[PATCH v3 05/17] cxl: docs/platform/acpi reference documentation

Gregory Price
2025-05-12T16:21:51Z

Add basic ACPI table information needed to understand the CXL
driver probe process.

Signed-off-by: Gregory Price <gourry@gourry.net>
---
Documentation/driver-api/cxl/index.rst | 1 +
.../driver-api/cxl/platform/acpi.rst | 76 +++++++++++++++++++
.../driver-api/cxl/platform/acpi/cedt.rst | 62 +++++++++++++++
.../driver-api/cxl/platform/acpi/dsdt.rst | 28 +++++++
.../driver-api/cxl/platform/acpi/hmat.rst | 32 ++++++++
.../driver-api/cxl/platform/acpi/slit.rst | 21 +++++
.../driver-api/cxl/platform/acpi/srat.rst | 44 +++++++++++
7 files changed, 264 insertions(+)
create mode 100644 Documentation/driver-api/cxl/platform/acpi.rst
create mode 100644 Documentation/driver-api/cxl/platform/acpi/cedt.rst
create mode 100644 Documentation/driver-api/cxl/platform/acpi/dsdt.rst
create mode 100644 Documentation/driver-api/cxl/platform/acpi/hmat.rst
create mode 100644 Documentation/driver-api/cxl/platform/acpi/slit.rst
create mode 100644 Documentation/driver-api/cxl/platform/acpi/srat.rst

diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
index ffa0462ad950..336322dc35a0 100644
--- a/Documentation/driver-api/cxl/index.rst
+++ b/Documentation/driver-api/cxl/index.rst
@@ -26,6 +26,7 @@ that have impacts on each other. The docs here break up configurations steps.
:caption: Platform Configuration

