Date:   Tue, 26 May 2020 21:04:32 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Yicong Yang <yangyicong@...ilicon.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 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.

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.

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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