The gs_usb kernel module (/drivers/net/can/usb/gs_usb.c) provides a SocketCAN interface for the "Geschwister Schneider USB/CAN devices" and also some other open hardware / open firmware USB-CAN adapters like the bytewerk.org candleLight and candleZero as well as the "cantact" and "canable" hardware when using the candleLight firmware.

As SocketCAN is included in the raspbian default kernel and the gs_usb module is enabled in popular desktop distributions (e.g. Ubuntu), I would love to see the module enabled in future raspbian default kernels.

This should be as easy as setting "CONFIG_CAN_GS_USB = m".

References:
http://schneidersoft.net/en//node/2
https://github.com/HubertD/candleLight
https://github.com/HubertD/candleZero
https://github.com/HubertD/candleLight_fw
http://linklayer.github.io/cantact/
http://canable.io/