Re: [PATCH v1] PCI: pciehp: Make sure DPC trigger status is reset in PDC handler

[Sathyanarayanan Kuppuswamy]

Hi Lukas,

Thanks for the review.

On 6/15/23 11:35 AM, Lukas Wunner wrote:
> On Wed, Jun 14, 2023 at 11:25:59PM -0700, Kuppuswamy Sathyanarayanan wrote:
>> During the EDR-based DPC recovery process, for devices with persistent
>> issues, the firmware may choose not to handle the DPC error and leave
>> the port in DPC triggered state. In such scenarios, if the user
>> replaces the faulty device with a new one, the OS is expected to clear
>> the DPC trigger status in the hotplug error handler to enable the new
>> device enumeration.
> 
> You're clearing the DPC trigger status upon a PDC event, yet are saying
> here the purpose is to reset port state for a future hotplugged device.

Sorry, it is a typo. I meant "hotplug interrupt handler".

The goal is to ensure that when a new device presence is detected, the
old DPC trigger status is cleared.

> 
> A PDC event may be synthesized, e.g. to trigger slot bringup via
> sysfs, so using a PDC event to clear DPC trigger status feels wrong.

IMO, it is harmless. We just want to make sure the previous DPC trigger
status is cleared before enumerating a new device.

> pciehp_unconfigure_device() seems like a more appropriate place to me.
> 

I initially thought to add it there. Spec also recommends clearing it
when removing the device. But I wasn't sure if pciehp_unconfigure_device()
would be called only during device removal. 

Let me test this path and get back to you.

> 
>> More details about this issue can be found in PCIe
>> firmware specification, r3.3, sec titled "DPC Event Handling"
>> Implementation note.
> 
> That Implementation Note contains a lot of text and a fairly complex
> flow chart. If you could point to specific paragraphs or numbers in
> the Implementation Note that would make life easier for a reviewer
> to make the connection between your code and the spec.

It is the text at the end of the flowchart. Copied it here for reference.

For devices with persistent errors, a port may be kept in the DPC triggered
state (disabled) to keep those devices from continuing to generate errors. For
hot-plug slots, the errant device may be removed and replaced with a new device.
If the DPC trigger state is not cleared, then the port above the newly inserted
device will still be disabled and will be non-operational. Therefore, operating
systems may need to modify their hot-plug interrupt handling code to clear DPC
Trigger Status when a device is removed so that a subsequent insertion
will succeed.

> 
> 
>> Similar issue might also happen if the DPC or EDR recovery handler
>> exits before clearing the trigger status. To fix this issue, clear the
>> DPC trigger status in PDC interrupt handler.
> 
> I was about to ask why the code is added to dpc.c, not edr.c,
> and why it's not constrained to CONFIG_PCIE_EDR, but I assume
> that's the reason?  Because it "might" happen for OS-native DPC
> as well?

Yes. There are code paths in the DPC driver where error recover handler
can exit before clearing the DPC trigger status. So I think this fix is
applicable for native code as well.

> 
> 
>> +/**
>> + * pci_reset_trigger - Clear DPC trigger status
>> + * @pdev: PCI device
>> + *
>> + * It is called from the PCIe hotplug driver to clean the DPC
>> + * trigger status in the PDC interrupt handler.
>> + */
>> +void pci_dpc_reset_trigger(struct pci_dev *pdev)
>> +{
>> +	if (!pdev->dpc_cap)
>> +		return;
>> +
>> +	pci_write_config_word(pdev, pdev->dpc_cap + PCI_EXP_DPC_STATUS,
>> +			      PCI_EXP_DPC_STATUS_TRIGGER);
>> +}
> 
> This may run concurrently to dpc_reset_link(), so I'd expect that
> you need some kind of serialization.  What happens if pciehp clears
> trigger status behind the DPC driver's back while it is handling an
> error?

Currently, we only call pci_dpc_reset_trigger() in PDC interrupt handler.

Do you think there would be a race between error handler and PDC handler?

> 
> Thanks,
> 
> Lukas

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer