[PATCH v9] PCI: Add device-specific reset for Qualcomm devices

[Jose Ignacio Tornos Martinez]

Hi Mani,

Let me clarify the exact scenario and where the reset is necessary:

* For the commented WiFi devices (WCN6855/WCN7850):

Standard VFIO passthrough flow (this works fine):
  1. Unbind native driver (ath11k/ath12k/MHI)
  2. Bind vfio-pci driver
  3. Assign device to VM
  4. VM boots, loads its own driver → device works perfectly
  5. VM shuts down cleanly → device can be reassigned → works fine

The problem occurs with unclean VM termination:
  1. VM crashes or is force-terminated
  2. VFIO tries to reset the device before reassignment
  3. Without a working PCI reset method, reset fails
  4. Device stuck in undefined state → cannot be reassigned to another VM
  
     Unbinding the driver again doesn't help because the device hardware
     itself is in a bad state. From hypervisor:
     $ lspci -vvv -s 0000:03:00.0
        03:00.0 Network controller: Qualcomm Technologies, Inc (rev ff) (prog-if ff)
            !!! Unknown header type 7f
     And a full host power-cycle is necessary to recover.
     
* For the commented modem devices (SDX62/SDX65): 

Even worse because it fails during the first VM boot without proper reset
capability, standard VFIO passthrough flow:
  1. Unbind native driver (MHI)
  2. Bind vfio-pci driver
  3. Assign device to VM
  4. VM boots, loads its own driver and crashes:
     [   24.024165] mhi mhi0: Device failed to enter MHI Ready
     [   24.024168] mhi mhi0: MHI did not enter READY state
     
     Unbind/rebind attempts fail:
     [  352.643601] mhi mhi0: Requested to power ON
     [  352.643611] mhi mhi0: Power on setup success
     [  373.442954] mhi mhi0: Device failed to clear MHI Reset
     [  373.442970] mhi mhi0: MHI did not enter READY state
     And requires a full host power cycle to recover,
     even outside of VFIO scenarios.

* MHI Host driver's remove callback may handle clean software state
teardown, but it doesn't provide a PCI reset capability that VFIO can
invoke. VFIO needs a reset method registered in the PCI reset hierarchy
(device_specific, pm, flr, bus, etc.). VFIO invokes this reset both during
initial device binding (before the VM starts) and when reassigning the
device between VMs - without a working reset method, the device cannot
reach a clean state for initialization.



I hope this clarifies the scenario better. Please let me know if I can
provide more information or run any specific tests to help investigate this
further.

Thanks

Best regards
José Ignacio