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cxl: docs/platform/cdat reference documentation

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Add documentation for CDAT structures for CXL usages.

Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20250515000923.2590820-2-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
This commit is contained in:
Dave Jiang
2025-05-15 16:39:12 -07:00
parent f97bdc61c7
commit 49cee8fe5e
2 changed files with 119 additions and 0 deletions
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Documentation/driver-api/cxl/index.rst
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@@ -27,6 +27,7 @@ that have impacts on each other. The docs here break up configurations steps.
platform/bios-and-efi
platform/acpi
platform/cdat
platform/example-configs
.. toctree::
Documentation/driver-api/cxl/platform/cdat.rst
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.. SPDX-License-Identifier: GPL-2.0
======================================
Coherent Device Attribute Table (CDAT)
======================================
The CDAT provides functional and performance attributes of devices such
as CXL accelerators, switches, or endpoints. The table formatting is
similar to ACPI tables. CDAT data may be parsed by BIOS at boot or may
be enumerated at runtime (after device hotplug, for example).
Terminology:
DPA - Device Physical Address, used by the CXL device to denote the address
it supports for that device.
DSMADHandle - A device unique handle that is associated with a DPA range
defined by the DSMAS table.
===============================================
Device Scoped Memory Affinity Structure (DSMAS)
===============================================
The DSMAS contains information such as DSMADHandle, the DPA Base, and DPA
Length.
This table is used by Linux in conjunction with the Device Scoped Latency and
Bandwidth Information Structure (DSLBIS) to determine the performance
attributes of the CXL device itself.
Example ::
Structure Type : 00 [DSMAS]
Reserved : 00
Length : 0018 <- 24d, size of structure
DSMADHandle : 01
Flags : 00
Reserved : 0000
DPA Base : 0000000040000000 <- 1GiB base
DPA Length : 0000000080000000 <- 2GiB size
==================================================================
Device Scoped Latency and Bandwidth Information Structure (DSLBIS)
==================================================================
This table is used by Linux in conjunction with DSMAS to determine the
performance attributes of a CXL device. The DSLBIS contains latency
and bandwidth information based on DSMADHandle matching.
Example ::
Structure Type : 01 [DSLBIS]
Reserved : 00
Length : 18 <- 24d, size of structure
Handle : 0001 <- DSMAS handle
Flags : 00 <- Matches flag field for HMAT SLLBIS
Data Type : 00 <- Latency
Entry Basee Unit : 0000000000001000 <- Entry Base Unit field in HMAT SSLBIS
Entry : 010000000000 <- First byte used here, CXL LTC
Reserved : 0000
Structure Type : 01 [DSLBIS]
Reserved : 00
Length : 18 <- 24d, size of structure
Handle : 0001 <- DSMAS handle
Flags : 00 <- Matches flag field for HMAT SLLBIS
Data Type : 03 <- Bandwidth
Entry Basee Unit : 0000000000001000 <- Entry Base Unit field in HMAT SSLBIS
Entry : 020000000000 <- First byte used here, CXL BW
Reserved : 0000
==================================================================
Switch Scoped Latency and Bandwidth Information Structure (SSLBIS)
==================================================================
The SSLBIS contains information about the latency and bandwidth of a switch.
The table is used by Linux to compute the performance coordinates of a CXL path
from the device to the root port where a switch is part of the path.
Example ::
Structure Type : 05 [SSLBIS]
Reserved : 00
Length : 20 <- 32d, length of record, including SSLB entries
Data Type : 00 <- Latency
Reserved : 000000
Entry Base Unit : 00000000000000001000 <- Matches Entry Base Unit in HMAT SSLBIS
<- SSLB Entry 0
Port X ID : 0100 <- First port, 0100h represents an upstream port
Port Y ID : 0000 <- Second port, downstream port 0
Latency : 0100 <- Port latency
Reserved : 0000
<- SSLB Entry 1
Port X ID : 0100
Port Y ID : 0001
Latency : 0100
Reserved : 0000
Structure Type : 05 [SSLBIS]
Reserved : 00
Length : 18 <- 24d, length of record, including SSLB entry
Data Type : 03 <- Bandwidth
Reserved : 000000
Entry Base Unit : 00000000000000001000 <- Matches Entry Base Unit in HMAT SSLBIS
<- SSLB Entry 0
Port X ID : 0100 <- First port, 0100h represents an upstream port
Port Y ID : FFFF <- Second port, FFFFh indicates any port
Bandwidth : 1200 <- Port bandwidth
Reserved : 0000
The CXL driver uses a combination of CDAT, HMAT, SRAT, and other data to
generate "whole path performance" data for a CXL device.