Re: [PATCH v10 0/3] Support zero-sized HDM decoders

[Dave Jiang]

On 9/14/26 2:08 AM, Richard Cheng wrote:
> 
> 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. It clarifies
> the error handling and iteration control in poison_by_decoder(), and
> shares the check for a forgiven -EFAULT on a RAM partition with
> cxl_get_poison_unmapped().
> 
> 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.
> 
> Testing:
> 
> As reported in v9, the series was tested on arm64 with cxl_test and
> the ndctl v85 unit suite, with KASAN and lockdep enabled. With
> mock_zero_size_decoders disabled, the full CXL suite reported 15 tests
> passed and 1 skipped.
> 
> cxl-poison.sh was skipped because its unaligned translation section
> was unreachable; both run_poison_test() rounds passed. Details:
> https://lore.kernel.org/linux-cxl/anKpY6Y1Sqne1f7Y@MWDK4CY14F/
> 
> The core patch was also previously validated on a Montage CXL Type 3
> device.
> 
> The corresponding ndctl test:
> https://lore.kernel.org/linux-cxl/20260709022857.18732-1-icheng@xxxxxxxxxx/
> 
> Runtime-tested v10 on a CXL Type 3 device and confirmed that normal
> device operation continues to work with the series applied.
> 
> Changelog:
> 
> v9->v10 (Alison Schofield):
> - Patch 2: propagate -ENOMEM when allocation of the standalone
>   zero-sized resource fails.
> - Patch 2: retain the original !cxled->dpa_res guard in cxl_dpa_free().
>   An existing zero-sized reservation must reach the region,
>   enabled-state, and release-order checks.
> - Patch 2: update the commit message.
> - Patches 1 and 3 are unchanged.
> 
> Previous version:
> https://lore.kernel.org/linux-cxl/20260805055524.22311-1-icheng@xxxxxxxxxx/
> 
> 
> 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       |  58 +++++++++++++------
>  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, 195 insertions(+), 57 deletions(-)
> 
> 
> base-commit: 899648a2707d0405f573dfc7fe15385d9ba3af55

Applied to cxl/next:
057dc2892d1f
00f9ef9ad1ce
fef22d37d470