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Documentation/cxl: Document DPA partition layout and ordering rules

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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@gourry.net>
Signed-off-by: Anisa Su <anisa.su@samsung.com>
Tested-by: Wonjae Lee <wj28.lee@samsung.com>
Tested-by: Junhee Park <jh9934.park@samsung.com>
Tested-by: Heesoo Kim <habil.kim@samsung.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Link: https://patch.msgid.link/20261010225852.10727-9-anisa.su@samsung.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
This commit is contained in:
Anisa Su 2026-10-11 07:54:59 +09:00 committed by Dave Jiang
parent c801107605
commit e4478834aa
2 changed files with 39 additions and 0 deletions
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35
Documentation/driver-api/cxl/linux/cxl-driver.rst
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@ -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

4
Documentation/driver-api/cxl/maturity-map.rst
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@ -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
------------------