xref: /linux/Documentation/driver-api/cxl/theory-of-operation.rst (revision 29e9359005dd1ac5f9683608891718e6a32a20a3)

.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>

===============================================
Compute Express Link Driver Theory of Operation
===============================================

A Compute Express Link Memory Device is a CXL component that implements the
CXL.mem protocol. It contains some amount of volatile memory, persistent memory,
or both. It is enumerated as a PCI device for configuration and passing
messages over an MMIO mailbox. Its contribution to the System Physical
Address space is handled via HDM (Host Managed Device Memory) decoders
that optionally define a device's contribution to an interleaved address
range across multiple devices underneath a host-bridge or interleaved
across host-bridges.

The CXL Bus
===========
Similar to how a RAID driver takes disk objects and assembles them into a new
logical device, the CXL subsystem is tasked to take PCIe and ACPI objects and
assemble them into a CXL.mem decode topology. The need for runtime configuration
of the CXL.mem topology is also similar to RAID in that different environments
with the same hardware configuration may decide to assemble the topology in
contrasting ways. One may choose performance (RAID0) striping memory across
multiple Host Bridges and endpoints while another may opt for fault tolerance
and disable any striping in the CXL.mem topology.

Platform firmware enumerates a menu of interleave options at the "CXL root port"
(Linux term for the top of the CXL decode topology). From there, PCIe topology
dictates which endpoints can participate in which Host Bridge decode regimes.
Each PCIe Switch in the path between the root and an endpoint introduces a point
at which the interleave can be split. For example platform firmware may say at a
given range only decodes to 1 one Host Bridge, but that Host Bridge may in turn
interleave cycles across multiple Root Ports. An intervening Switch between a
port and an endpoint may interleave cycles across multiple Downstream Switch
Ports, etc.

Here is a sample listing of a CXL topology defined by 'cxl_test'. The 'cxl_test'
module generates an emulated CXL topology of 2 Host Bridges each with 2 Root
Ports. Each of those Root Ports are connected to 2-way switches with endpoints
connected to those downstream ports for a total of 8 endpoints::

    # cxl list -BEMPu -b cxl_test
    {
      "bus":"root3",
      "provider":"cxl_test",
      "ports:root3":[
        {
          "port":"port5",
          "host":"cxl_host_bridge.1",
          "ports:port5":[
            {
              "port":"port8",
              "host":"cxl_switch_uport.1",
              "endpoints:port8":[
                {
                  "endpoint":"endpoint9",
                  "host":"mem2",
                  "memdev":{
                    "memdev":"mem2",
                    "pmem_size":"256.00 MiB (268.44 MB)",
                    "ram_size":"256.00 MiB (268.44 MB)",
                    "serial":"0x1",
                    "numa_node":1,
                    "host":"cxl_mem.1"
                  }
                },
                {
                  "endpoint":"endpoint15",
                  "host":"mem6",
                  "memdev":{
                    "memdev":"mem6",
                    "pmem_size":"256.00 MiB (268.44 MB)",
                    "ram_size":"256.00 MiB (268.44 MB)",
                    "serial":"0x5",
                    "numa_node":1,
                    "host":"cxl_mem.5"
                  }
                }
              ]
            },
            {
              "port":"port12",
              "host":"cxl_switch_uport.3",
              "endpoints:port12":[
                {
                  "endpoint":"endpoint17",
                  "host":"mem8",
                  "memdev":{
                    "memdev":"mem8",
                    "pmem_size":"256.00 MiB (268.44 MB)",
                    "ram_size":"256.00 MiB (268.44 MB)",
                    "serial":"0x7",
                    "numa_node":1,
                    "host":"cxl_mem.7"
                  }
                },
                {
                  "endpoint":"endpoint13",
                  "host":"mem4",
                  "memdev":{
                    "memdev":"mem4",
                    "pmem_size":"256.00 MiB (268.44 MB)",
                    "ram_size":"256.00 MiB (268.44 MB)",
                    "serial":"0x3",
                    "numa_node":1,
                    "host":"cxl_mem.3"
                  }
                }
              ]
            }
          ]
        },
        {
          "port":"port4",
          "host":"cxl_host_bridge.0",
          "ports:port4":[
            {
              "port":"port6",
              "host":"cxl_switch_uport.0",
              "endpoints:port6":[
                {
                  "endpoint":"endpoint7",
                  "host":"mem1",
                  "memdev":{
                    "memdev":"mem1",
                    "pmem_size":"256.00 MiB (268.44 MB)",
                    "ram_size":"256.00 MiB (268.44 MB)",
                    "serial":"0",
                    "numa_node":0,
                    "host":"cxl_mem.0"
                  }
                },
                {
                  "endpoint":"endpoint14",
                  "host":"mem5",
                  "memdev":{
                    "memdev":"mem5",
                    "pmem_size":"256.00 MiB (268.44 MB)",
                    "ram_size":"256.00 MiB (268.44 MB)",
                    "serial":"0x4",
                    "numa_node":0,
                    "host":"cxl_mem.4"
                  }
                }
              ]
            },
            {
              "port":"port10",
              "host":"cxl_switch_uport.2",
              "endpoints:port10":[
                {
                  "endpoint":"endpoint16",
                  "host":"mem7",
                  "memdev":{
                    "memdev":"mem7",
                    "pmem_size":"256.00 MiB (268.44 MB)",
                    "ram_size":"256.00 MiB (268.44 MB)",
                    "serial":"0x6",
                    "numa_node":0,
                    "host":"cxl_mem.6"
                  }
                },
                {
                  "endpoint":"endpoint11",
                  "host":"mem3",
                  "memdev":{
                    "memdev":"mem3",
                    "pmem_size":"256.00 MiB (268.44 MB)",
                    "ram_size":"256.00 MiB (268.44 MB)",
                    "serial":"0x2",
                    "numa_node":0,
                    "host":"cxl_mem.2"
                  }
                }
              ]
            }
          ]
        }
      ]
    }

