Re: [PATCH V2] PCI: Clear errors logged in Secondary Status Register

[Vidya Sagar <vidyas@nvidia.com>]

On 23-01-2024 04:30, Bjorn Helgaas wrote:
> External email: Use caution opening links or attachments
>
>
> On Tue, Jan 16, 2024 at 08:02:58PM +0530, Vidya Sagar wrote:
>> The enumeration process leaves the 'Received Master Abort' bit set in
>> the Secondary Status Register of the downstream port in the following
>> scenarios.
>>
>> (1) The device connected to the downstream port has ARI capability
>>      and that makes the kernel set the 'ARI Forwarding Enable' bit in
>>      the Device Control 2 Register of the downstream port. This
>>      effectively makes the downstream port forward the configuration
>>      requests targeting the devices downstream of it, even though they
>>      don't exist in reality. It causes the downstream devices return
>>      completions with UR set in the status in turn causing 'Received
>>      Master Abort' bit set.
>>
>>      In contrast, if the downstream device doesn't have ARI capability,
>>      the 'ARI Forwarding Enable' bit in the downstream port is not set
>>      and any configuration requests targeting the downstream devices
>>      that don't exist are terminated (section 6.13 of PCI Express Base
>>      6.0 spec) in the downstream port itself resulting in no change of
>>      the 'Received Master Abort' bit.
>>
>> (2) A PCIe switch is connected to the downstream port and when the
>>      enumeration flow tries to explore the presence of devices that
>>      don't really exist downstream of the switch, the downstream
>>      port receives the completions with UR set causing the 'Received
>>      Master Abort' bit set.
> Are these the only possible ways this error is logged?  I expected
> them to be logged when we enumerate below a Root Port that has nothing
> attached, for example.
In this case, there won't be any TLP sent downstream. I talked about 
this scenario in the
second paragraph of point (1) above.
> Does clearing them in pci_scan_bridge_extend() cover all ways this
> error might be logged during enumeration?  I can't remember whether
> all enumeration goes through this path.
So far in my testing, clearing it in pci_scan_bridge_extend() covers all 
the cases.

>> Clear 'Received Master Abort' bit to keep the bridge device in a clean
>> state post enumeration.
>>
>> Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
>> ---
>> V2:
>> * Changed commit message based on Bjorn's feedback
>>
>>   drivers/pci/probe.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index 795534589b98..640d2871b061 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -1470,6 +1470,9 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
>>        }
>>
>>   out:
>> +     /* Clear errors in the Secondary Status Register */
>> +     pci_write_config_word(dev, PCI_SEC_STATUS, 0xffff);
>> +
>>        pci_write_config_word(dev, PCI_BRIDGE_CONTROL, bctl);
>>
>>        pm_runtime_put(&dev->dev);
>> --
>> 2.25.1
>>