ARK-OS/arkos/kernel/Documentation/userspace-api/accelerators/ocxl.rst

========================================================
OpenCAPI (Open Coherent Accelerator Processor Interface)
========================================================

OpenCAPI is an interface between processors and accelerators. It aims
at being low-latency and high-bandwidth.

The specification was developed by the OpenCAPI Consortium, and is now
available from the `Compute Express Link Consortium
<https://computeexpresslink.org/resource/opencapi-specification-archive/>`_.

It allows an accelerator (which could be an FPGA, ASICs, ...) to access
the host memory coherently, using virtual addresses. An OpenCAPI
device can also host its own memory, that can be accessed from the
host.

OpenCAPI is known in linux as 'ocxl', as the open, processor-agnostic
evolution of 'cxl' (the driver for the IBM CAPI interface for
powerpc), which was named that way to avoid confusion with the ISDN
CAPI subsystem.


High-level view
===============

OpenCAPI defines a Data Link Layer (DL) and Transaction Layer (TL), to
be implemented on top of a physical link. Any processor or device
implementing the DL and TL can start sharing memory.

::

  +-----------+                         +-------------+
  |           |                         |             |
  |           |                         | Accelerated |
  | Processor |                         |  Function   |
  |           |  +--------+             |    Unit     |  +--------+
  |           |--| Memory |             |    (AFU)    |--| Memory |
  |           |  +--------+             |             |  +--------+
  +-----------+                         +-------------+
       |                                       |
  +-----------+                         +-------------+
  |    TL     |                         |    TLX      |
  +-----------+                         +-------------+
       |                                       |
  +-----------+                         +-------------+
  |    DL     |                         |    DLX      |
  +-----------+                         +-------------+
       |                                       |
       |                   PHY                 |
       +---------------------------------------+



Device discovery
================

OpenCAPI relies on a PCI-like configuration space, implemented on the
device. So the host can discover AFUs by querying the config space.

OpenCAPI devices in Linux are treated like PCI devices (with a few
caveats). The firmware is expected to abstract the hardware as if it
was a PCI link. A lot of the existing PCI infrastructure is reused:
devices are scanned and BARs are assigned during the standard PCI
enumeration. Commands like 'lspci' can therefore be used to see what
devices are available.

The configuration space defines the AFU(s) that can be found on the
physical adapter, such as its name, how many memory contexts it can
work with, the size of its MMIO areas, ...



MMIO
====

OpenCAPI defines two MMIO areas for each AFU:

* the global MMIO area, with registers pertinent to the whole AFU.
* a per-process MMIO area, which has a fixed size for each context.



AFU interrupts
==============

OpenCAPI includes the possibility for an AFU to send an interrupt to a
host process. It is done through a 'intrp_req' defined in the
Transaction Layer, specifying a 64-bit object handle which defines the
interrupt.

The driver allows a process to allocate an interrupt and obtain its
64-bit object handle, that can be passed to the AFU.



char devices
============

The driver creates one char device per AFU found on the physical
device. A physical device may have multiple functions and each
function can have multiple AFUs. At the time of this writing though,
it has only been tested with devices exporting only one AFU.

Char devices can be found in /dev/ocxl/ and are named as:
/dev/ocxl/<AFU name>.<location>.<index>

where <AFU name> is a max 20-character long name, as found in the
config space of the AFU.
<location> is added by the driver and can help distinguish devices
when a system has more than one instance of the same OpenCAPI device.
<index> is also to help distinguish AFUs in the unlikely case where a
device carries multiple copies of the same AFU.



Sysfs class
===========

An ocxl class is added for the devices representing the AFUs. See
/sys/class/ocxl. The layout is described in
Documentation/ABI/testing/sysfs-class-ocxl



User API
========

open
----

Based on the AFU definition found in the config space, an AFU may
support working with more than one memory context, in which case the
associated char device may be opened multiple times by different
processes.


ioctl
-----

OCXL_IOCTL_ATTACH:

  Attach the memory context of the calling process to the AFU so that
  the AFU can access its memory.

OCXL_IOCTL_IRQ_ALLOC:

  Allocate an AFU interrupt and return an identifier.

OCXL_IOCTL_IRQ_FREE:

  Free a previously allocated AFU interrupt.

OCXL_IOCTL_IRQ_SET_FD:

  Associate an event fd to an AFU interrupt so that the user process
  can be notified when the AFU sends an interrupt.

OCXL_IOCTL_GET_METADATA:

  Obtains configuration information from the card, such at the size of
  MMIO areas, the AFU version, and the PASID for the current context.

OCXL_IOCTL_ENABLE_P9_WAIT:

  Allows the AFU to wake a userspace thread executing 'wait'. Returns
  information to userspace to allow it to configure the AFU. Note that
  this is only available on POWER9.

OCXL_IOCTL_GET_FEATURES:

  Reports on which CPU features that affect OpenCAPI are usable from
  userspace.


mmap
----

A process can mmap the per-process MMIO area for interactions with the
AFU.