code: def ppp_read(self, ppp_fd):
          '''called from main select loop in mavproxy when the pppd child
          sends us some data'''
          buf = os.read(ppp_fd, 100)
          if len(buf) == 0:
              # EOF on the child fd
              self.stop_ppp_link()
              return
          print("ppp packet len=%u" % len(buf))
          master = self.master
          master.mav.ppp_send(len(buf), buff ppp_read(self, ppp_fd):
          '''called from main select loop in mavproxy when the pppd child
          sends us some data'''
          buf = os.read(ppp_fd, 100)
          if len(buf) == 0:
              # EOF on the child fd
              self.stop_ppp_link()
              return
          print("ppp packet len=%u" % len(buf))
          master = self.master
          master.mav.ppp_send(len(buf), buf)
docs: called from main select loop in mavproxy when the pppd child
          sends us some data