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+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+===================================
+Compute Express Link Memory Devices
+===================================
+
+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.
+
+CXL Bus: Theory of Operation
+============================
+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
+
+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/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/regs.c
+ :doc: cxl registers
+
+.. kernel-doc:: drivers/cxl/core/mbox.c
+ :doc: cxl mbox
+
+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: