cxl/hdm: Avoid incorrect DVSEC fallback when HDM decoders are enabled

cxl/hdm: Avoid incorrect DVSEC fallback when HDM decoders are enabled

Check the global CXL_HDM_DECODER_ENABLE bit instead of looping over
per-decoder COMMITTED bits to determine whether to fall back to DVSEC
range emulation. When the HDM decoder capability is globally enabled,
ignore DVSEC range registers regardless of individual decoder commit
state.

should_emulate_decoders() currently loops over per-decoder COMMITTED
bits, which leads to an incorrect DVSEC fallback when those bits are
zero. One way to trigger this is to destroy a region and bounce the
memdev:

  cxl disable-region region0
  cxl destroy-region region0
  cxl disable-memdev mem0
  cxl enable-memdev mem0

Region teardown zeroes the HDM decoder registers including the committed
bits. The subsequent memdev re-probe finds uncommitted decoders and falls
back to DVSEC emulation, even though HDM remains globally enabled.

Observed failures:

  should_emulate_decoders: cxl_port endpoint6: decoder6.0: committed: 0 base: 0x0_00000000 size: 0x0_00000000
  devm_cxl_setup_hdm: cxl_port endpoint6: Fallback map 1 range register
  ..
  devm_cxl_add_region: cxl_acpi ACPI0017:00: decoder0.0: created region0
  __construct_region: cxl_pci 0000:e1:00.0: mem1:decoder6.0:
  __construct_region region0 res: [mem 0x850000000-0x284fffffff flags 0x200] iw: 1 ig: 4096
  cxl region0: pci0000:e0:port1 cxl_port_setup_targets expected iw: 1 ig: 4096 ..
  cxl region0: pci0000:e0:port1 cxl_port_setup_targets got iw: 1 ig: 256 state: disabled ..
  cxl_port endpoint6: failed to attach decoder6.0 to region0: -6
  ..
  devm_cxl_add_region: cxl_acpi ACPI0017:00: decoder0.0: created region4
  alloc_hpa: cxl region4: HPA allocation error (-34) ..

Fixes: 52cc48ad2a76 ("cxl/hdm: Limit emulation to the number of range registers")
Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20260316201950.224567-1-Smita.KoralahalliChannabasappa@amd.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>