--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+===========================================
+Compute Express Link Subsystem Maturity Map
+===========================================
+
+The Linux CXL subsystem tracks the dynamic `CXL specification
+<https://computeexpresslink.org/cxl-specification-landing-page>`_ that
+continues to respond to new use cases with new features, capability
+updates and fixes. At any given point some aspects of the subsystem are
+more mature than others. While the periodic pull requests summarize the
+`work being incorporated each merge window
+<https://lore.kernel.org/linux-cxl/?q=s%3APULL+s%3ACXL+tc%3Atorvalds+NOT+s%3ARe>`_,
+those do not always convey progress relative to a starting point and a
+future end goal.
+
+What follows is a coarse breakdown of the subsystem's major
+responsibilities along with a maturity score. The expectation is that
+the change-history of this document provides an overview summary of the
+subsystem maturation over time.
+
+The maturity scores are:
+
+- [3] Mature: Work in this area is complete and no changes on the horizon.
+ Note that this score can regress from one kernel release to the next
+ based on new test results or end user reports.
+
+- [2] Stabilizing: Major functionality operational, common cases are
+ mature, but known corner cases are still a work in progress.
+
+- [1] Initial: Capability that has exited the Proof of Concept phase, but
+ may still have significant gaps to close and fixes to apply as real
+ world testing occurs.
+
+- [0] Known gap: Feature is on a medium to long term horizon to
+ implement. If the specification has a feature that does not even have
+ a '0' score in this document, there is a good chance that no one in
+ the linux-cxl@vger.kernel.org community has started to look at it.
+
+- X: Out of scope for kernel enabling, or kernel enabling not required
+
+Feature and Capabilities
+========================
+
+Enumeration / Provisioning
+--------------------------
+All of the fundamental enumeration an object model of the subsystem is
+in place, but there are several corner cases that are pending closure.
+
+
+* [2] CXL Window Enumeration
+
+ * [0] :ref:`Extended-linear memory-side cache <extended-linear>`
+ * [0] Low Memory-hole
+ * [0] Hetero-interleave
+
+* [2] Switch Enumeration
+
+ * [0] CXL register enumeration link-up dependency
+
+* [2] HDM Decoder Configuration
+
+ * [0] Decoder target and granularity constraints
+
+* [2] Performance enumeration
+
+ * [3] Endpoint CDAT
+ * [3] Switch CDAT
+ * [1] CDAT to Core-mm integration
+
+ * [1] x86
+ * [0] Arm64
+ * [0] All other arch.
+
+ * [0] Shared link
+
+* [2] Hotplug
+ (see CXL Window Enumeration)
+
+ * [0] Handle Soft Reserved conflicts
+
+* [0] :ref:`RCH link status <rch-link-status>`
+* [0] Fabrics / G-FAM (chapter 7)
+* [0] Global Access Endpoint
+
+
+RAS
+---
+In many ways CXL can be seen as a standardization of what would normally
+be handled by custom EDAC drivers. The open development here is
+mainly caused by the enumeration corner cases above.
+
+* [3] Component events (OS)
+* [2] Component events (FFM)
+* [1] Endpoint protocol errors (OS)
+* [1] Endpoint protocol errors (FFM)
+* [0] Switch protocol errors (OS)
+* [1] Switch protocol errors (FFM)
+* [2] DPA->HPA Address translation
+
+ * [1] XOR Interleave translation
+ (see CXL Window Enumeration)
+
+* [1] Memory Failure coordination
+* [0] Scrub control
+* [2] ACPI error injection EINJ
+
+ * [0] EINJ v2
+ * [X] Compliance DOE
+
+* [2] Native error injection
+* [3] RCH error handling
+* [1] VH error handling
+* [0] PPR
+* [0] Sparing
+* [0] Device built in test
+
+
+Mailbox commands
+----------------
+
+* [3] Firmware update
+* [3] Health / Alerts
+* [1] :ref:`Background commands <background-commands>`
+* [3] Sanitization
+* [3] Security commands
+* [3] RAW Command Debug Passthrough
+* [0] CEL-only-validation Passthrough
+* [0] Switch CCI
+* [3] Timestamp
+* [1] PMEM labels
+* [0] PMEM GPF / Dirty Shutdown
+* [0] Scan Media
+
+PMU
+---
+* [1] Type 3 PMU
+* [0] Switch USP/ DSP, Root Port
+
+Security
+--------
+
+* [X] CXL Trusted Execution Environment Security Protocol (TSP)
+* [X] CXL IDE (subsumed by TSP)
+
+Memory-pooling
+--------------
+
+* [1] Hotplug of LDs (via PCI hotplug)
+* [0] Dynamic Capacity Device (DCD) Support
+
+Multi-host sharing
+------------------
+
+* [0] Hardware coherent shared memory
+* [0] Software managed coherency shared memory
+
+Multi-host memory
+-----------------
+
+* [0] Dynamic Capacity Device Support
+* [0] Sharing
+
+Accelerator
+-----------
+
+* [0] Accelerator memory enumeration HDM-D (CXL 1.1/2.0 Type-2)
+* [0] Accelerator memory enumeration HDM-DB (CXL 3.0 Type-2)
+* [0] CXL.cache 68b (CXL 2.0)
+* [0] CXL.cache 256b Cache IDs (CXL 3.0)
+
+User Flow Support
+-----------------
+
+* [0] HPA->DPA Address translation (need xormaps export solution)
+
+Details
+=======
+
+.. _extended-linear:
+
+* **Extended-linear memory-side cache**: An HMAT proposal to enumerate the presence of a
+ memory-side cache where the cache capacity extends the SRAT address
+ range capacity. `See the ECN
+ <https://lore.kernel.org/linux-cxl/6650e4f835a0e_195e294a8@dwillia2-mobl3.amr.corp.intel.com.notmuch/>`_
+ for more details:
+
+.. _rch-link-status:
+
+* **RCH Link Status**: RCH (Restricted CXL Host) topologies, end up
+ hiding some standard registers like PCIe Link Status / Capabilities in
+ the CXL RCRB (Root Complex Register Block).
+
+.. _background-commands:
+
+* **Background commands**: The CXL background command mechanism is
+ awkward as the single slot is monopolized potentially indefinitely by
+ various commands. A `cancel on conflict
+ <http://lore.kernel.org/r/66035c2e8ba17_770232948b@dwillia2-xfh.jf.intel.com.notmuch>`_
+ facility is needed to make sure the kernel can ensure forward progress
+ of priority commands.
projects / nvme.git / commitdiff