Documentation/driver-api/cxl: Introduce conventions.rst

There exists shipping platforms that bend, break, or otherwise lean on
ambiguities in the CXL specification. Without driver changes to accommodate
these deviations, end users are left without CXL subsystem RAS features.
Specifically, provisioning, error translation, and other flows require the
CXL subsystem to understand the platforms CXL topology beyond undecorated
memory address ranges.

Those isolated compatibility problems risk growing into deeper upstream
maintenance burden if different platform vendors arrive at diverging
solutions. For example, there are multiple options for resolving
low-memory-mmio intersecting large-interleave-ways CXL windows. Linux
should only entertain one solution to that problem.

Now, with the ACPI Specification Working Group, situations like this would
be resolved with the "Code First ECN" process to codify Linux expectations
in a specification. In the absence of such a process for the CXL
specification, create a file in Linux documentation to detail the
motivations, assumptions, tradeoffs, and proposals for amending
specification language.

The goal is to capture the issues such that platform vendors arrive at
compatible solutions for these problems and serve as a repository for
potential specification updates. The expectation is to update
conventions.rst along with CXL subsystem code changes to accommodate the
platform topology.

[ dj: Rebased against v6.16-rc1 ]

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Robert Richter <rrichter@amd.com>
Link: https://patch.msgid.link/20250603185254.3730099-1-dan.j.williams@intel.com


Signed-off-by: Dave Jiang <dave.jiang@intel.com>