[PATCH v3 03/17] cxl: docs/devices - add cxl device and protocol reference

Gregory Price
2025-05-12T16:22:00Z

Add a simple device primer sufficient to understand the theory
of operation documentation.

Signed-off-by: Gregory Price <gourry@gourry.net>
---
.../driver-api/cxl/devices/device-types.rst | 165 ++++++++++++++++++
Documentation/driver-api/cxl/index.rst | 6 +
2 files changed, 171 insertions(+)
create mode 100644 Documentation/driver-api/cxl/devices/device-types.rst

diff --git a/Documentation/driver-api/cxl/devices/device-types.rst b/Documentation/driver-api/cxl/devices/device-types.rst
new file mode 100644
index 000000000000..c70564cf0be3
--- /dev/null
+++ b/Documentation/driver-api/cxl/devices/device-types.rst
@@ -0,0 +1,165 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Devices and Protocols
+=====================
+
+The type of CXL device (Memory, Accelerator, etc) dictates many configuration steps. This section
+covers some basic background on device types and on-device resources used by the platform and OS
+which impact configuration.
+
+Protocols
+=========
+
+There are three core protocols to CXL. For the purpose of this documentation,
+we will only discuss very high level definitions as the specific hardware
+details are largely abstracted away from Linux. See the CXL specification
+for more details.
+
+CXL.io
+------
+The basic interaction protocol, similar to PCIe configuration mechanisms.
+Typically used for initialization, configuration, and I/O access for anything
+other than memory (CXL.mem) or cache (CXL.cache) operations.
+
+The Linux CXL driver exposes access to .io functionalty via the various sysfs
+interfaces and /dev/cxl/ devices (which exposes direct access to device
+mailboxes).
+
+CXL.cache
+---------
+The mechanism by which a device may coherently access and cache host memory.
+
+Largely transparent to Linux once configured.
+
+CXL.mem
+---------
+The mechanism by which the CPU may coherently access and cache device memory.
+
+Largely transparent to Linux once configured.
+
+
+Device Types
+============
+
+Type-1
+------
+
+A Type-1 CXL device:
+
+* Supports cxl.io and cxl.cache protocols
+* Implements a fully coherent cache
+* Allows Device-to-Host coherence and Host-to-Device snoops.
+* Does NOT have host-managed device memory (HDM)
+
+Typical examples of type-1 devices is a Smart NIC - which may want to
+directly operate on host-memory (DMA) to store incoming packets. These
+devices largely rely on CPU-attached memory.
+
+Type-2
+------
+
+A Type-2 CXL Device:
+
+* Supports cxl.io, cxl.cache, and cxl.mem protocols
+* Optionally implements coherent cache and Host-Managed Device Memory
+* Is typically an accelerator device w/ high bandwidth memory.
+
+The primary difference between a type-1 and type-2 device is the presence
+of host-managed device memory, which allows the device to operate on a
+local memory bank - while the CPU sill has coherent DMA to the same memory.
+
+The allows things like GPUs to expose their memory via DAX devices or file
+descriptors, allows drivers and programs direct access to device memory
+rather than use block-transfer semantics.
+
+Type-3
+------
+
+A Type-3 CXL Device
+
+* Supports cxl.io and cxl.mem
+* Implements Host-Managed Device Memory
+* May provide either Volatile or Persistent memory capacity (or both).
+
+A basic example of a type-3 device is a simple memory expander, whose
+local memory capacity is exposed to the CPU for access directly via
+basic coherent DMA.
+
+Switch
+------
+
+A CXL switch is a device capacity of routing any CXL (and by extension, PCIe)
+protocol between an upstream, downstream, or peer devices. Many devices, such
+as Multi-Logical Devices, imply the presence of switching in some manner.
+
+Logical Devices and Heads
+-------------------------
+
+A CXL device may present one or more "Logical Devices" to one or more hosts
+(via physical "Heads").
+
+A Single-Logical Device (SLD) is a device which presents a single device to
+one or more heads.
+
+A Multi-Logical Device (MLD) is a device which may present multiple devices
+to one or more devices.
+
+A Single-Headed Device exposes only a single physical connection.
+
+A Multi-Headed Device exposes multiple physical connections.
+
+MHSLD
+~~~~~
+A Multi-Headed Single-Logical Device (MHSLD) exposes a single logical
+device to multiple heads which may be connected to one or more discrete
+hosts. An example of this would be a simple memory-pool which may be
+statically configured (prior to boot) to expose portions of its memory
+to Linux via the CEDT ACPI table.
+
+MHMLD
+~~~~~
+A Multi-Headed Multi-Logical Device (MHMLD) exposes multiple logical
+devices to multiple heads which may be connected to one or more discrete
+hosts. An example of this would be a Dynamic Capacity Device or which
+may be configured at runtime to expose portions of its memory to Linux.
+
+Example Devices
+===============
+
+Memory Expander
+---------------
+The simplest form of Type-3 device is a memory expander. A memory expander
+exposes Host-Managed Device Memory (HDM) to Linux. This memory may be
+Volatile or Non-Volatile (Persistent).
+
+Memory Expanders will typically be considered a form of Single-Headed,
+Single-Logical Device - as its form factor will typically be an add-in-card
+(AIC) or some other similar form-factor.
+
+The Linux CXL driver provides support for static or dynamic configuration of
+basic memory expanders. The platform may program decoders prior to OS init
+(e.g. auto-decoders), or the user may program the fabric if the platform
+defers these operations to the OS.
+
+Multiple Memory Expanders may be added to an external chassis and exposed to
+a host via a head attached to a CXL switch. This is a "memory pool", and
+would be considered an MHSLD or MHMLD depending on the management capabilities
+provided by the switch platform.
+
+As of v6.14, Linux does not provide a formalized interface to manage non-DCD
+MHSLD or MHMLD devices.
+
+Dynamic Capacity Device (DCD)
+-----------------------------
+
+A Dynamic Capacity Device is a Type-3 device which provides dynamic management
+of memory capacity. The basic premise of a DCD to provide an allocator-like
+interface for physical memory capacity to a "Fabric Manager" (an external,
+privileged host with privileges to change configurations for other hosts).
+
+A DCD manages "Memory Extents", which may be volatile or persistent. Extents
+may also be exclusive to a single host or shared across multiple hosts.
+
+As of v6.14, Linux does not provide a formalized interface to manage DCD
+devices, however there is active work on LKML targeting future release.
diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
index fe1594dc6778..a2d1c5b18a8a 100644
--- a/Documentation/driver-api/cxl/index.rst
+++ b/Documentation/driver-api/cxl/index.rst
@@ -15,6 +15,12 @@ that have impacts on each other. The docs here break up configurations steps.
theory-of-operation
maturity-map

+.. toctree::
+ :maxdepth: 2
+ :caption: Device Reference
+
+ devices/device-types
+
.. toctree::
:maxdepth: 1
:caption: Linux Kernel Configuration
--
2.49.0