Hey, just wanted to provide some corrections (and give some background) for SMBus trackpads.
You group PS/2 and SMBus together, which is a bit weird. SMBus is really a derivative of I2C, which both have much higher data bandwidth compared to PS/2. The main benefit to SMBus (and I2C) is that they send all finger coordinates at once, which the PS/2 bus is not able to do. While 99% of SMBus trackpads I've seen so far do support PS/2 connectivity, this relationship does not hold true the other way around. Many Synaptics PS/2 trackpads (generally Skylake era and older) are not connected over SMBus. Use of SMBus over PS/2 is basically always preferred for Synaptics, especially since Acidanthera's VoodooPS2 has some bugs with how buttons are handled.
SMBus trackpads are not that easy to detect. Best way in Windows is to check the name of the SMBus Controller in Device Manager. Normally, it'll be named "Intel SMBus Controller". When renamed to "Elans SMBus Controller" or "Synaptics SMBus Controller", there is an SMBus trackpad. The HID devices will always be attached to the PS/2 interface due to how the drivers work, but they really are using the higher bandwidth SMBus. In macOS, Acidanthera's VoodooPS2Trackpad will put a property in IORegistry which mentions "Intertouch Support". In Linux, just search for "RMI4" or "Intertouch" in sudo dmesg
. If RMI4 appears, then it's an SMBus trackpad. The Linux method though is not that accurate, as many trackpads have to be added manually to a list in the PS/2 Synaptics driver for SMBus support.
Many Thinkpads and HP ProBooks (Ivy Bridge and newer) will use SMBus, though this is not a rule. The Thinkpad 2-in-1s like to use I2C for example. My Thinkpad X1 Extreme (Gen 1 - Coffeelake) uses SMBus, and I believe that was the case for gen 2 as well.
VoodooElans is not complete. Elans SMBus trackpads should use VoodooSMBus from the VoodooSMBus repo. The VoodooSMBus that comes with VoodooRMI is built on a different branch and has different functionality.
Also, VoodooRMI works for Synaptics I2C trackpads. Probably best to replace VoodooI2CSynaptics with it, as VoodooRMI fixes some bugs and has more functionality in general. It also supports the F12 trackpads.
Edit:
If you've got extra questions, feel free to ask them. It can get confusing.