mouse-fs

Abuses a Logitech mouse's DPI register to store 2 bytes of data. As long as the mouse stays powered.

Usage

mouse-fs write "hi"   # store 2 bytes
mouse-fs read         # retrieve them
mouse-fs reset        # restore 1000 DPI
mouse-fs raw          # show raw DPI value

How it works

A Logitech mouse communicates via the HID++ 2.0 protocol over a Unifying receiver.

By enumerating the device's feature table, we can locate the AdjustableDPI feature (0x2201). It accepts arbitrary u16 values without validation, which allows encoding 2 bytes of data into the DPI value.

Persistence behavior:

The stored value behaves like this:

• survives switching computers while the mouse stays powered
• survives unplugging/replugging the receiver
• is lost when the mouse loses power (power off or battery removal)

Important details:

• Writing DPI only affects the active DPI
• The device also maintains a fallback/default DPI
• On power cycles, the fallback value is restored

So while you can store arbitrary data in the DPI register, it does not persist across power cycles.

Tested on macOS. Long HID++ reports are blocked by IOHIDManager so storage is limited to one u16 (2 bytes) via short reports only.

Requirements

• Logitech mouse connected via Unifying receiver
• Logi Options+ must not be running (it holds the HID++ interface)
• brew install hidapi

Would be cool

• Resolve HID++ feature index automatically so you can use this on every Logitech mouse? Idk about this
• Look where fallback DPI is actually stored