NMEA_MAX_LEN
#define NMEA_MAX_DATA_LEN (NMEA_MAX_LEN - NMEA_TRAIL_LEN)
s = skb_put(skb, NMEA_MAX_LEN + 1); /* +'\0' */
len = (p - s) - (NMEA_MAX_LEN + 1); /* exp. vs real length diff */
skb = alloc_skb(NMEA_MAX_LEN * 2 + 2, GFP_ATOMIC); /* GGA + RMC */