PCI: Avoid FLR for Mediatek MT7922 WiFi

PCI: Avoid FLR for Mediatek MT7922 WiFi

commit 81f64e925c29fe6e99f04b131fac1935ac931e81 upstream.

The Mediatek MT7922 WiFi device advertises FLR support, but it apparently
does not work, and all subsequent config reads return ~0:

  pci 0000:01:00.0: [14c3:0616] type 00 class 0x028000 PCIe Endpoint
  pciback 0000:01:00.0: not ready 65535ms after FLR; giving up

After an FLR, pci_dev_wait() waits for the device to become ready.  Prior
to d591f6804e7e ("PCI: Wait for device readiness with Configuration RRS"),
it polls PCI_COMMAND until it is something other that PCI_POSSIBLE_ERROR
(~0).  If it times out, pci_dev_wait() returns -ENOTTY and
__pci_reset_function_locked() tries the next available reset method.
Typically this is Secondary Bus Reset, which does work, so the MT7922 is
eventually usable.

After d591f6804e7e, if Configuration Request Retry Status Software
Visibility (RRS SV) is enabled, pci_dev_wait() polls PCI_VENDOR_ID until it
is something other than the special 0x0001 Vendor ID that indicates a
completion with RRS status.

When RRS SV is enabled, reads of PCI_VENDOR_ID should return either 0x0001,
i.e., the config read was completed with RRS, or a valid Vendor ID.  On the
MT7922, it seems that all config reads after FLR return ~0 indefinitely.
When pci_dev_wait() reads PCI_VENDOR_ID and gets 0xffff, it assumes that's
a valid Vendor ID and the device is now ready, so it returns with success.

After pci_dev_wait() returns success, we restore config space and continue.
Since the MT7922 is not actually ready after the FLR, the restore fails and
the device is unusable.

We considered changing pci_dev_wait() to continue polling if a
PCI_VENDOR_ID read returns either 0x0001 or 0xffff.  This "works" as it did
before d591f6804e7e, although we have to wait for the timeout and then fall
back to SBR.  But it doesn't work for SR-IOV VFs, which *always* return
0xffff as the Vendor ID.

Mark Mediatek MT7922 WiFi devices to avoid the use of FLR completely.  This
will cause fallback to another reset method, such as SBR.

Link: https://lore.kernel.org/r/20250212193516.88741-1-helgaas@kernel.org
Fixes: d591f6804e7e ("PCI: Wait for device readiness with Configuration RRS")
Link: https://github.com/QubesOS/qubes-issues/issues/9689#issuecomment-2582927149
Link: https://lore.kernel.org/r/Z4pHll_6GX7OUBzQ@mail-itl
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>