Commit 9bd8546e authored by Gregory Price's avatar Gregory Price Committed by Dave Jiang
Browse files

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: default avatarGregory Price <gourry@gourry.net>
Reviewed-by: default avatarDave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20250512162134.3596150-8-gourry@gourry.net


Signed-off-by: default avatarDave Jiang <dave.jiang@intel.com>
parent 8545940b

Documentation/driver-api/cxl/index.rst

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@@ -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

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.. 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