Re: [RESEND PATCH v13 8/8] Documentation/cxl: Document DPA partition layout and ordering rules

[Jonathan Cameron <jic23@kernel.org>]

On Tue,  8 Sep 2026 03:15:12 -0700
Anisa Su <anisa.su887@gmail.com> wrote:

> DC Partitions complicate DPA ordering. Add a DPA Partitions section to
> cxl-driver.rst describing spec-mandated and Linux requirements for the
> layout.
> 
> Suggested-by: Gregory Price <gourry@gourry.net>
> Signed-off-by: Anisa Su <anisa.su@samsung.com>
A couple of minor potential tweaks.  Otherwise LGTM
Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

> 
> ---
> New patch in v13.
> ---
>  .../driver-api/cxl/linux/cxl-driver.rst       | 38 +++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/Documentation/driver-api/cxl/linux/cxl-driver.rst b/Documentation/driver-api/cxl/linux/cxl-driver.rst
> index dd6dd17dc536..f0742d6c86c3 100644
> --- a/Documentation/driver-api/cxl/linux/cxl-driver.rst
> +++ b/Documentation/driver-api/cxl/linux/cxl-driver.rst
> @@ -181,6 +181,44 @@ 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 follows the static precedent anyway for the partition it maps: the

Maybe "Linux chooses to only support...

> +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`.
> +
> +Support for additional dynamic partitions may be added if devices appear
> +that need it, which is what the `dynamic_ram_1` name leaves room for.  Until
> +then a device offering more than one is still usable, just not in full.
I'd drop this last paragraph. Predicting the future is tricky, even with a may.
Hopefully anyone realises that if something is needed Linux doesn't support
they should propose patches to add it!

Jonathan

> +
>  Port Relationships
>  ~~~~~~~~~~~~~~~~~~
>  In our example described above, there are four host bridges attached to the