Geraint, basically the way that I turn a driver on and off is that I CreateFile() on it to start and then CloseHandle when I'm done.  You'll need a driver permanently loaded in the background even when I'm not using you, though you don't have to have any threads or anything active in your driver when I'm not using you.  To get ontifications from GPSID, when it calls CreateFile() the kernel will map the call to your driver's xxx_Open() and when I call CloseHandle() it'll map to your xxx_Close().  You have the theoretical problem that another app could also call CreateFile() on your driver, so you can do ref counting if you want to support them or alternately just fail any but the first call.  If you're on a Windows Mobile platform you can make yourself as trusted only driver so you can be pretty sure random stuff won't be messing with you.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnce50/html/implement_network_service.asp?frame=true has some background about writing a service in services.exe, a bunch (but not all!) also applies to device driver writing.

You don't have to monitor the "CurrentDriver" key at all -- GPSID takes care of all this.  If someone changes the value on me, what'll happen is that on the next time I startup the GPS hardware a different driver will get the xxx_Open() call.

I'm very happy you're actively integrating GPSID with your GPS chip - having the OEM tie closely with GPSID so there's a good app development & end user experience is why I created did GPSID in the 1st place.

John

You're correct, I only do CreateFile() once no matter how many apps do GPSOpenDevice() -- the reference counting is totally done at GPSID's layer.  1st app using GPSID we do CreateFile on GPS device, when the # of apps using GPSID goes to 0 we close GPS device.

In your setup, is "GPD1" the underlying GPS driver or is it the MUX that GPSID is doing for it?  The GPS driver and GPSID MUX'r have to have different registrations (like "GPD1:" for mux and "GPS4:" for real GPS driver, for instance).  I take it you're writing a GPS driver here, Anil?  The one area where you would be worried about ref counting is if an application called CreateFile("Anil's Driver") before any app called GPSOpenDevice() or CreateFile("GPS Multiplexer").  In this case that app could block GPSID based apps from accessing your data if you only let one app call your driver at same time.  So if you're paranoid, you can either do a multiplexer at your level along with ref counting.  If you have faith in apps being behaved, then you don't need that -- you should probably just hard-code it so that if more than 1 guy calls CreateFile() on your driver then you fail, but nothing harder.  A bunch of existing GPS technologies, like Bluetooth GPS, have the same problem where an app can effectively Denial of Service GPSID if it doesn't behave.  You can protect against this to an extent by in your xxx_Open() by doing <if (PSLGetCallerTrust() != OEM_CERTIFY_TRUST) {fail open;}> -> this would keep untrusted apps from opening your driver, and since gpsid.dll is in device.exe it'll always be trusted.

I'm happy to help you with this if you have more questions, but it may be easier if we continue this discussion one one of the newsgroups (http://blogs.msdn.com/cenet/archive/2005/12/05/500181.aspx).  I check newsgroups everyday and it's easier for me to track them than it is using this blog software.

John