Work around corruped Forerunner 305 packets.

diff --git a/jeeps/gpsusbread.c b/jeeps/gpsusbread.c
index ed075a55303e28082c5572801e44a625d0b32917..17a933140b6e482c89db36b9d8c845c680756aa7 100644 (file)
--- a/jeeps/gpsusbread.c
+++ b/jeeps/gpsusbread.c
@@ -62,6 +62,28 @@ do_over:
                goto do_over;
        }
        
+       /* We sometimes get corrupted packets during a track log transfer
+        * where the first byte in a packet is lost, causing all remaining
+        * bytes in this packet to be shifted. So far, this has only been
+        * observed on a Forerunner 305 (both on Linux and Windows). The
+        * cause is unknown, but it seems to be timing dependent.
+        * We try to detect the corruption mainly by checking reserved bytes
+        * 3 and 7 which normally should be 0, the remaining comparisons are
+        * only sanity checks and they alone could also trigger in case of
+        * valid packets. Note: We can't detect corrupted packets with an ID
+        * or length that's a multiple of 256, but such corrupted packets
+        * haven't been observed so far.
+        */
+       if (gps_save_id == 484
+           && pkt.gusb_pkt.type == 0 && pkt.gusb_pkt.reserved1 == 0
+           && pkt.gusb_pkt.reserved2 == 0 && pkt.gusb_pkt.reserved3 != 0
+           && pkt.gusb_pkt.pkt_id[0] == 0 && pkt.gusb_pkt.pkt_id[1] == 0
+           && pkt.gusb_pkt.reserved6 == 0 && pkt.gusb_pkt.reserved7 != 0) {
+               GPS_Warning("Received USB packet looks corrupted, trying to fix");
+               memmove(&pkt.dbuf[1], &pkt.dbuf[0], sizeof(pkt) - 1);
+               pkt.gusb_pkt.type = 20;
+       }
+       
        /* 
         * Populate members of serial packet from USB packet.   The
         * copy here seems wasteful, but teaching all the callers about
@@ -73,6 +95,10 @@ do_over:
        payload_size = le_read32(&pkt.gusb_pkt.datasz);
        if (payload_size<0 || payload_size>MAX_GPS_PACKET_SIZE)
        {
+               /* If you get this, the packet might have been corrupted
+                * by the unit. Have a look at the corruption detection
+                * code above.
+                */
                GPS_Error("GPS_Packet_Read_usb: Bad payload size %d", payload_size);
                gps_errno = FRAMING_ERROR;
                return 0;