platform/bios-and-efi
+ platform/acpi

.. toctree::
:maxdepth: 1
diff --git a/Documentation/driver-api/cxl/platform/acpi.rst b/Documentation/driver-api/cxl/platform/acpi.rst
new file mode 100644
index 000000000000..ee7e6bd4c43d
--- /dev/null
+++ b/Documentation/driver-api/cxl/platform/acpi.rst
@@ -0,0 +1,76 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========
+ACPI Tables
+===========
+
+ACPI is the "Advanced Configuration and Power Interface", which is a standard
+that defines how platforms and OS manage power and configure computer hardware.
+For the purpose of this theory of operation, when referring to "ACPI" we will
+usually refer to "ACPI Tables" - which are the way a platform (BIOS/EFI)
+communicates static configuration information to the operation system.
+
+The Following ACPI tables contain *static* configuration and performance data
+about CXL devices.
+
+.. toctree::
+ :maxdepth: 1
+
+ acpi/cedt.rst
+ acpi/srat.rst
+ acpi/hmat.rst
+ acpi/slit.rst
+ acpi/dsdt.rst
+
+The SRAT table may also contain generic port/initiator content that is intended
+to describe the generic port, but not information about the rest of the path to
+the endpoint.
+
+Linux uses these tables to configure kernel resources for statically configured
+(by BIOS/EFI) CXL devices, such as:
+
+- NUMA nodes
+- Memory Tiers
+- NUMA Abstract Distances
+- SystemRAM Memory Regions
+- Weighted Interleave Node Weights
+
+ACPI Debugging
+==============
+
+The :code:`acpidump -b` command dumps the ACPI tables into binary format.
+
+The :code:`iasl -d` command disassembles the files into human readable format.
+
+Example :code:`acpidump -b && iasl -d cedt.dat` ::
+
+ [000h 0000 4] Signature : "CEDT" [CXL Early Discovery Table]
+
+Common Issues
+-------------
+Most failures described here result in a failure of the driver to surface
+memory as a DAX device and/or kmem.
+
+* CEDT CFMWS targets list UIDs do not match CEDT CHBS UIDs.
+* CEDT CFMWS targets list UIDs do not match DSDT CXL Host Bridge UIDs.
+* CEDT CFMWS Restriction Bits are not correct.
+* CEDT CFMWS Memory regions are poorly aligned.
+* CEDT CFMWS Memory regions spans a platform memory hole.
+* CEDT CHBS UIDs do not match DSDT CXL Host Bridge UIDs.
+* CEDT CHBS Specification version is incorrect.
+* SRAT is missing regions described in CEDT CFMWS.
+
+ * Result: failure to create a NUMA node for the region, or
+ region is placed in wrong node.
+
+* HMAT is missing data for regions described in CEDT CFMWS.
+
+ * Result: NUMA node being placed in the wrong memory tier.
+
+* SLIT has bad data.
+
+ * Result: Lots of performance mechanisms in the kernel will be very unhappy.
+
+All of these issues will appear to users as if the driver is failing to
+support CXL - when in reality they are all the failure of a platform to
+configure the ACPI tables correctly.
diff --git a/Documentation/driver-api/cxl/platform/acpi/cedt.rst b/Documentation/driver-api/cxl/platform/acpi/cedt.rst
new file mode 100644
index 000000000000..1d9c9d3592dc
--- /dev/null
+++ b/Documentation/driver-api/cxl/platform/acpi/cedt.rst
@@ -0,0 +1,62 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+CEDT - CXL Early Discovery Table
+================================
+
+The CXL Early Discovery Table is generated by BIOS to describe the CXL memory
+regions configured at boot by the BIOS.
+
+CHBS
+====
+The CXL Host Bridge Structure describes CXL host bridges. Other than describing
+device register information, it reports the specific host bridge UID for this
+host bridge. These host bridge ID's will be referenced in other tables.
+
+Example ::
+
+ Subtable Type : 00 [CXL Host Bridge Structure]
+ Reserved : 00
+ Length : 0020
+ Associated host bridge : 00000007 <- Host bridge _UID
+ Specification version : 00000001
+ Reserved : 00000000
+ Register base : 0000010370400000
+ Register length : 0000000000010000
+
+CFMWS
+=====
+The CXL Fixed Memory Window structure describes a memory region associated
+with one or more CXL host bridges (as described by the CHBS). It additionally
+describes any inter-host-bridge interleave configuration that may have been
+programmed by BIOS.
+
+Example ::
+
+ Subtable Type : 01 [CXL Fixed Memory Window Structure]
+ Reserved : 00
+ Length : 002C
+ Reserved : 00000000
+ Window base address : 000000C050000000 <- Memory Region
+ Window size : 0000003CA0000000
+ Interleave Members (2^n) : 01 <- Interleave configuration
+ Interleave Arithmetic : 00
+ Reserved : 0000
+ Granularity : 00000000
+ Restrictions : 0006
+ QtgId : 0001
+ First Target : 00000007 <- Host Bridge _UID
+ Next Target : 00000006 <- Host Bridge _UID
+
+The restriction field dictates what this SPA range may be used for (memory type,
+voltile vs persistent, etc). One or more bits may be set. ::
+
+ Bit[0]: CXL Type 2 Memory
+ Bit[1]: CXL Type 3 Memory
+ Bit[2]: Volatile Memory
+ Bit[3]: Persistent Memory
+ Bit[4]: Fixed Config (HPA cannot be re-used)
+
+INTRA-host-bridge interleave (multiple devices on one host bridge) is NOT
+reported in this structure, and is solely defined via CXL device decoder
+programming (host bridge and endpoint decoders).
