cxl: docs/linux - overview

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>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20250512162134.3596150-8-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

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
 
 

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