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

[Jose Ignacio Tornos Martinez]

Hi Mani and Alex,

Thank you both for the guidance. 


I think your insight about the BAR-space reset mechanisms is a much better
approach to have a "real" reset. With that, I've implemented device-specific
reset functions based on the driver code Mani referenced:

WCN6855/WCN7850 WiFi (reset_qualcomm_wifi):
- Uses SoC global reset via BAR0 offset 0x3008
- Follows the sequence from ath11k_pci_soc_global_reset() /
  ath12k_pci_soc_global_reset() / ath11k_pci_sw_reset()
- Set reset bit → 10ms delay → clear reset bit → 10ms delay → verify link
  recovery (up to 5s)
- Includes MHI controller reset cleanup (offset 0x38) to clear SYSERR after SoC
  reset
- Both WCN6855 and WCN7850 share identical register offsets and reset sequence

SDX62/SDX65 modems (reset_qualcomm_modem):
- Uses MHI SoC reset via BAR0 offset 0xb0
- Follows the sequence from mhi_soc_reset() / mhi_pci_reset_prepare()
- Write reset request → 2s delay for reset completion

All implementations follow Alex's pattern: save command register ->
enable memory decode -> ioremap BAR -> perform reset sequence ->
iounmap -> restore command.
Testing results: The implementation shows successful reset behavior for both
WiFi and modem devices in VFIO passthrough scenarios.


Regarding the approach (quirks.c vs dedicated VFIO driver):

Mani, I understand your concern about potential conflicts with the native
drivers. I want to make sure I implement this correctly. Could you clarify
when you envision the conflict occurring? 
>From what I can see, the native drivers call their reset functions directly
without using pci_try_reset_function(), so the device-specific reset in
quirks.c could be enough, but I may be missing some error recovery paths or
other scenarios.

Alex, Mani - would you prefer a dedicated VFIO PCI variant driver (similar
to mlx5-vfio-pci) to ensure the reset logic only runs in VFIO scenarios, or
is the quirks.c approach acceptable? I can implement either approach.

With the decision I will send v10.


Thank you again for all your help

Best regards
José Ignacio