kplex: add support for Sierra Wireless Gobi GPS

Sierra Wireless modems need the string '$GPS_START' to be sent to the
GPS tty device as only then the modem firmware starts emitting
NMEA-0183 sentences.
Add an option 'sierragpsstart' to kplex' serial driver to support that
quirk as kplex can be very useful to spread GPS data over the network
while also supplying 'ugps' using a PTY, allowing for correct system
time to be set automatically on boot up from GPS.

This patch is also PR'ed at the upstream project:
https://github.com/stripydog/kplex/pull/54

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

net/kplex/Makefile

@@ -8,7 +8,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=kplex
 PKG_VERSION:=1.4
-PKG_RELEASE=2
+PKG_RELEASE=3
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tgz
 PKG_SOURCE_URL:=http://www.stripydog.com/download

net/kplex/patches/100-add-support-for-Sierra-Wireless-qcserial-NMEA-0183-i.patch

@@ -0,0 +1,65 @@
+From a3dec2cbe5e539b5a270bed86eed78b283c79cdb Mon Sep 17 00:00:00 2001
+From: Daniel Golle <daniel@makrotopia.org>
+Date: Thu, 27 May 2021 01:18:20 +0200
+Subject: [PATCH] add support for Sierra Wireless qcserial NMEA-0183 interface
+
+Sierra Wireless EM 74xx modems come with a serial port outputting
+NMEA-0183 GPS sentences. In order to make it work, the magic string
+'$GPS_START' needs to be written to the modem, as only then the modem
+firmware starts sending NMEA-0183 output.
+Add option 'sierragpsstart' which if set to anything else than 0 will
+make kplex send the magic string when the device is opened.
+---
+ serial.c | 13 ++++++++++---
+ 1 file changed, 10 insertions(+), 3 deletions(-)
+
+--- a/serial.c
++++ b/serial.c
+@@ -24,6 +24,7 @@
+ #include <pwd.h>
+ 
+ #define DEFSERIALQSIZE 32
++#define SIERRA_GPS_START "$GPS_START\n"
+ 
+ struct if_serial {
+     int fd;
+@@ -290,7 +291,8 @@ struct iface *init_serial (struct iface
+     int ret;
+     struct kopts *opt;
+     int qsize=DEFSERIALQSIZE;
+-    
++    int send_gps_start = 0;
++
+     for(opt=ifa->options;opt;opt=opt->next) {
+         if (!strcasecmp(opt->var,"filename"))
+             devname=opt->val;
+@@ -324,7 +326,9 @@ struct iface *init_serial (struct iface
+                 logerr(0,"Invalid queue size specified: %s",opt->val);
+                 return(NULL);
+             }
+-        } else  {
++        } else if (!strcasecmp(opt->var, "sierragpsstart")) {
++            send_gps_start=atoi(opt->val);
++        } else {
+             logerr(0,"unknown interface option %s",opt->var);
+             return(NULL);
+         }
+@@ -337,7 +341,7 @@ struct iface *init_serial (struct iface
+     }
+ 
+     /* Open interface or die */
+-    if ((ifs->fd=ttyopen(devname,ifa->direction)) < 0) {
++    if ((ifs->fd=ttyopen(devname, send_gps_start?BOTH:ifa->direction)) < 0) {
+         return(NULL);
+     }
+     DEBUG(3,"%s: opened serial device %s for %s",ifa->name,devname,
+@@ -358,6 +362,9 @@ struct iface *init_serial (struct iface
+     ifs->saved=1;
+     ifs->slavename=NULL;
+ 
++    if (send_gps_start)
++        write(ifs->fd, SIERRA_GPS_START, strlen(SIERRA_GPS_START));
++
+     /* Assign pointers to read, write and cleanup routines */
+     ifa->read=do_read;
+     ifa->readbuf=read_serial;