mirror of https://gitee.com/openkylin/linux.git
161 lines
4.9 KiB
ReStructuredText
161 lines
4.9 KiB
ReStructuredText
========================================================
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OpenCAPI (Open Coherent Accelerator Processor Interface)
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========================================================
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OpenCAPI is an interface between processors and accelerators. It aims
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at being low-latency and high-bandwidth. The specification is
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developed by the `OpenCAPI Consortium <http://opencapi.org/>`_.
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It allows an accelerator (which could be a FPGA, ASICs, ...) to access
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the host memory coherently, using virtual addresses. An OpenCAPI
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device can also host its own memory, that can be accessed from the
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host.
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OpenCAPI is known in linux as 'ocxl', as the open, processor-agnostic
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evolution of 'cxl' (the driver for the IBM CAPI interface for
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powerpc), which was named that way to avoid confusion with the ISDN
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CAPI subsystem.
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High-level view
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===============
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OpenCAPI defines a Data Link Layer (DL) and Transaction Layer (TL), to
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be implemented on top of a physical link. Any processor or device
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implementing the DL and TL can start sharing memory.
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::
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+-----------+ +-------------+
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| | | |
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| | | Accelerated |
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| Processor | | Function |
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| | +--------+ | Unit | +--------+
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| |--| Memory | | (AFU) |--| Memory |
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| | +--------+ | | +--------+
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+-----------+ +-------------+
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| |
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+-----------+ +-------------+
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| TL | | TLX |
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+-----------+ +-------------+
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| |
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+-----------+ +-------------+
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| DL | | DLX |
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+-----------+ +-------------+
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| PHY |
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+---------------------------------------+
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Device discovery
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================
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OpenCAPI relies on a PCI-like configuration space, implemented on the
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device. So the host can discover AFUs by querying the config space.
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OpenCAPI devices in Linux are treated like PCI devices (with a few
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caveats). The firmware is expected to abstract the hardware as if it
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was a PCI link. A lot of the existing PCI infrastructure is reused:
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devices are scanned and BARs are assigned during the standard PCI
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enumeration. Commands like 'lspci' can therefore be used to see what
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devices are available.
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The configuration space defines the AFU(s) that can be found on the
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physical adapter, such as its name, how many memory contexts it can
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work with, the size of its MMIO areas, ...
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MMIO
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====
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OpenCAPI defines two MMIO areas for each AFU:
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* the global MMIO area, with registers pertinent to the whole AFU.
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* a per-process MMIO area, which has a fixed size for each context.
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AFU interrupts
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==============
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OpenCAPI includes the possibility for an AFU to send an interrupt to a
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host process. It is done through a 'intrp_req' defined in the
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Transaction Layer, specifying a 64-bit object handle which defines the
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interrupt.
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The driver allows a process to allocate an interrupt and obtain its
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64-bit object handle, that can be passed to the AFU.
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char devices
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============
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The driver creates one char device per AFU found on the physical
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device. A physical device may have multiple functions and each
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function can have multiple AFUs. At the time of this writing though,
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it has only been tested with devices exporting only one AFU.
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Char devices can be found in /dev/ocxl/ and are named as:
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/dev/ocxl/<AFU name>.<location>.<index>
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where <AFU name> is a max 20-character long name, as found in the
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config space of the AFU.
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<location> is added by the driver and can help distinguish devices
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when a system has more than one instance of the same OpenCAPI device.
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<index> is also to help distinguish AFUs in the unlikely case where a
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device carries multiple copies of the same AFU.
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Sysfs class
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===========
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An ocxl class is added for the devices representing the AFUs. See
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/sys/class/ocxl. The layout is described in
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Documentation/ABI/testing/sysfs-class-ocxl
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User API
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========
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open
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----
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Based on the AFU definition found in the config space, an AFU may
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support working with more than one memory context, in which case the
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associated char device may be opened multiple times by different
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processes.
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ioctl
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-----
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OCXL_IOCTL_ATTACH:
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Attach the memory context of the calling process to the AFU so that
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the AFU can access its memory.
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OCXL_IOCTL_IRQ_ALLOC:
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Allocate an AFU interrupt and return an identifier.
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OCXL_IOCTL_IRQ_FREE:
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Free a previously allocated AFU interrupt.
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OCXL_IOCTL_IRQ_SET_FD:
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Associate an event fd to an AFU interrupt so that the user process
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can be notified when the AFU sends an interrupt.
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mmap
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----
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A process can mmap the per-process MMIO area for interactions with the
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AFU.
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