Cumulus contains a large portion of closed-source code, including all the driver code provided by Broadcom, which is where much of the interesting stuff happens. (I assume their Mellanox products are the same, though I'm not familiar with them)

I'm not aware of any efforts to write open source drivers for any high speed ethernet chips, though I haven't looked much. Intel's (formerly Fulcrum) switch chips have open data sheets (but not the reference code, simulators, etc) so it would at least be plausible to start there.