diff --git a/Documentation/driver-api/cxl/platform/acpi/dsdt.rst b/Documentation/driver-api/cxl/platform/acpi/dsdt.rst
new file mode 100644
index 000000000000..b4583b01d67d
--- /dev/null
+++ b/Documentation/driver-api/cxl/platform/acpi/dsdt.rst
@@ -0,0 +1,28 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================================
+DSDT - Differentiated system Description Table
+==============================================
+
+This table describes what peripherals a machine has.
+
+This table's UIDs for CXL devices - specifically host bridges, must be
+consistent with the contents of the CEDT, otherwise the CXL driver will
+fail to probe correctly.
+
+Example Compute Express Link Host Bridge ::
+
+ Scope (_SB)
+ {
+ Device (S0D0)
+ {
+ Name (_HID, "ACPI0016" /* Compute Express Link Host Bridge */) // _HID: Hardware ID
+ Name (_CID, Package (0x02) // _CID: Compatible ID
+ {
+ EisaId ("PNP0A08") /* PCI Express Bus */,
+ EisaId ("PNP0A03") /* PCI Bus */
+ })
+ ...
+ Name (_UID, 0x05) // _UID: Unique ID
+ ...
+ }
diff --git a/Documentation/driver-api/cxl/platform/acpi/hmat.rst b/Documentation/driver-api/cxl/platform/acpi/hmat.rst
new file mode 100644
index 000000000000..095a26f02a37
--- /dev/null
+++ b/Documentation/driver-api/cxl/platform/acpi/hmat.rst
@@ -0,0 +1,32 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================================
+HMAT - Heterogeneous Memory Attribute Table
+===========================================
+
+The Heterogeneous Memory Attributes Table contains information such as cache
+attributes and bandwidth and latency details for memory proximity domains.
+For the purpose of this document, we will only discuss the SSLIB entry.
+
+SLLBI
+=====
+The System Locality Latency and Bandwidth Information records latency and
+bandwidth information for proximity domains.
+
+This table is used by Linux to configure interleave weights and memory tiers.
+
+Example (Heavily truncated for brevity) ::
+
+ Structure Type : 0001 [SLLBI]
+ Data Type : 00 <- Latency
+ Target Proximity Domain List : 00000000
+ Target Proximity Domain List : 00000001
+ Entry : 0080 <- DRAM LTC
+ Entry : 0100 <- CXL LTC
+
+ Structure Type : 0001 [SLLBI]
+ Data Type : 03 <- Bandwidth
+ Target Proximity Domain List : 00000000
+ Target Proximity Domain List : 00000001
+ Entry : 1200 <- DRAM BW
+ Entry : 0200 <- CXL BW
diff --git a/Documentation/driver-api/cxl/platform/acpi/slit.rst b/Documentation/driver-api/cxl/platform/acpi/slit.rst
new file mode 100644
index 000000000000..a56768e8fe41
--- /dev/null
+++ b/Documentation/driver-api/cxl/platform/acpi/slit.rst
@@ -0,0 +1,21 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========================================
+SLIT - System Locality Information Table
+========================================
+
+The system locality information table provides "abstract distances" between
+accessor and memory nodes. Node without initiators (cpus) are infinitely (FF)
+distance away from all other nodes.
+
+The abstract distance described in this table does not describe any real
+latency of bandwidth information.
+
+Example ::
+
+ Signature : "SLIT" [System Locality Information Table]
+ Localities : 0000000000000004
+ Locality 0 : 10 20 20 30
+ Locality 1 : 20 10 30 20
+ Locality 2 : FF FF 0A FF
+ Locality 3 : FF FF FF 0A
diff --git a/Documentation/driver-api/cxl/platform/acpi/srat.rst b/Documentation/driver-api/cxl/platform/acpi/srat.rst
new file mode 100644
index 000000000000..56d7bbb18c3b
--- /dev/null
+++ b/Documentation/driver-api/cxl/platform/acpi/srat.rst
@@ -0,0 +1,44 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+SRAT - Static Resource Affinity Table
+=====================================
+
+The System/Static Resource Affinity Table describes resource (CPU, Memory)
+affinity to "Proximity Domains". This table is technically optional, but for
+performance information (see "HMAT") to be enumerated by linux it must be
+present.
+
+There is a careful dance between the CEDT and SRAT tables and how NUMA nodes are
+created. If things don't look quite the way you expect - check the SRAT Memory
+Affinity entries and CEDT CFMWS to determine what your platform actually
+supports in terms of flexible topologies.
+
+The SRAT may statically assign portions of a CFMWS SPA range to a specific
+proximity domains. See linux numa creation for more information about how
+this presents in the NUMA topology.
+
+Proximity Domain
+================
+A proximity domain is ROUGHLY equivalent to "NUMA Node" - though a 1-to-1
+mapping is not guaranteed. There are scenarios where "Proximity Domain 4" may
+map to "NUMA Node 3", for example. (See "NUMA Node Creation")
+
+Memory Affinity
+===============
+Generally speaking, if a host does any amount of CXL fabric (decoder)
+programming in BIOS - an SRAT entry for that memory needs to be present.
+
+Example ::
+
+ Subtable Type : 01 [Memory Affinity]
+ Length : 28
+ Proximity Domain : 00000001 <- NUMA Node 1
+ Reserved1 : 0000
+ Base Address : 000000C050000000 <- Physical Memory Region
+ Address Length : 0000003CA0000000
+ Reserved2 : 00000000
+ Flags (decoded below) : 0000000B
+ Enabled : 1
+ Hot Pluggable : 1
+ Non-Volatile : 0
--
2.49.0