The length arguments on some of the functions declared in the netdev2 API are signed, which does not make sense and causes warnings in some driver implementations when -Wsign-compare is used.

https://github.com/RIOT-OS/RIOT/blob/master/drivers/include/net/netdev2.h#L140
https://github.com/RIOT-OS/RIOT/blob/master/drivers/include/net/netdev2.h#L161

Warning in at86rf2xx:
https://github.com/RIOT-OS/RIOT/blob/master/drivers/at86rf2xx/at86rf2xx_netdev.c#L156

riot/drivers/at86rf2xx/at86rf2xx_netdev.c:156:17: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
     if (pkt_len > len) {
                 ^