Re: [PATCH v1 1/1] PCI/ERR: Handle fatal error recovery for non-hotplug capable devices

[Yicong Yang]

Hi,


On 2020/5/27 9:31, Kuppuswamy, Sathyanarayanan wrote:
> Hi,
>
> On 5/21/20 7:56 PM, Yicong Yang wrote:
>>
>>
>> On 2020/5/22 3:31, Kuppuswamy, Sathyanarayanan wrote:
>>>
>>>
>>> On 5/21/20 3:58 AM, Yicong Yang wrote:
>>>> On 2020/5/21 1:04, Kuppuswamy, Sathyanarayanan wrote:
>>>>>
>>>>>
>>>>> On 5/20/20 1:28 AM, Yicong Yang wrote:
>>>>>> On 2020/5/7 11:32, sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx wrote:
>>>>>>> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
>>>>>>>
>>>>>>> If there are non-hotplug capable devices connected to a given
>>>>>>> port, then during the fatal error recovery(triggered by DPC or
>>>>>>> AER), after calling reset_link() function, we cannot rely on
>>>>>>> hotplug handler to detach and re-enumerate the device drivers
>>>>>>> in the affected bus. Instead, we will have to let the error
>>>>>>> recovery handler call report_slot_reset() for all devices in
>>>>>>> the bus to notify about the reset operation. Although this is
>>>>>>> only required for non hot-plug capable devices, doing it for
>>>>>>> hotplug capable devices should not affect the functionality.
>>>>>>>
>>>>>>> Along with above issue, this fix also applicable to following
>>>>>>> issue.
>>>>>>>
>>>>>>> Commit 6d2c89441571 ("PCI/ERR: Update error status after
>>>>>>> reset_link()") added support to store status of reset_link()
>>>>>>> call. Although this fixed the error recovery issue observed if
>>>>>>> the initial value of error status is PCI_ERS_RESULT_DISCONNECT
>>>>>>> or PCI_ERS_RESULT_NO_AER_DRIVER, it also discarded the status
>>>>>>> result from report_frozen_detected. This can cause a failure to
>>>>>>> recover if _NEED_RESET is returned by report_frozen_detected and
>>>>>>> report_slot_reset is not invoked.
>>>>>>>
>>>>>>> Such an event can be induced for testing purposes by reducing the
>>>>>>> Max_Payload_Size of a PCIe bridge to less than that of a device
>>>>>>> downstream from the bridge, and then initiating I/O through the
>>>>>>> device, resulting in oversize transactions.  In the presence of DPC,
>>>>>>> this results in a containment event and attempted reset and recovery
>>>>>>> via pcie_do_recovery.  After 6d2c89441571 report_slot_reset is not
>>>>>>> invoked, and the device does not recover.
>>>>>>>
>>>>>>> [original patch is from jay.vosburgh@xxxxxxxxxxxxx]
>>>>>>> [original patch link https://lore.kernel.org/linux-pci/18609.1588812972@famine/]
>>>>>>> Fixes: 6d2c89441571 ("PCI/ERR: Update error status after reset_link()")
>>>>>>> Signed-off-by: Jay Vosburgh <jay.vosburgh@xxxxxxxxxxxxx>
>>>>>>> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
>>>>>>> ---
>>>>>>>     drivers/pci/pcie/err.c | 19 +++++++++++++++----
>>>>>>>     1 file changed, 15 insertions(+), 4 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/pci/pcie/err.c b/drivers/pci/pcie/err.c
>>>>>>> index 14bb8f54723e..db80e1ecb2dc 100644
>>>>>>> --- a/drivers/pci/pcie/err.c
>>>>>>> +++ b/drivers/pci/pcie/err.c
>>>>>>> @@ -165,13 +165,24 @@ pci_ers_result_t pcie_do_recovery(struct pci_dev *dev,
>>>>>>>         pci_dbg(dev, "broadcast error_detected message\n");
>>>>>>>         if (state == pci_channel_io_frozen) {
>>>>>>>             pci_walk_bus(bus, report_frozen_detected, &status);
>>>>>>> -        status = reset_link(dev);
>>>>>>> -        if (status != PCI_ERS_RESULT_RECOVERED) {
>>>>>>> +        status = PCI_ERS_RESULT_NEED_RESET;
>>>>>>> +    } else {
>>>>>>> +        pci_walk_bus(bus, report_normal_detected, &status);
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    if (status == PCI_ERS_RESULT_NEED_RESET) {
>>>>>>> +        if (reset_link) {
>>>>>>> +            if (reset_link(dev) != PCI_ERS_RESULT_RECOVERED)
>>>>>>
>>>>>> we'll call reset_link() only if link is frozen. so it may have problem here.
>>>>> you mean before this change right?
>>>>> After this change, reset_link() will be called as long as status is
>>>>> PCI_ERS_RESULT_NEED_RESET.
>>>>
>>>> Yes. I think we should reset the link only if the io is blocked as before. There's
>>>> no reason to reset a normal link.
>>> Currently, only AER and DPC driver uses pcie_do_recovery() call. So the
>>> possible reset_link options are dpc_reset_link() and aer_root_reset().
>>>
>>> In dpc_reset_link() case, the link is already disabled and hence we
>>> don't need to do another reset. In case of aer_root_reset() it
>>> uses pci_bus_error_reset() to reset the slot.
>>
>> Not exactly. In pci_bus_error_reset(), we call pci_slot_reset() only if it's
>> hotpluggable. But we always call pci_bus_reset() to perform a secondary bus
>> reset for the bridge. That's what I think is unnecessary for a normal link,
>> and that's what reset link indicates us to do. The slot reset is introduced
>> in the process only to solve side effects. (c4eed62a2143, PCI/ERR: Use slot reset if available)
>
> IIUC, pci_bus_reset() will do slot reset if its supported (hot-plug
> capable slots). If its not supported then it will attempt secondary
> bus reset. So secondary bus reset will be attempted only if slot
> reset is not supported.
>
> Since reported_error_detected() requests us to do reset, we will have
> to attempt some kind of reset before we call ->slot_reset() right?
> What is the side effect in calling secondary bus reset?

