Some PCIe devices lack working reset methods for VFIO passthrough scenarios.
These devices typically have no FLR, advertise NoSoftRst+ (blocking PM reset),
and have broken or unavailable bus reset. When a VM crashes, VFIO cannot reset
the device for reuse without a working reset method.

This series addresses the problem by adding general d3cold infrastructure and
device-specific reset for Qualcomm devices:

**Patch 1/3: d3cold reset method**
Adds D3cold as a general reset method with strict _PR3 requirement. Only
attempts true power cycling via platform control (ACPI _PR3), never falls
back to D3hot. This provides genuine power-off reset on platforms that
support it.

**Patch 2/3: device-specific reset for Qualcomm devices**
Adds device-specific reset entries for Qualcomm ath11k/ath12k WiFi and
SDX62/SDX65 modems using D3cold power cycling with automatic D3hot fallback.
Uses pci_set_power_state(D3cold) which automatically falls back to D3hot on
platforms without _PR3. Extracts shared pci_dev_d3cold_d0_cycle() helper to
avoid code duplication with general d3cold method. Device-specific reset is
position #1 in reset hierarchy, providing immediate working reset for these
Qualcomm devices.

**Patch 3/3: Qualcomm quirk_no_bus_reset**
Disables broken bus reset for Qualcomm devices. Testing proves this is
device-specific: MediaTek MT7925e works correctly with bus reset using the
same passive M.2-to-PCIe adapters where Qualcomm devices fail, confirming
PERST# is properly wired and the issue is not deployment-specific. Acts as
safety net to prevent broken SBR if users override via sysfs.

v5: - Follow the advice from Alex Williamson, replace v4's general "soft"
    reset method (patch 2/3) as too broad by a device-specific reset using
    pci_dev_reset_methods[]
    - Address the suggestion of AI review (sashiko.dev), add IOMMU handling
    (pci_dev_reset_iommu_prepare/done) to prevent IOMMU faults during power
    state transitions 
v4: https://lore.kernel.org/all/20260518124836.460805-1-jtornosm@redhat.com/

Jose Ignacio Tornos Martinez (3):
  PCI: Add d3cold as general reset method
  PCI: Add device-specific reset for Qualcomm devices
  PCI: Disable broken bus reset on Qualcomm devices

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2.53.0