Re: [PATCH v3 1/1] PCI/ERR: Fix reset logic in pcie_do_recovery() call

From: "Kuppuswamy, Sathyanarayanan"  <sathyanarayanan.kuppuswamy@linux.intel.com>
To: Sinan Kaya <okaya@kernel.org>, Bjorn Helgaas <helgaas@kernel.org>
Cc: bhelgaas@google.com, linux-pci@vger.kernel.org,
	linux-kernel@vger.kernel.org, ashok.raj@intel.com,
	Jay Vosburgh <jay.vosburgh@canonical.com>
Subject: Re: [PATCH v3 1/1] PCI/ERR: Fix reset logic in pcie_do_recovery() call
Date: Mon, 28 Sep 2020 10:15:44 -0700	[thread overview]
Message-ID: <8a3aeb3c-83c4-8626-601d-360946d55dd8@linux.intel.com> (raw)
In-Reply-To: <a2bbdfed-fb17-51dc-8ae4-55d924c13211@kernel.org>

On 9/28/20 4:17 AM, Sinan Kaya wrote:
> On 9/27/2020 10:43 PM, Kuppuswamy, Sathyanarayanan wrote:
>> FATAL + no-hotplug - In this case, link will still be reseted. But
>> currently driver state is not properly restored. So I attempted
>> to restore it using pci_reset_bus().
>>
>>           status = reset_link(dev);
>> -        if (status != PCI_ERS_RESULT_RECOVERED) {
>> +        if (status == PCI_ERS_RESULT_RECOVERED) {
>> +            status = PCI_ERS_RESULT_NEED_RESET;
>>
>> ...
>>
>>       if (status == PCI_ERS_RESULT_NEED_RESET) {
>>           /*
>> -         * TODO: Should call platform-specific
>> -         * functions to reset slot before calling
>> -         * drivers' slot_reset callbacks?
>> +         * TODO: Optimize the call to pci_reset_bus()
>> +         *
>> +         * There are two components to pci_reset_bus().
>> +         *
>> +         * 1. Do platform specific slot/bus reset.
>> +         * 2. Save/Restore all devices in the bus.
>> +         *
>> +         * For hotplug capable devices and fatal errors,
>> +         * device is already in reset state due to link
>> +         * reset. So repeating platform specific slot/bus
>> +         * reset via pci_reset_bus() call is redundant. So
>> +         * can optimize this logic and conditionally call
>> +         * pci_reset_bus().
>>            */
>> +        pci_reset_bus(dev);
> 
> I think we have to go to remove/rescan for this case as you also
> mentioned above. There is no state to save. All BAR assignments
> are gone. Entire device programming is also lost.
> 
> I don't think pci_reset_bus() can recover from this situation safely.
> It will make things worse by saving/restoring the hardware default
> state.
> 
> This should remove/rescan logic should be inside DPC's slot_reset()
> function BTW. Not here.
Since there is no state restoration for FATAL errors, I am wondering whether
calls to ->error_detected(), ->mmio_enabled() and ->slot_reset() are
required?

Let me know your comments about following pseudo code.

if (fatal error & hotplug_supported)
    do nothing // if fatal triggered by DPC, clear DPC state.

if (fatal error & no-hotplug)
   perform slot_reset and renumerate affected devices.

> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer