This directory contains the codes shared among different drivers on the different architectures. They are hence supposed not to be architecture-specific. For example, the dummy pcm driver and the serial MIDI driver are found in this directory. In the sub-directories, there are the codes for components which are independent from bus and cpu architectures.
The MPU401 and MPU401-UART modules are stored here.
The OPL3 and OPL4 FM-synth stuff is found here.