pcie_clear_device_status() resets the error bits in the PCIe Device Status
Register (PCI_EXP_DEVSTA).

Previously we did this unconditionally, but on ACPI systems, the _OSC AER
bit negotiates control of the AER capability.  Per sec 4.5.1 of the System
Firmware Intermediary _OSC and DPC Updates ECN [1], this bit also covers
other error enable/status bits including the following:

  Correctable Error Reporting Enable
  Non-Fatal Error Reporting Enable
  Fatal Error Reporting Enable
  Unsupported Request Reporting Enable

These bits are all in the PCIe Device Control register (the ECN omitted
"Reporting", but I think that's a typo), so by implication the _OSC AER bit
also applies to the error status bits in the PCIe Device Status register:

  Correctable Error Detected
  Non-Fatal Error Detected
  Fatal Error Detected
  Unsupported Request Detected

Clear the PCIe Device Status error bits only when the OS controls the AER
capability and related error enable/status bits.  If platform firmware
controls the AER capability, firmware is responsible for clearing these
bits.

One call path leading here is:

  ghes_do_proc
    ghes_handle_aer
      aer_recover_queue
        schedule_work(&aer_recover_work)
  ...
  aer_recover_work_func
    pcie_do_recovery
      pcie_clear_device_status

[1] System Firmware Intermediary (SFI) _OSC and DPC Updates ECN, Feb 24,
    2020, affecting PCI Firmware Specification, Rev. 3.2
    https://members.pcisig.com/wg/PCI-SIG/document/14076
[bhelgaas: commit log, move test from pcie_clear_device_status() to callers]
Link: https://lore.kernel.org/r/20200622113523.891666-1-Jonathan.Cameron@huawei.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>