Linux 7.4 adds initial support for CXL Dynamic Capacity Devices

Changes to the CXL subsystem were merged into the Linux 7.4 kernel on October 26. These updates include the first stage of support for Dynamic Capacity Devices (DCD), which are Type-3 CXL memory expansion hardware. DCD allows server memory capacity to scale up or down dynamically without requiring a hardware reconfiguration or a reboot. This technology can reduce idle memory by pooling resources across a rack through multi-host memory sharing.

Image: Phoronix

DCD support in this merge includes flagging support in the mailbox, reading dynamic capacity configurations from the device, configuring dynamic capacity interrupts, and gathering DSMAS data for DCD partitions. DCD is not enabled yet, and some code is still undergoing review before it is fully upstream.

Other changes to the CXL subsystem include allowing zero sized HDM decoders and reworking GPF timeout programming. Latency calculations now use consistent units and account for link width.