In that listing each "root", "port", and "endpoint" object correspond a kernel
'struct cxl_port' object. A 'cxl_port' is a device that can decode CXL.mem to
its descendants. So "root" claims non-PCIe enumerable platform decode ranges and
decodes them to "ports", "ports" decode to "endpoints", and "endpoints"
represent the decode from SPA (System Physical Address) to DPA (Device Physical
Address).

Continuing the RAID analogy, disks have both topology metadata and on device
metadata that determine RAID set assembly. CXL Port topology and CXL Port link
status is metadata for CXL.mem set assembly. The CXL Port topology is enumerated
by the arrival of a CXL.mem device. I.e. unless and until the PCIe core attaches
the cxl_pci driver to a CXL Memory Expander there is no role for CXL Port
objects. Conversely for hot-unplug / removal scenarios, there is no need for
the Linux PCI core to tear down switch-level CXL resources because the endpoint
->remove() event cleans up the port data that was established to support that
Memory Expander.

The port metadata and potential decode schemes that a give memory device may
participate can be determined via a command like::

    # cxl list -BDMu -d root -m mem3
    {
      "bus":"root3",
      "provider":"cxl_test",
      "decoders:root3":[
        {
          "decoder":"decoder3.1",
          "resource":"0x8030000000",
          "size":"512.00 MiB (536.87 MB)",
          "volatile_capable":true,
          "nr_targets":2
        },
        {
          "decoder":"decoder3.3",
          "resource":"0x8060000000",
          "size":"512.00 MiB (536.87 MB)",
          "pmem_capable":true,
          "nr_targets":2
        },
        {
          "decoder":"decoder3.0",
          "resource":"0x8020000000",
          "size":"256.00 MiB (268.44 MB)",
          "volatile_capable":true,
          "nr_targets":1
        },
        {
          "decoder":"decoder3.2",
          "resource":"0x8050000000",
          "size":"256.00 MiB (268.44 MB)",
          "pmem_capable":true,
          "nr_targets":1
        }
      ],
      "memdevs:root3":[
        {
          "memdev":"mem3",
          "pmem_size":"256.00 MiB (268.44 MB)",
          "ram_size":"256.00 MiB (268.44 MB)",
          "serial":"0x2",
          "numa_node":0,
          "host":"cxl_mem.2"
        }
      ]
    }

...which queries the CXL topology to ask "given CXL Memory Expander with a kernel
device name of 'mem3' which platform level decode ranges may this device
participate". A given expander can participate in multiple CXL.mem interleave
sets simultaneously depending on how many decoder resource it has. In this
example mem3 can participate in one or more of a PMEM interleave that spans to
Host Bridges, a PMEM interleave that targets a single Host Bridge, a Volatile
memory interleave that spans 2 Host Bridges, and a Volatile memory interleave
that only targets a single Host Bridge.

