cxl: Add KUnit tests and documentation

Changes in v2:
- Use explicit hex designators in the decoder count table to match the spec (JonathanC)
- Generalize arch option and add help ref in testing page (AnisaS)
- Explicitly initialize the mock return_code to SUCCESS (MingL)
- Add a 'Maintaining KUnit tests' section to testing page
- Wrap long lines in testing page (JonathanC)
- Rebase on 7.4-rc1
v1: https://lore.kernel.org/linux-cxl/[email protected]/

The CXL subsystem has no KUnit tests. Its automated test coverage comes from cxl_test and the ndctl suite, which exercise the driver stack through an emulated CXL topology. KUnit complements that coverage by testing self- contained logic and error paths in isolation.

This series introduces CXL KUnit infrastructure and documentation describing how it complements the existing cxl_test coverage.

The first KUnit suite, a collection of related test cases, exercises the HDM decoder interleave encoding helpers, providing a starting point for testing self-contained CXL logic. The second suite exercises mailbox command handling, including error paths that are difficult to reach through cxl_test.

The mailbox tests also demonstrate the value of regression coverage. An unknown device return code can cause an out-of-bounds read and allow a failed command to be reported as successful. The series corrects that behavior, then adds tests that reproduce the failure without the correction. With KASAN enabled, the tests also expose the out-of-bounds read.

Together, the two suites establish a pattern for adding KUnit coverage to CXL, both for existing functionality and for bugs as they are found.

Alison Schofield (3):
Documentation/driver-api/cxl: Add a testing page
cxl: Add KUnit tests for HDM decoder encoding helpers cxl/mbox: Add KUnit tests for cxl_internal_send_cmd()

Gaobin Huang (1):
cxl/mbox: Treat unknown device return codes as errors

 Documentation/driver-api/cxl/index.rst   |   1 +
 Documentation/driver-api/cxl/testing.rst | 139 +++++++++++++
 drivers/cxl/.kunitconfig                 |   4 +
 drivers/cxl/Kconfig                      |  16 ++
 drivers/cxl/Makefile                     |   1 +
 drivers/cxl/core/Makefile                |   1 +
 drivers/cxl/core/tests/Makefile          |   5 +
 drivers/cxl/core/tests/mbox_kunit.c      | 250 +++++++++++++++++++++++
 drivers/cxl/cxlmem.h                     |   6 +
 drivers/cxl/tests/Makefile               |   5 +
 drivers/cxl/tests/hdm_encoding_kunit.c   | 159 ++++++++++++++
 11 files changed, 587 insertions(+)
 create mode 100644 Documentation/driver-api/cxl/testing.rst
 create mode 100644 drivers/cxl/.kunitconfig
 create mode 100644 drivers/cxl/core/tests/Makefile
 create mode 100644 drivers/cxl/core/tests/mbox_kunit.c
 create mode 100644 drivers/cxl/tests/Makefile
 create mode 100644 drivers/cxl/tests/hdm_encoding_kunit.c

base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4

2.37.3