitorious.org:

$> git clone git://gitorious.org/lanedo/openwrt.git
$> git clone git://gitorious.org/lanedo/openwrt-packages.git


You’ll want both ‘modemmanager-support‘ branches in those git repositories.

ModemManager will be available under the ‘Network’ section in the OpenWRT build configuration; but only after explicitly selecting the following kernel modules (all under Kernel Modules/USB support):

• kmod-usb-net
• kmod-usb-serial
• kmod-usb-acm

Once all above are enabled, ModemManager can be selected, and it will itself pull all its other required dependencies (systemd-udev, glib, dbus, libqmi…).

 

No LuCI? No connection manager?

Well, not yet :). If you want to use ModemManager you still need to have your own connection manager software to manage the connectivity. ModemManager will get your mobile broadband modem connected, but you still need to either call pppd or setup the wwan interface yourself (depending on the data port type). This is really not a big deal; and you can just setup a shell script using the provided mmcli command line interface to talk to ModemManager.