Lathe spindle speed PWM control

Does anybody know of a plug-in (does LinuxCNC have these?) that will control lathe spindle speed.
In theory the calculation is simple - the spindle RPM should be inversely proportional to the tool position X value (absolute so that it allows for both front & rear tool posts) and it would be useful to have an option to stop the spindle during a tool change. There should be a variable to allow for different materials too. Simple enough but how would I go about writing it if nothing already exists?
Thanks in advance

No plugins, but hal components written in C, fairly simple once you have a look at them.
I think you mean CSS or constant surface speed, i am pretty sure LinuxCNC can do that, but it does require having spindle feedback wired to LinuxCNC (physically and in hal), prefferably A, B, and Index to be able to also do threading (it can do with 2 of those, not sure exactly).
Stoping spindle during tool change is built in and can be set in the ini configuration file. Pretty sure it is by default as i had to change that on my Mazak, been over a year since i did that....

Hello Tommy
Thanks for that - I will look into hal components. I was really not sure where to start. It is only a model engineering lathe so will not be pushed as much as a production machine. I will fit a tachometer to the spindle but accurate feedback is not really of great concern. And yes it was CSS I was referring to. At least I will have continuously variable spindle speeds on the new lathe - the previous one was too much trouble to mess around with the spindle gears and in theory the spindle speed needs to change with the tool X position anyway for optimum results. I have been using LinuxCNC on the lathe (and mill) for at least a decade, but the old lathe was far too crude to be worth messing around with optimisation.

Have a look after 3 minute mark, that is CSS (i think) and that should be included in LinuxCNC