[PATCH v3 07/17] cxl: docs/linux - overview

Gregory Price
2025-05-12T16:21:54Z

Add type-3 device configuration overview that explains the probe
process for a type-3 device from early-boot through memory-hotplug.

Signed-off-by: Gregory Price <gourry@gourry.net>
---
Documentation/driver-api/cxl/index.rst | 3 +-
.../driver-api/cxl/linux/overview.rst | 103 ++++++++++++++++++
2 files changed, 105 insertions(+), 1 deletion(-)
create mode 100644 Documentation/driver-api/cxl/linux/overview.rst

diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
index 6a5fb7e00c52..bc2228c77c32 100644
--- a/Documentation/driver-api/cxl/index.rst
+++ b/Documentation/driver-api/cxl/index.rst
@@ -30,9 +30,10 @@ that have impacts on each other. The docs here break up configurations steps.
platform/example-configs

.. toctree::
- :maxdepth: 1
+ :maxdepth: 2
:caption: Linux Kernel Configuration

+ linux/overview
linux/access-coordinates

diff --git a/Documentation/driver-api/cxl/linux/overview.rst b/Documentation/driver-api/cxl/linux/overview.rst
new file mode 100644
index 000000000000..648beb2c8c83
--- /dev/null
+++ b/Documentation/driver-api/cxl/linux/overview.rst
@@ -0,0 +1,103 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========
+Overview
+========
+
+This section presents the configuration process of a CXL Type-3 memory device,
+and how it is ultimately exposed to users as either a :code:`DAX` device or
+normal memory pages via the kernel's page allocator.
+
+Portions marked with a bullet are points at which certain kernel objects
+are generated.
+
+1) Early Boot
+
+ a) BIOS, Build, and Boot Parameters
+
+ i) EFI_MEMORY_SP
+ ii) CONFIG_EFI_SOFT_RESERVE
+ iii) CONFIG_MHP_DEFAULT_ONLINE_TYPE
+ iv) nosoftreserve
+
+ b) Memory Map Creation
+
+ i) EFI Memory Map / E820 Consulted for Soft-Reserved
+
+ * CXL Memory is set aside to be handled by the CXL driver
+
+ * Soft-Reserved IO Resource created for CFMWS entry
+
+ c) NUMA Node Creation
+
+ * Nodes created from ACPI CEDT CFMWS and SRAT Proximity domains (PXM)
+
+ d) Memory Tier Creation
+
+ * A default memory_tier is created with all nodes.
+
+ e) Contiguous Memory Allocation
+
+ * Any requested CMA is allocated from Online nodes
+
+ f) Init Finishes, Drivers start probing
+
+2) ACPI and PCI Drivers
+
+ a) Detects PCI device is CXL, marking it for probe by CXL driver
+
+3) CXL Driver Operation
+
+ a) Base device creation
+
+ * root, port, and memdev devices created
+ * CEDT CFMWS IO Resource creation
+
+ b) Decoder creation
+
+ * root, switch, and endpoint decoders created
+
+ c) Logical device creation
+
+ * memory_region and endpoint devices created
+
+ d) Devices are associated with each other
+
+ * If auto-decoder (BIOS-programmed decoders), driver validates
+ configurations, builds associations, and locks configs at probe time.
+
+ * If user-configured, validation and associations are built at
+ decoder-commit time.
+
+ e) Regions surfaced as DAX region
+
+ * dax_region created
+
+ * DAX device created via DAX driver
+
+4) DAX Driver Operation
+
+ a) DAX driver surfaces DAX region as one of two dax device modes
+
+ * kmem - dax device is converted to hotplug memory blocks
+
+ * DAX kmem IO Resource creation
+
+ * hmem - dax device is left as daxdev to be accessed as a file.
+
+ * If hmem, journey ends here.
+
+ b) DAX kmem surfaces memory region to Memory Hotplug to add to page
+ allocator as "driver managed memory"
+
+5) Memory Hotplug
+
+ a) mhp component surfaces a dax device memory region as multiple memory
+ blocks to the page allocator
+
+ * blocks appear in :code:`/sys/bus/memory/devices` and linked to a NUMA node
+
+ b) blocks are onlined into the requested zone (NORMAL or MOVABLE)
+
+ * Memory is marked "Driver Managed" to avoid kexec from using it as region
+ for kernel updates
--
2.49.0