cxl: doc/linux/access-coordinates Update access coordinates calculation methods

Add documentation on how to calculate the access coordinates for a given
CXL region in detail.

Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20250515000923.2590820-4-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: davejiang

Documentation/driver-api/cxl/linux/access-coordinates.rst

@@ -5,6 +5,83 @@
 CXL Access Coordinates Computation
 ==================================
 
+Latency and Bandwidth Calculation
+=================================
+A memory region performance coordinates (latency and bandwidth) are typically
+provided via ACPI tables :doc:`SRAT <../platform/acpi/srat>` and
+:doc:`HMAT <../platform/acpi/hmat>`. However, the platform firmware (BIOS) is
+not able to annotate those for CXL devices that are hot-plugged since they do
+not exist during platform firmware initialization. The CXL driver can compute
+the performance coordinates by retrieving data from several components.
+
+The :doc:`SRAT <../platform/acpi/srat>` provides a Generic Port Affinity
+subtable that ties a proximity domain to a device handle, which in this case
+would be the CXL hostbridge. Using this association, the performance
+coordinates for the Generic Port can be retrieved from the
+:doc:`HMAT <../platform/acpi/hmat>` subtable. This piece represents the
+performance coordinates between a CPU and a Generic Port (CXL hostbridge).
+
+The :doc:`CDAT <../platform/cdat>` provides the performance coordinates for
+the CXL device itself. That is the bandwidth and latency to access that device's
+memory region. The DSMAS subtable provides a DSMADHandle that is tied to a
+Device Physical Address (DPA) range. The DSLBIS subtable provides the
+performance coordinates that's tied to a DSMADhandle and this ties the two
+table entries together to provide the performance coordinates for each DPA
+region. For example, if a device exports a DRAM region and a PMEM region,
+then there would be different performance characteristsics for each of those
+regions.
+
+If there's a CXL switch in the topology, then the performance coordinates for the
+switch is provided by SSLBIS subtable. This provides the bandwidth and latency
+for traversing the switch between the switch upstream port and the switch
+downstream port that points to the endpoint device.
+
+Simple topology example::
+
+ GP0/HB0/ACPI0016-0
+        RP0
+         |
+         | L0
+         |
+     SW 0 / USP0
+     SW 0 / DSP0
+         |
+         | L1
+         |
+        EP0
+
+In this example, there is a CXL switch between an endpoint and a root port.
+Latency in this example is calculated as such:
+L(EP0) - Latency from EP0 CDAT DSMAS+DSLBIS
+L(L1) - Link latency between EP0 and SW0DSP0
+L(SW0) - Latency for the switch from SW0 CDAT SSLBIS.
+L(L0) - Link latency between SW0 and RP0
+L(RP0) - Latency from root port to CPU via SRAT and HMAT (Generic Port).
+Total read and write latencies are the sum of all these parts.
+
+Bandwidth in this example is calculated as such:
+B(EP0) - Bandwidth from EP0 CDAT DSMAS+DSLBIS
+B(L1) - Link bandwidth between EP0 and SW0DSP0
+B(SW0) - Bandwidth for the switch from SW0 CDAT SSLBIS.
+B(L0) - Link bandwidth between SW0 and RP0
+B(RP0) - Bandwidth from root port to CPU via SRAT and HMAT (Generic Port).
+The total read and write bandwidth is the min() of all these parts.
+
+To calculate the link bandwidth:
+LinkOperatingFrequency (GT/s) is the current negotiated link speed.
+DataRatePerLink (MB/s) = LinkOperatingFrequency / 8
+Bandwidth (MB/s) = PCIeCurrentLinkWidth * DataRatePerLink
+Where PCIeCurrentLinkWidth is the number of lanes in the link.
+
+To calculate the link latency:
+LinkLatency (picoseconds) = FlitSize / LinkBandwidth (MB/s)
+
+See `CXL Memory Device SW Guide r1.0 <https://www.intel.com/content/www/us/en/content-details/643805/cxl-memory-device-software-guide.html>`_,
+section 2.11.3 and 2.11.4 for details.
+
+In the end, the access coordinates for a constructed memory region is calculated from one
+or more memory partitions from each of the CXL device(s).
+
 Shared Upstream Link Calculation
 ================================
 For certain CXL region construction with endpoints behind CXL switches (SW) or
@@ -91,5 +168,11 @@ Finally, the cxl_region_update_bandwidth() is called and the aggregated
 bandwidth from all the members of the last xarray is updated for the
 access coordinates residing in the cxl region (cxlr) context.
 
-.. kernel-doc:: drivers/cxl/acpi.c
-   :identifiers: cxl_acpi_evaluate_qtg_dsm
+QTG ID
+======
+Each :doc:`CEDT <../platform/acpi/cedt>` has a QTG ID field. This field provides
+the ID that associates with a QoS Throttling Group (QTG) for the CFMWS window.
+Once the access coordinates are calculated, an ACPI Device Specific Method can
+be issued to the ACPI0016 device to retrieve the QTG ID depends on the access
+coordinates provided. The QTG ID for the device can be used as guidance to match
+to the CFMWS to setup the best Linux root decoder for the device performance.