Re: [PATCH v14 8/8] Documentation/cxl: Document DPA partition layout and ordering rules

[Dave Jiang <dave.jiang@intel.com>]

On 9/18/26 1:30 PM, Anisa Su wrote:
> 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>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
> Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
> Reviewed-by: Alison Schofield <alison.schofield@intel.com>
> ---
> Changes:
> [jonathan]: drop paragraph describing "support for additional
> dc partitions may be added later..." to avoid predicting future work in
> documentation
> [davidlohr]: add DCD sub-items to maturity-map.rst so a multi-step
> upstreaming shows users what is there and sets expectations.
> ---
>  .../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..eb4de2a9a9af 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 and exposes it as
> +`dynamic_ram_1`. 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

Maybe hold off this commit for when everything goes in? Otherwise it's weird that there's documentation but no code.

> @@ -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

I would just update maturity map once when the entire DCD code is in.

> +
>  Multi-host sharing
>  ------------------
>