Date:   Wed, 27 May 2020 14:41:46 +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 2020/5/27 12:04, Kuppuswamy, Sathyanarayanan wrote:
>
>
> On 5/26/20 8:50 PM, Yicong Yang wrote:
>> 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@...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)
>>>
>>> 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.
> IIUC, your concern is whether it is correct to trigger reset for
> pci_channel_io_normal case right ? Please correct me if my
> assumption is incorrect.

right.

>
> If its true, then why would report_error_detected() will return
> PCI_ERS_*_NEED_RESET for pci_channel_io_normal case ? If
> report_error_detected() requests reset in pci_channel_io_normal
> case then I think we should give preference to it.

If we get PCI_ERS_*_NEED_RESET, we should do slot reset, no matter it's a
hotpluggable slot or not.  And we shouldn't do it here in reset_link(), that's
two separate things.  The `slot reset` done in aer_root_reset() is only for *link
reset*, as there may have some side effects to perform secondary bus reset directly
for hotpluggable slot, as mentioned in commit c4eed62a2143, so it use slot reset
to do the reset link things.

As for slot reset required by the driver, we should perform it later just before the
->slot_reset(). I noticed the TODO comments there and we should implement
it if it's necessary.

It lies in line 183, drivers/pcie/err.c:

    if (status == PCI_ERS_RESULT_NEED_RESET) {
        /*
         * TODO: Should call platform-specific
         * functions to reset slot before calling
         * drivers' slot_reset callbacks?
         */
        status = PCI_ERS_RESULT_RECOVERED;
        pci_dbg(dev, "broadcast slot_reset message\n");
        pci_walk_bus(bus, report_slot_reset, &status);
    }


>
> Let me know your comments.
>>
>> 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) {
>>>>>>>>
>>>>>>> .
>>>>>>>
>>>>>>
>>>>> .
>>>>>
>>>>
>>> .
>>>
>>
> .
>

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