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OpenCAPI stands for Open Coherent Accelerator Processor Interface. Linux kernel documentation describes it as a low-latency, high-bandwidth interface between processors and accelerators. A device such as an FPGA or ASIC can use it to reach host memory coherently with virtual addresses. The device can also offer its own memory to the host.
What OpenCAPI defines
OpenCAPI is an interface specification. It defines a Data Link Layer and a Transaction Layer, and these run over a physical link. It does not name a card, a connector or a slot. Which cable, card or host platform you have is a separate question from the protocol, and the answer depends on the system.
The OC-Accel project describes OpenCAPI as an open, high-performance coherent bus interface. It says the interface supports three kinds of attachment:
- coherent user-level accelerators
- I/O devices
- advanced memory
Its documentation says advanced memory can be accessed with read/write semantics or with user-level DMA semantics.
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The two ideas that matter most
Coherent access with virtual addresses
Per the Linux kernel documentation, an accelerator can reach host memory coherently using virtual addresses. The accelerator therefore works with the same view of memory as the host processor. It does not need separate copies or buffers managed by hand.
Device-hosted memory
An OpenCAPI device can also host memory that the host can access. That is why the documentation describes uses beyond compute accelerators.
CAPI versus OpenCAPI
The names are related but are not interchangeable. The table compares the generations that the sources distinguish.
| Item | What the sources say |
|---|---|
| IBM CAPI | IBM’s AIX documentation describes it as a high-bandwidth, low-latency path between external devices and the POWER8 core and system memory architecture. The CAPI Flash adapter it describes uses PCIe Gen3 as its underlying transport. |
| CAPI 1 and CAPI 2 | OpenPOWER’s SNAP repository says SNAP addresses these cards. |
| CAPI 3 (OpenCAPI) | The SNAP repository directs these users to OC-Accel. Linux calls OpenCAPI the open, processor-agnostic evolution of IBM’s CAPI. |
A POWER8 CAPI adapter, an OpenCAPI 3 device and a generic PCIe FPGA card are different things. When you discuss a device, say which generation and platform you mean.
Where it appears in practice
The OpenCAPI POWER Platform Architecture Guide, version 3.0 dated 2019-01-18, defines an interface structure for coherently attaching accelerators to IBM Power Systems over an OpenCAPI physical link. It is a platform architecture document. It does not guarantee that every OpenCAPI device works on every system.
OC-Accel is a documented FPGA development framework based on OpenCAPI 3.0. It covers developing and deploying FPGA accelerators on OpenCAPI-enabled systems. It is one example implementation and does not define the whole standard.
The OC-Accel installation notes for the IBM AC922 show how specific compatibility can be. They list slot and card restrictions. They also distinguish slots enabled for CAPI from slots intended for OpenCAPI mode. These notes are platform-specific and historical, so treat them as an illustration, not a current compatibility list.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where the specification lives now
The OpenCAPI Consortium developed the specification. Linux kernel documentation says it is now available from the Compute Express Link Consortium. The sources reviewed do not establish a current specification version. They also do not establish the original consortium’s present status, or current support for any particular host or adapter. Check the Compute Express Link Consortium directly before relying on any of these.
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