Re: [PATCH v9 0/3] Support zero-sized HDM decoders

[Richard Cheng]

On Wed, Aug 05, 2026 at 01:55:21PM +0800, Richard Cheng wrote:

Hello folks,

Just a gentle ping on the series, thanks.

Best regards,
Richard Cheng.

> CXL r4.0 §8.2.4.20.12 ("Committing Decoder Programming") and
> §14.13.10 ("CXL HDM Decoder Zero Size Commit") permit committing an
> HDM decoder with size 0. Firmware may commit and lock such decoders so
> the OS cannot program regions through them; this is a platform design
> choice rather than a specification requirement.
> 
> The kernel currently rejects these decoders with -ENXIO during port
> enumeration and aborts enumeration of the entire port. On affected
> systems, this prevents the CXL topology from appearing in userspace.
> 
> This series enumerates empty committed decoders into the topology while
> keeping them out of region assembly.
> 
> Patch 1 is a preparatory cleanup with no functional change. In
> poison_by_decoder(), "rc" carries both an error code and the loop
> control signal for device_for_each_child(), so returning it bare is
> misleading, the early guards mean "keep walking", not "no error".
> Zeroing "rc" to forgive an -EFAULT on a RAM partition adds to that by
> discarding what the device actually returned. Return a literal 0 where
> the walk should continue, and test the forgiven case directly instead
> of rewriting "rc". Patch 2 extends this function, so cleaning it up
> first keeps the functional change free of unrelated churn.
> 
> Patch 2 makes zero-size DPA reservations first-class. Since the resource
> tree cannot represent an empty range, an empty decoder is backed by a
> separately allocated resource. This keeps port->hdm_end, skip handling,
> and in-order DPA accounting consistent. Operations requiring non-empty
> capacity, including region attachment and poison queries, are guarded
> appropriately.
> 
> Patch 3 adds the mock_zero_size_decoders cxl_test module parameter,
> disabled by default. When enabled, it installs committed, locked,
> zero-sized endpoint, switch, and host-bridge decoders under
> host-bridge0.
> 
> Tested on arm64 with cxl_test and the ndctl unit suite (ndctl
> v85), on a kernel built with KASAN and lockdep enabled.
> 
> Full cxl suite with mock_zero_size_decoders off:
> """
>  1/16 ndctl:cxl / cxl-topology.sh       OK                7.50s
>  2/16 ndctl:cxl / cxl-region-sysfs.sh   OK                5.29s
>  3/16 ndctl:cxl / cxl-labels.sh         OK                3.44s
>  4/16 ndctl:cxl / cxl-create-region.sh  OK                9.97s
>  5/16 ndctl:cxl / cxl-xor-region.sh     OK                5.77s
>  6/16 ndctl:cxl / cxl-events.sh         OK                2.89s
>  7/16 ndctl:cxl / cxl-sanitize.sh       OK                5.74s
>  8/16 ndctl:cxl / cxl-destroy-region.sh OK                3.84s
>  9/16 ndctl:cxl / cxl-qos-class.sh      OK                3.22s
> 10/16 ndctl:cxl / cxl-translate.sh      OK                0.95s
> 11/16 ndctl:cxl / cxl-elc.sh            OK                4.22s
> 12/16 ndctl:cxl / cxl-dax-hmem.sh       OK                4.60s
> 13/16 ndctl:cxl / cxl-region-replay.sh  OK                8.74s
> 14/16 ndctl:cxl / cxl-security.sh       OK                1.83s
> 15/16 ndctl:cxl / cxl-features.sh       OK                1.45s
> 16/16 ndctl:cxl / cxl-poison.sh         SKIP             11.66s
> 
> Ok:                 15
> Expected Fail:      0
> Fail:               0
> Unexpected Pass:    0
> Skipped:            1
> Timeout:            0
> """
> 
> The reason that cxl-poison.sh is skipped is stated in
> https://lore.kernel.org/linux-cxl/anKpY6Y1Sqne1f7Y@MWDK4CY14F/
> 
> Both run_poison_test() rounds pass, so everything this series touches is
> covered, only the unaligned translation section is unreachable, and the
> series does not touch translation.
> 
> The core patch was also previously validated on a Montage CXL Type 3
> device.
> 
> The corresponding ndctl test is attached at
> https://lore.kernel.org/linux-cxl/20260709022857.18732-1-icheng@xxxxxxxxxx/
> 
> 
> Changelog:
> 
> v8->v9:
> - Patch 1: add a poison_efault_forgiven() helper and use it in
>   cxl_get_poison_unmapped() as well.
> - Patch 2: rebased on that helper, no functional change.
> - Patch 3: no change.
> - Fixed the test environment so cxl-security.sh runs and passes; the
>   suite is now 15 OK / 1 SKIP. cxl-poison.sh still skips, see above.
> 
> v7->v8:
> - Split the poison_by_decoder() readability cleanup out of the core
>   patch into a new preparatory patch 1.
> - cxl_dpa_request_region(): split the declaration from the allocation
>   so the allocation and its error check sit together.
> - Add the missing blank line between commit message paragraphs in
>   patch 2.
> - Patch 3 is unchanged.
> 
> v6->v7:
> - Rebase onto cxl/for-7.3/cxl-type2-test.
> - Patch 1 is unchanged.
> - Adapt patch 2 to the refactored Type-2/Type-3 decoder initialization
>   paths.
> - Integrate mock_zero_size_decoders with special-topology parameter
>   validation and reject its incompatible use with fail_autoassemble.
> 
> 
> Richard Cheng (3):
>   cxl/region: Simplify poison_by_decoder() error handling
>   cxl/hdm: Allow zero sized HDM decoders
>   tools/testing/cxl: Enable zero sized decoders under hb0
> 
>  drivers/cxl/core/hdm.c       |  55 ++++++++++++------
>  drivers/cxl/core/mbox.c      |   3 +
>  drivers/cxl/core/region.c    |  69 ++++++++++++++--------
>  drivers/cxl/cxl.h            |  10 ++++
>  drivers/cxl/port.c           |   3 +
>  tools/testing/cxl/test/cxl.c | 109 ++++++++++++++++++++++++++++++-----
>  6 files changed, 193 insertions(+), 56 deletions(-)
> 
> 
> base-commit: 1c6b4ceafc3b994871c29340e0c1ddb0af5800e7
> -- 
> 2.43.0
>