linux-kernel - Re: [PATCH v13 6/6] PCI/DPC: Do not do recovery for hotplug enabled system

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Date:   Mon, 16 Apr 2018 11:21:15 +0530
From:   poza@...eaurora.org
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Sinan Kaya <okaya@...eaurora.org>,
        Keith Busch <keith.busch@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dongdong Liu <liudongdong3@...wei.com>,
        Wei Zhang <wzhang@...com>, Timur Tabi <timur@...eaurora.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        linux-pci-owner@...r.kernel.org
Subject: Re: [PATCH v13 6/6] PCI/DPC: Do not do recovery for hotplug enabled
 system

On 2018-04-16 11:03, poza@...eaurora.org wrote:
> On 2018-04-16 08:47, Bjorn Helgaas wrote:
>> On Sat, Apr 14, 2018 at 11:53:17AM -0400, Sinan Kaya wrote:
>> 
>>> You indicated that you want to unify the AER and DPC behavior. Let's
>>> settle on what we want to do one more time. We have been going forth
>>> and back on the direction.
>> 
>> My thinking is that as much as possible, similar events should be
>> handled similarly, whether the mechanism is AER, DPC, EEH, etc.
>> Ideally, drivers shouldn't have to be aware of which mechanism is in
>> use.
>> 
>> Error recovery includes conventional PCI as well, but right now I
>> think we're only concerned with PCIe.  The following error types are
>> from PCIe r4.0, sec 6.2.2:
>> 
>>   ERR_COR
>>     Corrected by hardware with no software intervention.  Software
>>     involved for logging only.
>> 
>>     Handled by AER via pci_error_handlers; DPC is never involved.
>> 
>>     Link is unaffected.
>> 
>>   ERR_NONFATAL
>>     A transaction is unreliable but the link is fully functional.
>> 
>>     If DPC is not supported, handled by AER via pci_error_handlers and
>>     the link is unaffected.
>> 
>>     If DPC supported, handled by DPC (because we set
>>     PCI_EXP_DPC_CTL_EN_NONFATAL) via remove/re-enumerate.
>> 
>>   ERR_FATAL
>>     The link is unreliable.
>> 
>>     If DPC is not supported, handled by AER via pci_error_handlers and
>>     the link is reset.
>> 
>>     If DPC supported, handled by DPC via remove/re-enumerate.
>> 
>> It doesn't seem right to me that we handle both ERR_NONFATAL and
>> ERR_FATAL events differently if we happen to have DPC support in a
>> switch.
>> 
>> Maybe we should consider triggering DPC only on ERR_FATAL?  That would
>> keep DPC out of the ERR_NONFATAL cases.
>> 
>> For ERR_FATAL, maybe we should bite the bullet and use
>> remove/re-enumerate for AER as well as for DPC.  That would be painful
>> for higher-level software, but if we're willing to accept that pain
>> for new systems that support DPC, maybe life would be better overall
>> if it worked the same way on systems without DPC?
>> 
>> Bjorn
> 
> This had crossed my mind when I first looked at the code.
> DPC is getting triggered for both ERR_NONFATAL and ERR_FATAL case.
> I thought the primary purpose of DPC to recover fatal errors, by
> triggering HW recovery.
> but what if some platform wants to handle both FATAL and NON_FATAL with 
> DPC ?
> 
> As you said AER FATAL cases and DPC FATAL cases should be handled 
> similarly.
> e.g. remove/re-enumerate the devices.
> 
> while NON_FATAL case; only AER would come into picture.
> if some platform would like to handle DPC NON_FATAL then it should
> follow AER NON_FATAL path  (where it does not do remove/re-enumerate)
> 
> And the case where hotplug is enabled, remove/re-enumerate more sense
> in case of ERR_FATAL.
> And the case where hotplug is disabled, only re-enumeration is
> required. (no need to remove the devices)
> but then do we need to handle this case specifically, what is the harm
> in removing the devices in all the cases followed by re-enumerate ?

To Clarify the last line, what I meant here was, in case of ERR_FATAL we 
can always remove/re-enumerate the devices irrespective of hotplug is 
enabled or not.

and in case of ERR_NONFATAL, DPC will follow AER path (where it just 
tries to recover)
although I am not very sure that how to handle ERR_NONFATAL case if 
hotplug is enabled. Because as Keith suggested device might have been 
changed run-time.

> 
> Regards,
> Oza.