Messages in this thread | | | Subject | Re: [PATCH v17 09/12] PCI/AER: Allow clearing Error Status Register in FF mode | From | Kuppuswamy Sathyanarayanan <> | Date | Fri, 13 Mar 2020 13:26:28 -0700 |
| |
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) >> >>> If CONFIG_HOTPLUG_PCI_PCIE=n and CONFIG_HOTPLUG_PCI_ACPI=y, I could >>> believe that the situations are similar to the above. >>> >>> What if CONFIG_HOTPLUG_PCI_PCIE=n and CONFIG_HOTPLUG_PCI_ACPI=n? Then >>> 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] https://git.kernel.org/pub/scm/linux/kernel/git/helgaas/pci.git/log/?h=review/edr > [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
| |