Observations on HelenOS device drivers framework

Some time ago we merged Lenka's new Device Drivers Framework (DDF henceforth) to HelenOS mainline. When I finally read through her thesis and had a closer look at the code I promised to do a writeup of my observations. Here it is. A bit of a long read though, be warned.

Let's start with a quick overview. Lenka's contribution can be broken down into several parts. First there is the devman server which (a) manages the physical device topology, driver task life cycle, driver attaching and detaching, (b) maintains a 'logical' device topology (devices grouped by their class).

Second, Lenka proposed that for any IPC communication between two drivers or an application and driver should be strictly defined in terms of function calls and the client- and server-side IPC glue code should be kept strictly separate in a library to be reused by all servers and clients using such code.

Third, when mapping the function calls to IPC, Lenka reserved the first field of the message for an interface id. This allowed her to communicate with a driver using several different protocols, namely a protocol for talking to the driver as such (create new instance) and for talking to the device itself (e.g. read/write, etc.).