I agree we should do a slot reset if driver required. The question is if we apply
the patch, think of a situation that the io is normal, the slot is not hotpluggable but
driver reports a reset, then:
-->aer_root_reset()
----->pci_bus_error_reset()
---------> pci_bridge_secondary_bus_reset()  // Is it necessary to reset if the link is not blocked?

Before commit (c4eed62a2143, PCI/ERR: Use slot reset if available), the reset_link() for aer is
-->aer_root_reset()
----->pci_bridge_secondary_bus_reset()

As mentioned by the commit c4eed62a2143 "The secondary bus reset may have link side effects that a hotplug capable
port may incorrectly react to. Use the slot specific reset for hotplug ports, fixing the undesirable link
down-up handling during error recovering." So I assume it use hotplug slot reset rather than secondary
bus reset to recover the link. If the link is normal, it's unnecessary to do so. so we should add a check
before reset the link in the patch:

+ if (status == PCI_ERS_RESULT_NEED_RESET &&
+      state == pci_channel_io_frozen) {

We should do slot reset if driver required, but it's different from the `slot reset` in pci_bus_error_reset().
Previously we don't do a slot reset and call ->slot_reset() directly, I don't know the certain reason.

Thanks,
Yicong
>
>>
>> PCI_ERS_RESULT_NEED_RESET indicates that the driver
>> wants a platform-dependent slot reset and its ->slot_reset() method to be called then.
>> I don't think it's same as slot reset mentioned above, which is only for hotpluggable
>> ones.
> What you think is the correct reset implementation ? Is it something
> like this?
>
> if (hotplug capable)
>    try_slot_reset()
> else
>    do_nothing()
>>
>> Previously, if link is normal and the driver reports PCI_ERS_RESULT_NEED_RESET,
>> we'll only call ->slot_reset() without slot reset in reset_link(). Maybe it's better
>> to perform just like before.
>>
>> Thanks.
>>
>>
>>>>
>>>> Furthermore, PCI_ERS_RESULT_NEED_RESET means device driver requires a slot reset rather
>>>> than a link reset, so it maybe improper to use it to judge whether a link reset is needed.
>>>> We decide whether to do a link reset only by the io state.
>>>>
>>>> Thanks,
>>>> Yicong
>>>>
>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Yicong
>>>>>>
>>>>>>
>>>>>>> +                status = PCI_ERS_RESULT_DISCONNECT;
>>>>>>> +        } else {
>>>>>>> +            if (pci_bus_error_reset(dev))
>>>>>>> +                status = PCI_ERS_RESULT_DISCONNECT;
>>>>>>> +        }
>>>>>>> +
>>>>>>> +        if (status == PCI_ERS_RESULT_DISCONNECT) {
>>>>>>>                 pci_warn(dev, "link reset failed\n");
>>>>>>>                 goto failed;
>>>>>>>             }
>>>>>>> -    } else {
>>>>>>> -        pci_walk_bus(bus, report_normal_detected, &status);
>>>>>>>         }
>>>>>>>           if (status == PCI_ERS_RESULT_CAN_RECOVER) {
>>>>>>
>>>>> .
>>>>>
>>>>
>>> .
>>>
>>
> .
>