[gpsd-users] New driver switching logic

[Eric S. Raymond]

gpsmon is working correctly again in git head.

After wrestling with complicated deiver-switching rules for the last
week, I've arrived at some brutally simple ones:

Drivers are now sorted into two classes: sticky and non-sticky. A
driver is flagged "sticky" if it has control methods (speed changer,
mode changer, rate changer) or an identity we don't want to forget.
Importantly, Generic NMEA and AIVDM and the JSON slave driver are
non-sticky.

Here's a table:

.       hook    .       send    NMEA    .       Generic NMEA
.       hook    trigger send    NMEA    sticky  Ashtech
.       hook    trigger send    NMEA    sticky  Delorme TripMate
.       hook    trigger send    NMEA    sticky  Pre-2003 Delorme EarthMate
.       hook    trigger send    NMEA    sticky  Furuno Electric GH-79L4
control hook    trigger send    NMEA    sticky  Garmin NMEA
control hook    trigger send    NMEA    sticky  MTK-3301
.       hook    trigger send    NMEA    sticky  OceanServer Digital Compass
.       hook    trigger send    NMEA    sticky  San Jose Navigation FV18
control hook    trigger send    NMEA    sticky  True North
control hook    .       send    NMEA    sticky  Jackson Labs Fury
.       .       .       .       binary  .       AIVDM
control hook    .       send    binary  sticky  EverMore binary
control .       .       send    binary  sticky  Garmin Serial binary
.       hook    probe   send    binary  sticky  Garmin USB binary
control hook    probe   send    binary  sticky  GeoStar binary
.       .       .       .       binary  sticky  iTalk binary
.       hook    .       send    binary  sticky  Oncore binary
control hook    .       send    binary  sticky  Navcom binary
control hook    .       send    binary  sticky  SiRF binary
control hook    .       send    binary  sticky  SuperStarII binary
control hook    probe   send    binary  sticky  Trimble TSIP binary
control hook    .       send    binary  sticky  uBlox UBX binary
control .       .       send    binary  sticky  Zodiac binary
.       .       .       .       binary  .       RTCM104V2
.       .       .       .       binary  .       RTCM104V3
.       .       .       .       binary  .       Garmin Simple Text
.       .       .       .       binary  .       JSON slave driver

The rules are:

(1) Always switch drivers to match the incoming packet type. If the
new driver is sticky, save a pointer to it.

(2) After processing the packet, revert to the last sticky driver you
have seen.

The assumption here is that no device will ever match two *different*
sticky drivers. Once you've seen one, you keep it, and let packets
for non-sticky driver types pass through without changing it.
-- 
                <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>

The right to buy weapons is the right to be free.
        -- A.E. Van Vogt, "The Weapon Shops Of Isher", ASF December 1942