cxl/hdm: Allow zero sized HDM decoders

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. BIOS may commit and lock such decoders so the OS cannot
program regions through them, this is a design choice rather than a spec
requirement.

The kernel rejected these with -ENXIO during port enumeration and
aborted the whole port, so affected systems showed nothing under "cxl
list".

Treat empty decoders as first class reservations. Back them with a
separately allocated resource, since the resource tree cannot represent
an empty range, and keep the skip and hdm_end accounting intact. Exclude
empty decoders from region assembly and avoid zero-length poison queries.

Suggested-by: Dan Williams <djbw@kernel.org>
Signed-off-by: Vishal Aslot <vaslot@nvidia.com>
Signed-off-by: Richard Cheng <icheng@nvidia.com>
Reviewed-by: Dan Williams <djbw@kernel.org>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20260914090858.19181-3-icheng@nvidia.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

0 files changed, 0 insertions, 0 deletions