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

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Message-ID: <0c948351-9715-4c5c-ad0c-3727cd2ba8a8@nvidia.com>
Date: Mon, 1 Apr 2024 13:29:48 +0530
From: Vidya Sagar <vidyas@...dia.com>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: bhelgaas@...gle.com, linux-pci@...r.kernel.org,
 linux-kernel@...r.kernel.org, treding@...dia.com, jonathanh@...dia.com,
 kthota@...dia.com, mmaddireddy@...dia.com, sagar.tv@...il.com
Subject: Re: [PATCH V2] PCI: Clear errors logged in Secondary Status Register

Hi Bjorn,
Just checking on this thread.
Is there anything else you want me to clarify on?

Thanks,
Vidya Sagar

On 14-03-2024 06:09, Vidya Sagar wrote:
>
>
> 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@...dia.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
>>>
>