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diff --git a/Documentation/driver-api/cxl/access-coordinates.rst b/Documentation/driver-api/cxl/access-coordinates.rst deleted file mode 100644 index b07950ea30c9..000000000000 --- a/Documentation/driver-api/cxl/access-coordinates.rst +++ /dev/null @@ -1,91 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0 -.. include:: <isonum.txt> - -================================== -CXL Access Coordinates Computation -================================== - -Shared Upstream Link Calculation -================================ -For certain CXL region construction with endpoints behind CXL switches (SW) or -Root Ports (RP), there is the possibility of the total bandwidth for all -the endpoints behind a switch being more than the switch upstream link. -A similar situation can occur within the host, upstream of the root ports. -The CXL driver performs an additional pass after all the targets have -arrived for a region in order to recalculate the bandwidths with possible -upstream link being a limiting factor in mind. - -The algorithm assumes the configuration is a symmetric topology as that -maximizes performance. When asymmetric topology is detected, the calculation -is aborted. An asymmetric topology is detected during topology walk where the -number of RPs detected as a grandparent is not equal to the number of devices -iterated in the same iteration loop. The assumption is made that subtle -asymmetry in properties does not happen and all paths to EPs are equal. - -There can be multiple switches under an RP. There can be multiple RPs under -a CXL Host Bridge (HB). There can be multiple HBs under a CXL Fixed Memory -Window Structure (CFMWS). - -An example hierarchy: - -> CFMWS 0 -> | -> _________|_________ -> | | -> ACPI0017-0 ACPI0017-1 -> GP0/HB0/ACPI0016-0 GP1/HB1/ACPI0016-1 -> | | | | -> RP0 RP1 RP2 RP3 -> | | | | -> SW 0 SW 1 SW 2 SW 3 -> | | | | | | | | -> EP0 EP1 EP2 EP3 EP4 EP5 EP6 EP7 - -Computation for the example hierarchy: - -Min (GP0 to CPU BW, - Min(SW 0 Upstream Link to RP0 BW, - Min(SW0SSLBIS for SW0DSP0 (EP0), EP0 DSLBIS, EP0 Upstream Link) + - Min(SW0SSLBIS for SW0DSP1 (EP1), EP1 DSLBIS, EP1 Upstream link)) + - Min(SW 1 Upstream Link to RP1 BW, - Min(SW1SSLBIS for SW1DSP0 (EP2), EP2 DSLBIS, EP2 Upstream Link) + - Min(SW1SSLBIS for SW1DSP1 (EP3), EP3 DSLBIS, EP3 Upstream link))) + -Min (GP1 to CPU BW, - Min(SW 2 Upstream Link to RP2 BW, - Min(SW2SSLBIS for SW2DSP0 (EP4), EP4 DSLBIS, EP4 Upstream Link) + - Min(SW2SSLBIS for SW2DSP1 (EP5), EP5 DSLBIS, EP5 Upstream link)) + - Min(SW 3 Upstream Link to RP3 BW, - Min(SW3SSLBIS for SW3DSP0 (EP6), EP6 DSLBIS, EP6 Upstream Link) + - Min(SW3SSLBIS for SW3DSP1 (EP7), EP7 DSLBIS, EP7 Upstream link)))) - -The calculation starts at cxl_region_shared_upstream_perf_update(). A xarray -is created to collect all the endpoint bandwidths via the -cxl_endpoint_gather_bandwidth() function. The min() of bandwidth from the -endpoint CDAT and the upstream link bandwidth is calculated. If the endpoint -has a CXL switch as a parent, then min() of calculated bandwidth and the -bandwidth from the SSLBIS for the switch downstream port that is associated -with the endpoint is calculated. The final bandwidth is stored in a -'struct cxl_perf_ctx' in the xarray indexed by a device pointer. If the -endpoint is direct attached to a root port (RP), the device pointer would be an -RP device. If the endpoint is behind a switch, the device pointer would be the -upstream device of the parent switch. - -At the next stage, the code walks through one or more switches if they exist -in the topology. For endpoints directly attached to RPs, this step is skipped. -If there is another switch upstream, the code takes the min() of the current -gathered bandwidth and the upstream link bandwidth. If there's a switch -upstream, then the SSLBIS of the upstream switch. - -Once the topology walk reaches the RP, whether it's direct attached endpoints -or walking through the switch(es), cxl_rp_gather_bandwidth() is called. At -this point all the bandwidths are aggregated per each host bridge, which is -also the index for the resulting xarray. - -The next step is to take the min() of the per host bridge bandwidth and the -bandwidth from the Generic Port (GP). The bandwidths for the GP is retrieved -via ACPI tables SRAT/HMAT. The min bandwidth are aggregated under the same -ACPI0017 device to form a new xarray. - -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. |