[PATCH v15 8/8] Documentation/cxl: Document DPA partition layout and ordering rules

[Anisa Su]

DC Partitions complicate DPA ordering. Add a DPA Partitions section to
cxl-driver.rst describing spec-mandated and Linux requirements for the
layout.

maturity-map.rst breaks the DCD entry into sub-items so the map shows
what this series lands: partition enumeration is [1], event handling
and DC-backed DAX regions remain [0] until the follow-on series enables
them. The parent stays [0]; nothing is usable yet.

Suggested-by: Gregory Price <gourry@xxxxxxxxxx>
Signed-off-by: Anisa Su <anisa.su@xxxxxxxxxxx>
Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>
Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Reviewed-by: Alison Schofield <alison.schofield@xxxxxxxxx>
---
 .../driver-api/cxl/linux/cxl-driver.rst       | 35 +++++++++++++++++++
 Documentation/driver-api/cxl/maturity-map.rst |  4 +++
 2 files changed, 39 insertions(+)

diff --git a/Documentation/driver-api/cxl/linux/cxl-driver.rst b/Documentation/driver-api/cxl/linux/cxl-driver.rst
index dd6dd17dc536..212df4fb8d63 100644
--- a/Documentation/driver-api/cxl/linux/cxl-driver.rst
+++ b/Documentation/driver-api/cxl/linux/cxl-driver.rst
@@ -181,6 +181,41 @@ A Memory Device is a discrete base object that is not a port.  While the
 physical device it belongs to may also host an `endpoint`, the relationship
 between an `endpoint` and a `memdev` is not captured in sysfs.
 
+DPA Partitions
+~~~~~~~~~~~~~~
+A memory device presents its capacity as one flat `Device Physical Address`
+(DPA) space divided into `partitions`, which Linux lays out in a fixed
+order::
+
+  DPA 0                                                     end
+  +---------------+---------------+---------------------------+
+  |      ram      |     pmem      |       dynamic_ram_1       |
+  +---------------+---------------+---------------------------+
+  part[0]         part[1]         part[2]
+
+Part of that order is required by the CXL specification and part of it is a
+Linux choice.
+
+The `ram` and `pmem` order is mandated.  CXL r4.0 section 8.2.10.9.2.1 "Get
+Partition Info" (4100h), Table 8-310, mandates that volatile capacity starts
+at DPA 0 and pmem starts at the DPA immediately following it.
+
+Dynamic Capacity partitions only need to be 256MB aligned according to
+CXL r4.0 section 8.2.10.9.9.1 "Get Dynamic Capacity Configuration"
+(opcode 4800h), Table 8-347. So a device could leave a gap between ram/pmem
+(static) capacity and its first DC partition, or between one DC partition
+and the next.
+
+Linux chooses not to support gaps between static and dynamic capacity: the
+first DC partition must begin at the DPA immediately following static
+capacity -- after pmem, after ram on a device with no pmem, or at DPA 0 on
+a device with no static capacity at all.
+
+Currently, only one dynamic partition is supported. A device may report up
+to eight (CXL r4.0 Table 8-346); Linux configures the first as its
+`dynamic_ram_1` partition. Gaps between DC partitions are not checked, since
+only the first is used.
+
 Port Relationships
 ~~~~~~~~~~~~~~~~~~
 In our example described above, there are four host bridges attached to the
diff --git a/Documentation/driver-api/cxl/maturity-map.rst b/Documentation/driver-api/cxl/maturity-map.rst
index 282c1102dd81..fefd31899229 100644
--- a/Documentation/driver-api/cxl/maturity-map.rst
+++ b/Documentation/driver-api/cxl/maturity-map.rst
@@ -150,6 +150,10 @@ Memory-pooling
 * [1] Hotplug of LDs (via PCI hotplug)
 * [0] Dynamic Capacity Device (DCD) Support
 
+  * [1] DC partition enumeration / configuration (Get DC Config, CDAT DSMAS, event interrupts)
+  * [0] Extent add / release event handling
+  * [0] DC-backed DAX regions
+
 Multi-host sharing
 ------------------
 
-- 
2.43.0