Date:   Fri, 22 May 2020 10:56:03 +0800
From:   Yicong Yang <yangyicong@...ilicon.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>, <bhelgaas@...gle.com>
CC:     <jay.vosburgh@...onical.com>, <linux-pci@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <ashok.raj@...el.com>
Subject: Re: [PATCH v1 1/1] PCI/ERR: Handle fatal error recovery for
 non-hotplug capable devices



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@...ux.intel.com wrote:
>>>>> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
>>>>>
>>>>> 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@...onical.com]
>>>>> [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@...onical.com>
>>>>> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
>>>>> ---
>>>>>    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)

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.

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) {
>>>>
>>> .
>>>
>>
> .
>

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