Linux Kernel Developer Assesses Asymmetric Multiprocessing Foundations as Sound, Notes Testing Gaps

Assessment of Linux AMP Building Blocks

At Embedded Linux Conference Europe 2026, Wolfram Sang, a Linux kernel developer and maintainer of the Linux I2C subsystem, evaluated the Linux building blocks for asymmetric multiprocessing (AMP). His analysis indicated that these foundations are robust enough for continued development. However, he identified areas needing improvement, specifically thin contributor coverage and fragmented testing.

Context and AMP System Overview

Sang's examination stemmed from an effort to upstream a hardware-spinlock driver, which prompted a broader review of the AMP stack's health. He focused on the Linux aspects of AMP, where Linux application CPUs and smaller real-time cores operate within the same system-on-chip. Communication typically involves mailboxes for signaling and shared SRAM or DDR for data. More complex AMP systems require coordination for shared resource ownership, with SCMI serving as a communication method between Linux and a dedicated system-control processor for resource management.

Communication Mechanisms and Layers

AMP communication can be intricate; for instance, two unidirectional mailboxes can form a request-and-response channel, while a fully bidirectional SCMI setup might require four. Firmware can also dictate communication arrangements, as seen in a Keystone example where existing firmware used two unidirectional mailboxes for bidirectional SCMI, lacking an acknowledgment path for notifications.

Other layers like remoteproc handle firmware loading and remote core management, using firmware binaries to describe resource needs. RPMsg facilitates inter-processor communication via shared memory and driver callbacks. Virtio provides an open communication standard using shared-memory virtqueues for work notifications. Sang differentiated RPMsg from the proposed virtio-over-message transport, which is not yet upstream.

Contributor and Development Trends

Sang's survey of subsystem commits revealed growth in remoteproc core commits, from 175 at Linux 5.0 to approximately 1,550, though growth has slowed. Mailbox commits and drivers saw more recent growth, but with only about four authors contributing more than five patches. Hardware spinlocks showed minimal development, with core commits increasing from 19 to 30 over seven years and driver counts remaining flat.