The ReadoutCard Userspace Driver for the New Alice O2 Computing System

A large ion collider experiment (ALICE) experiment focuses on the study of the quark-gluon plasma as a product of heavy-ion collisions at the CERN large hadron collider (LHC). During the long shutdown 2 of the LHC in 2019-2020, a major upgrade is underway in order to cope with a hundredfold input data rate increase with peaks of up to 3.5 TB/s. This upgrade includes the new online-offline computing system called O². The O² readout chain runs on commodity Linux servers equipped with custom peripheral component interconnect express (PCIe) field-programmable gate array (FPGA)-based readout cards: the PCIe Gen 3 ×16, Intel Arria 10-based common readout unit (CRU), and the PCIe Gen 2 ×8, Xilinx Vertex 6-based Common ReadOut Receiver Card (CRORC). Access to the cards is provided through the O² ReadoutCard userspace driver, which handles synchronization and communication for direct memory access (DMA) transfers, provides base address registers (BAR) access, and facilitates card configuration and monitoring. The ReadoutCard driver is the lowest level interface to the readout cards within O² and is in use by all central systems and detector teams of the ALICE experiment. This communication presents the architecture of the driver and the suite of tools used for card configuration and monitoring. It also discusses its interaction with the tangent subsystems within the O² framework.