Switches on Embedded Devices

On many embedded devices, switches solve a common issue: many embedded applications require multiple Ethernet ports on a device, e.g. for redundant data connection, or to interface to multiple other devices. Since many SoCs have only a single Ethernet controller, switch ICs are used to multiplex the multiple network ports to the on-SoC Ethernet controller.

While for some applications forwarding packets between the external ports may be acceptable or even desirable, some applications need the external network ports to behave such as if they were connected to different network controllers. This implies that the ports are isolated one from another and traffic is not forwarded, but all traffic is sent to the SoC only.

Switch Support in a Bootloader

Especially kernel developers prefer network boot for loading new kernel or device-tree images to an embedded board. Not having to flash them on a storage medium saves some more time, but the most important speedup can be gained by just mounting the root file system over NFS instead of flashing an image with several MB to a storage device. After a rebuild, all that is required to use the new images is just a reboot of the device under test. In an earlier blog article we described how this helps developers to get close-to-instant turnaround times for testing code changes on embedded devices.

While DSA was introduced in the kernel quite some time ago, handling switches in barebox was quite cumbersome until DSA support was merged. Previously, custom board code was required to configure the different ports to whatever modus is required during boot. Now, it is possible to just configure IP addresses on the individual ports and call boot net from the barebox shell, which tries any available interface to find a server for booting.

Synergy from Porting the Subsystem

Because the barebox DSA subsystem is ported from Linux, it uses the same internal data structures and facilities. This allows reusing kernel device-tree descriptions of on-board switches for the switch setup in barebox as well. Also, porting DSA drivers from the kernel to barebox is simplified by the fact that the code shares all important data structure definitions.

This shows off one of the unique advantages of barebox: because its driver model adheres strongly to the Linux driver model, porting device drivers is as easy as can be, with only minor adjustments now and then.

Already supported are the Microchip KSZ8873, KSZ9477 and KSZ8873 and their compatible variants as well as the NXP SJA1105, for which the manufacturers have provided open documentation, which is still quire rare for network switch ICs. Support for Realtek rtl8365mb and rtl8366rb is on the way and will land upstream soon.

Unexpected Applications in the Wild

While network-booting embedded boards with switches was originally intended as a feature to speed up development in our daily work, some of our customers have found interesting applications of the possibilities offered by easily setting up network ports as network devices with DSA in barebox.