linux-kernel - Re: [PATCH v3 1/1] PCI/ERR: Fix reset logic in pcie_do

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Date:   Sun, 27 Sep 2020 19:43:46 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Sinan Kaya <okaya@...nel.org>, Bjorn Helgaas <helgaas@...nel.org>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, ashok.raj@...el.com,
        Jay Vosburgh <jay.vosburgh@...onical.com>
Subject: Re: [PATCH v3 1/1] PCI/ERR: Fix reset logic in pcie_do_recovery()
 call

Hi,

On 9/25/20 11:30 AM, Sinan Kaya wrote:
> On 9/25/2020 2:16 PM, Kuppuswamy, Sathyanarayanan wrote:
>>>
>>> If this is a too involved change, DPC driver should restore state
>>> when hotplug is not supported.
>> Yes. we can add a condition for hotplug capability check.
>>>
>>> DPC driver should be self-sufficient by itself.
>>>
> 
> Sounds good.
> 
>>>> Also for non-fatal errors, if reset is requested then we still need
>>>> some kind of bus reset call here
>>>
>>> DPC should handle both fatal and non-fatal cases
>> Currently DPC is only triggered for FATAL errors.
>>   and cause a bus reset
> 
> Thanks for the heads up.
> This seems to have changed since I looked at the DPC code.
> 
>>> in hardware already before triggering an interrupt.
>> Error recovery is not triggered only DPC driver. AER also uses the
>> same error recovery code. If DPC is not supported, then we still need
>> reset logic.
> 
> It sounds like we are cross-talking two issues.
> 
> 1. no state restore on DPC after FATAL error.
> Let's fix this.
Agree. Few more detail about the above issue is,

There are two cases under FATAL error.

FATAL + hotplug - In this case, link will be reseted. And hotplug handler
will remove the driver state. This case works well with current code.

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);

> 
> 2. no bus reset on NON_FATAL error through AER driver path.
> This already tells me that you need to split your change into
> multiple patches.
> 
> Let's talk about this too. bus reset should be triggered via
> AER driver before informing the recovery.
But as per error recovery documentation, any call to
->error_detected() or ->mmio_enabled() can request
PCI_ERS_RESULT_NEED_RESET. So we need to add code
to do the actual reset before calling ->slot_reset()
callback. So call to pci_reset_bus() fixes this
issue.

  	if (status == PCI_ERS_RESULT_NEED_RESET) {
+		pci_reset_bus(dev);


> 
> 	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);
> 	}
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer