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SubjectRe: [PATCH v17 09/12] PCI/AER: Allow clearing Error Status Register in FF mode
Hi Bjorn,

On 3/13/20 12:28 PM, Bjorn Helgaas wrote:
> [+cc Russell, Sam, Oliver since we're talking about the error recovery
> flow. The code we're talking about is at [1]]
> On Thu, Mar 12, 2020 at 11:22:13PM -0700, Kuppuswamy, Sathyanarayanan wrote:
>> On 3/12/2020 3:32 PM, Bjorn Helgaas wrote:
>>> On Thu, Mar 12, 2020 at 02:59:15PM -0700, Kuppuswamy Sathyanarayanan wrote:
>>>> On 3/12/20 12:53 PM, Bjorn Helgaas wrote:
>>>>> On Wed, Mar 11, 2020 at 04:07:59PM -0700, Kuppuswamy Sathyanarayanan wrote:
>>>>>> On 3/11/20 3:23 PM, Bjorn Helgaas wrote:
>>>>>>> Is any synchronization needed here between the EDR path and the
>>>>>>> hotplug/enumeration path?
>>>>>> If we want to follow the implementation note step by step (in
>>>>>> sequence) then we need some synchronization between EDR path and
>>>>>> enumeration path. But if it's OK to achieve the same end result by
>>>>>> following steps out of sequence then we don't need to create any
>>>>>> dependency between EDR and enumeration paths. Currently we follow
>>>>>> the latter approach.
>>>>> What would the synchronization look like?
>>>> we might need some way to disable the enumeration path till
>>>> we get response from firmware.
>>>> In native hot plug case, I think we can do it in two ways.
>>>> 1. Disable hotplug notification in slot ctl registers.
>>>>     (pcie_disable_notification())
>>>> 2. Some how block hotplug driver from processing the new
>>>>     events (not sure how feasible its).
>>>> Following method 1 would be easy, But I am not sure whether
>>>> its alright to disable them randomly. I think, unless we
>>>> clear the status as well, we might get some issues due to stale
>>>> notification history.
>>>> For ACPI event case, I am not sure whether we have some
>>>> communication protocol in place to disable receiving ACPI
>>>> events temporarily.
>>>> For polling model, we need to disable to the polling
>>>> timer thread till we receive _OST response from firmware.
>>>>> Ideally I think it would be better to follow the order in the
>>>>> flowchart if it's not too onerous.
>>>> None of the above changes will be pretty and I think it will
>>>> not be simple as well.
>>>>> That will make the code easier to
>>>>> understand. The current situation with this dependency on pciehp and
>>>>> what it will do leaves a lot of things implicit.
>>>>> What happens if CONFIG_PCIE_EDR=y but CONFIG_HOTPLUG_PCI_PCIE=n?
>>>>> IIUC, when DPC triggers, pciehp is what fields the DLLSC interrupt and
>>>>> unbinds the drivers and removes the devices.
>>>>> If that doesn't happen, and Linux clears the DPC trigger to bring
>>>>> the link back up, will those drivers try to operate uninitialized
>>>>> devices?
>>>> I don't think this will happen. In DPC reset_link before we bring up
>>>> the device we wait for link to go down first using
>>>> pcie_wait_for_link(pdev, false) function.
>>> I understand that, but these child devices were reset when DPC
>>> disabled the link. When the link comes back up, their BARs
>>> contain zeros.
>>> If CONFIG_HOTPLUG_PCI_PCIE=y, the DLLSC interrupt will cause
>>> pciehp to unbind the driver. It seems like the unbind races with
>>> the EDR notify handler.
>> Agree. But even if there is a race condition, after clearing DPC
>> trigger status, if hotplug driver properly removes/re-enumerates the
>> driver then the end result will still be same. There should be no
>> functional impact.
>>> If pciehp unbinds the driver before edr_handle_event() calls
>>> pcie_do_recovery(), this seems fine -- we'll call
>>> dpc_reset_link(), which brings up the link, we won't call any
>>> driver callbacks because there's no driver, and another DLLSC
>>> interrupt will cause pciehp to re-enumerate, which will
>>> re-initialize the device, then rebind the driver.
>>> If the EDR notify handler runs before pciehp unbinds the driver,
>> In the above case, from the kernel perspective device is still
>> accessible and IIUC, it will try to recover it in pcie_do_recovery()
>> using one of the callbacks.
>> int (*mmio_enabled)(struct pci_dev *dev);
>> int (*slot_reset)(struct pci_dev *dev);
>> void (*resume)(struct pci_dev *dev);
>> One of these callbacks will do pci_restore_state() to restore the
>> device, and IO will not attempted in these callbacks until the device
>> is successfully recovered.
> That might be what *should* happen, but I don't think it's what
> *does* happen.
> I don't think we use .mmio_enabled() and .slot_reset() for EDR
> because Linux EDR currently depends on DPC, so we'll be using
> dpc_reset_link(), which normally returns PCI_ERS_RESULT_RECOVERED,
> so pcie_do_recovery() skips .mmio_enabled() and .slot_reset().
After our discussion about non-hotplug cases, I am thinking
that reset_link() callback should not return
PCI_ERS_RESULT_RECOVERED in non hotplug cases. If
successfully reset-ed, it should return PCI_ERS_RESULT_NEED_RESET.
This will enable pcie_do_recovery() to proceed to .slot_reset() to
successfully recover the device.

Any comments ?
> I looked at the first few .resume() implementations (FWIW, I used [2]
> to find them), and none of them calls pci_restore_state() before doing
> I/O to the device:
> ioat_pcie_error_resume()
> pci_resume() (hfi1)
> qib_pci_resume()
> cxl_pci_resume()
> genwqe_err_resume()
> ...
> But I assume you've tested EDR with some driver that *does* call
> pci_restore_state()? Or maybe you have pciehp enabled,
Yes. I have tested it only with hotplug enabled. Let me try to disable
hotplug and verify the cases.
> and it always
> wins the race and unbinds the driver before the EDR notification? It
> would be interesting to make pciehp *lose* the race and see if
> anything breaks.
> pci-error-recovery.rst does not mention any requirement for the driver
> to call pci_restore_state(), and I think any state restoration like
> that should be the responsibility of the PCI core, not the driver.
>>> couldn't EDR bring up the link and call driver .mmio_enabled() before
>>> the device has been initialized?
>> Calling mmio_enabled in this case should not be a problem right?
>> Please check the following content from
>> Documentation/PCI/pci-error-recovery.rst. IIUC (following content),
>> IO will not be attempted until the device is successfully
>> re-configured.
>> STEP 2: MMIO Enabled
>> --------------------
>> This callback is made if all drivers on a segment agree that they can
>> try to recover and if no automatic link reset was performed by the HW.
>> If the platform can't just re-enable IOs without a slot reset or a link
>> reset, it will not call this callback, and instead will have gone
>> directly to STEP 3 (Link Reset) or STEP 4 (Slot Reset)
>>> believe that the situations are similar to the above.
>>> I assume there's nothing to unbind the driver, so pcie_do_recovery()
>>> will call the driver .mmio_enabled() and other recovery callbacks on a
>>> device that hasn't been initialized?
>> probably in .slot_reset() callback device config will be restored and it
>> will make the device functional again.
> I don't think .mmio_enabled() is a problem because IIUC, the device
> should not have been reset before calling .mmio_enabled().
In hotplug case, it is possible. since reset_link() is called before
.mmio_enabled, the device might be in reset state by the time
.mmio_enabled is called.
> But I think .slot_reset() *is* a problem. I looked at several
> .slot_reset() implementations ([3]); some called pci_restore_state(),
> but many did not.
> If no hotplug driver is enabled, I think the .slot_reset() callbacks
> that do not call pci_restore_state() are broken.
Yes. Agree. May be the documentation needs to be explicit about it ?
>> Also since in above case hotplug is not supported, topology change will
>> not be supported.
> [1]
> [2] F='\.resume'; git grep -A 10 "struct pci_error_handlers" | grep "$F\s*=" | sed -e "s/.*$F\s*=\s*//" -e 's/,\s*$//'
> [3] F='\.slot_reset'; git grep -A 10 "struct pci_error_handlers" | grep "$F\s*=" | sed -e "s/.*$F\s*=\s*//" -e 's/,\s*$//'

Sathyanarayanan Kuppuswamy
Linux kernel developer

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