CXL: Support region granularities finer than the root

Changes in v7:
- Rebase onto for-7.5/cxl-port-refactor
- Patch 3: adjust unaligned translate to work w new layouts (Sashiko)
- Patch 4: commit log wordsmith only
v6: https://lore.kernel.org/linux-cxl/[email protected]/

Begin Cover Letter:

Linux requires a region's interleave granularity to equal the interleave granularity of its interleaving root decoder. This series relaxes that one
restriction: a region granularity may be finer than the root granularity, provided the interleave levels between them refine monotonically toward the endpoints.

This series does not add general support for every granularity arrangement the CXL Specification permits. A region granularity coarser than the root remains invalid, and configurations whose granularity refines and then coarsens along the decoder path remain unsupported. Of the Mod3 configurations listed in CXL 4.0 Section 9.13.1.1, all are supported except Table 9-6 row 6, the refine-then-coarsen 12-way composition.

The use case driving this series is 6- and 12-way regions behind a 3-way host bridge interleave, CXL 4.0 Tables 9-6 and 9-7, which have no same-granularity alternative. Power-of-two regions finer than the root fall out of the same implementation.

Support applies to both firmware programmed auto regions and user created regions.

Two prior proposals addressed parts of this gap:

AlisonS introduced position arithmetic and sysfs gating for auto and user-created regions to support the 6- and 12-way interleave configurations without a same-granularity alternative [1].

RobertR introduced an HPA selector-bit model for auto regions that allows multi-level power-of-two interleaves regardless of granularity ordering [2].

Series structure in v7:
-----------------------
Patch 1 documents which region granularities Linux supports relative to the root and the monotonic refinement model.

Patch 2 generalizes endpoint position calculation to account for granularity changes between decoder levels.

Patch 3 enables auto regions with a granularity finer than the root and validates their decoder geometry.

Patch 4 enables user created regions with a granularity finer than the root by relaxing the existing root/region granularity restriction.

Patch 5 adds a cxl_test topology and coverage for region granularity finer than the root.

The companion NDCTL patchset that allows these regions in 'cxl create-region' and adds the unit test support is posted [3].

[1] https://lore.kernel.org/all/[email protected]/
[2] https://lore.kernel.org/all/[email protected]/
[3] https://lore.kernel.org/linux-cxl/[email protected]/

Alison Schofield (5):
Documentation/cxl: Describe region granularity finer than the root cxl/region: Generalize endpoint position mapping cxl/region: Support auto region granularity finer than the root cxl/region: Support user region granularity finer than the root cxl/test: Add a topology for region granularity finer than the root

 .../driver-api/cxl/linux/cxl-driver.rst       | 119 +++++
 drivers/cxl/core/region.c                     | 224 +++++++---
 tools/testing/cxl/test/cxl.c                  | 420 +++++++++++++++++-
 3 files changed, 683 insertions(+), 80 deletions(-)

base-commit: 0e3e8b653ca3ab145aced729861cf0d1a06da69e

2.37.3