linux-kernel - [PATCH 9/9] Documentation/driver-api/cxl: add dax and sysram driver documentation

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Message-ID: <20260129210442.3951412-10-gourry@gourry.net>
Date: Thu, 29 Jan 2026 16:04:42 -0500
From: Gregory Price <gourry@...rry.net>
To: linux-mm@...ck.org
Cc: linux-cxl@...r.kernel.org,
	nvdimm@...ts.linux.dev,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
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Subject: [PATCH 9/9] Documentation/driver-api/cxl: add dax and sysram driver documentation

Explain the binding process for sysram and daxdev regions which are
explicit about which dax driver to use during region creation.

Jonathan Corbet <corbet@....net> 
Signed-off-by: Gregory Price <gourry@...rry.net>
---
 .../driver-api/cxl/linux/cxl-driver.rst       | 43 +++++++++++++++++++
 .../driver-api/cxl/linux/dax-driver.rst       | 29 +++++++++++++
 2 files changed, 72 insertions(+)

diff --git a/Documentation/driver-api/cxl/linux/cxl-driver.rst b/Documentation/driver-api/cxl/linux/cxl-driver.rst
index dd6dd17dc536..1f857345e896 100644
--- a/Documentation/driver-api/cxl/linux/cxl-driver.rst
+++ b/Documentation/driver-api/cxl/linux/cxl-driver.rst
@@ -445,6 +445,49 @@ for more details. ::
     dax0.0      devtype  modalias   uevent
     dax_region  driver   subsystem
 
+DAX regions are created when a CXL RAM region is bound to one of the
+following drivers:
+
+* :code:`cxl_devdax_region` - Creates a dax_region for device_dax mode.
+  The resulting DAX device provides direct userspace access via
+  :code:`/dev/daxN.Y`.
+
+* :code:`cxl_dax_kmem_region` - Creates a dax_region for kmem mode via a
+  sysram_region intermediate device.  See `Sysram Region`_ below.
+
+Sysram Region
+~~~~~~~~~~~~~
+A `Sysram Region` is an intermediate device between a CXL `Memory Region`
+and a `DAX Region` for kmem mode.  It is created when a CXL RAM region is
+bound to the :code:`cxl_sysram_region` driver.
+
+The sysram_region device provides an interposition point where users can
+configure memory hotplug policy before the underlying dax_region is created
+and memory is hotplugged to the system.
+
+The device hierarchy for kmem mode is::
+
+  regionX -> sysram_regionX -> dax_regionX -> daxX.Y
+
+The sysram_region exposes an :code:`online_type` attribute that controls
+how memory will be onlined when the dax_kmem driver binds:
+
+* :code:`invalid` - Not configured (default). Blocks driver binding.
+* :code:`offline` - Memory will not be onlined automatically.
+* :code:`online` - Memory will be onlined in ZONE_NORMAL.
+* :code:`online_movable` - Memory will be onlined in ZONE_MOVABLE.
+
+Example two-stage binding process::
+
+  # Bind region to sysram_region driver
+  echo region0 > /sys/bus/cxl/drivers/cxl_sysram_region/bind
+
+  # Configure memory online type
+  echo online_movable > /sys/bus/cxl/devices/sysram_region0/online_type
+
+  # Bind sysram_region to dax_kmem_region driver
+  echo sysram_region0 > /sys/bus/cxl/drivers/cxl_dax_kmem_region/bind
+
 Mailbox Interfaces
 ------------------
 A mailbox command interface for each device is exposed in ::
diff --git a/Documentation/driver-api/cxl/linux/dax-driver.rst b/Documentation/driver-api/cxl/linux/dax-driver.rst
index 10d953a2167b..2b8e21736292 100644
--- a/Documentation/driver-api/cxl/linux/dax-driver.rst
+++ b/Documentation/driver-api/cxl/linux/dax-driver.rst
@@ -17,6 +17,35 @@ The DAX subsystem exposes this ability through the `cxl_dax_region` driver.
 A `dax_region` provides the translation between a CXL `memory_region` and
 a `DAX Device`.
 
+CXL DAX Region Drivers
+======================
+CXL provides multiple drivers for creating DAX regions, each suited for
+different use cases:
+
+cxl_devdax_region
+-----------------
+The :code:`cxl_devdax_region` driver creates a dax_region configured for
+device_dax mode.  When a CXL RAM region is bound to this driver, the
+resulting DAX device provides direct userspace access via :code:`/dev/daxN.Y`.
+
+Device hierarchy::
+
+  regionX -> dax_regionX -> daxX.Y
+
+This is the simplest path for applications that want to manage CXL memory
+directly from userspace.
+
+cxl_dax_kmem_region
+-------------------
+For kmem mode, CXL provides a two-stage binding process that allows users
+to configure memory hotplug policy before memory is added to the system.
+
+The :code:`cxl_dax_kmem_region` driver then binds a sysram_region
+device and creates a dax_region configured for kmem mode.
+
+The :code:`online_type` policy will be passed from sysram_region to
+the dax kmem driver for use when hotplugging the memory.
+
 DAX Device
 ==========
 A `DAX Device` is a file-like interface exposed in :code:`/dev/daxN.Y`. A
-- 
2.52.0