What model servo?
Most modern servos close the loop in the servo drive itself. The encoder goes back to the servo drive. They don't work without this.
Some people feel better about closing the loop with LinuxCNC, but reality is dedicated hardware within the servo drive is usually much higher bandwidth (faster) than involving the PC.
An error signal can be wired from the drive to LinuxCNC.
If you add linear encoders you would need encoder inputs (Mesa/Pico) unless you have a more advanced servo drive with linear encoder feedback (usually only high end servo drives).