Conversely the memory devices that can participate in a given platform level
decode scheme can be determined via a command like the following::

    # cxl list -MDu -d 3.2
    [
      {
        "memdevs":[
          {
            "memdev":"mem1",
            "pmem_size":"256.00 MiB (268.44 MB)",
            "ram_size":"256.00 MiB (268.44 MB)",
            "serial":"0",
            "numa_node":0,
            "host":"cxl_mem.0"
          },
          {
            "memdev":"mem5",
            "pmem_size":"256.00 MiB (268.44 MB)",
            "ram_size":"256.00 MiB (268.44 MB)",
            "serial":"0x4",
            "numa_node":0,
            "host":"cxl_mem.4"
          },
          {
            "memdev":"mem7",
            "pmem_size":"256.00 MiB (268.44 MB)",
            "ram_size":"256.00 MiB (268.44 MB)",
            "serial":"0x6",
            "numa_node":0,
            "host":"cxl_mem.6"
          },
          {
            "memdev":"mem3",
            "pmem_size":"256.00 MiB (268.44 MB)",
            "ram_size":"256.00 MiB (268.44 MB)",
            "serial":"0x2",
            "numa_node":0,
            "host":"cxl_mem.2"
          }
        ]
      },
      {
        "root decoders":[
          {
            "decoder":"decoder3.2",
            "resource":"0x8050000000",
            "size":"256.00 MiB (268.44 MB)",
            "pmem_capable":true,
            "nr_targets":1
          }
        ]
      }
    ]

...where the naming scheme for decoders is "decoder<port_id>.<instance_id>".

Driver Infrastructure
=====================

This section covers the driver infrastructure for a CXL memory device.

CXL Memory Device
-----------------

.. kernel-doc:: drivers/cxl/pci.c
   :doc: cxl pci

.. kernel-doc:: drivers/cxl/pci.c
   :internal:

.. kernel-doc:: drivers/cxl/mem.c
   :doc: cxl mem

.. kernel-doc:: drivers/cxl/cxlmem.h
   :internal:

.. kernel-doc:: drivers/cxl/core/memdev.c
   :identifiers:

CXL Port
--------
.. kernel-doc:: drivers/cxl/port.c
   :doc: cxl port

CXL Core
--------
.. kernel-doc:: drivers/cxl/cxl.h
   :doc: cxl objects

.. kernel-doc:: drivers/cxl/cxl.h
   :internal:

.. kernel-doc:: drivers/cxl/acpi.c
   :identifiers: add_cxl_resources

.. kernel-doc:: drivers/cxl/core/hdm.c
   :doc: cxl core hdm

.. kernel-doc:: drivers/cxl/core/hdm.c
   :identifiers:

.. kernel-doc:: drivers/cxl/core/cdat.c
   :identifiers:

.. kernel-doc:: drivers/cxl/core/port.c
   :doc: cxl core

.. kernel-doc:: drivers/cxl/core/port.c
   :identifiers:

.. kernel-doc:: drivers/cxl/core/pci.c
   :doc: cxl core pci

.. kernel-doc:: drivers/cxl/core/pci.c
   :identifiers:

.. kernel-doc:: drivers/cxl/core/pmem.c
   :doc: cxl pmem

.. kernel-doc:: drivers/cxl/core/pmem.c
   :identifiers:

.. kernel-doc:: drivers/cxl/core/regs.c
   :doc: cxl registers

.. kernel-doc:: drivers/cxl/core/regs.c
   :identifiers:

.. kernel-doc:: drivers/cxl/core/mbox.c
   :doc: cxl mbox

.. kernel-doc:: drivers/cxl/core/mbox.c
   :identifiers:

.. kernel-doc:: drivers/cxl/core/features.c
   :doc: cxl features

See :c:func:`devm_cxl_setup_features` for API details.

CXL Regions
-----------
.. kernel-doc:: drivers/cxl/core/region.c
   :doc: cxl core region

.. kernel-doc:: drivers/cxl/core/region.c
   :identifiers:

External Interfaces
===================

CXL IOCTL Interface
-------------------

.. kernel-doc:: include/uapi/linux/cxl_mem.h
   :doc: UAPI

.. kernel-doc:: include/uapi/linux/cxl_mem.h
   :internal: