Real-time USB communication in the Quest operating system

This paper describes a real-time USB 2 subsystem for the Quest operating system. Quest is designed for real-time embedded systems. Such systems need to interact with their environment using sensors and actuators. On many embedded platforms today there is support for basic serial, USB 2.0 and 100 Mbps Ethernet. Of these, USB 2.0 supports the highest throughput, while also supporting real-time communication. We show how the Quest USB 2.0 sub-system improves upon some of the deficiencies in USB software stacks in systems such as Linux through experimental evaluation. We demonstrate that the Quest USB sub-system is capable of predictable bandwidth allocation and increased overall performance. By dynamically reordering transaction requests, Quest's USB sub-system is able to avoid unnecessary admission control rejections. Additionally, we are able to provide real-time guarantees for asynchronous USB transactions such as bulk transfers, which are typically treated in a best-effort manner. Real-time guarantees for bulk transactions are necessary for any system interacting with devices that implement bulk endpoints such as in a real-time file system. The paper also introduces an algorithm for USB scheduling that accepts more requests and provides bulk transfer guarantees, for cases where Linux fails.