Re: [PATCH v12 10/15] PCI/ERR: Limit AER resets in pcie_do_recovery()

[Bjorn Helgaas]

On Mon, Nov 23, 2020 at 11:57:35PM +0000, Kelley, Sean V wrote:
> > On Nov 23, 2020, at 3:28 PM, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > On Fri, Nov 20, 2020 at 04:10:31PM -0800, Sean V Kelley wrote:
> >> In some cases a bridge may not exist as the hardware controlling may be
> >> handled only by firmware and so is not visible to the OS. This scenario is
> >> also possible in future use cases involving non-native use of RCECs by
> >> firmware.
> >> 
> >> Explicitly apply conditional logic around these resets by limiting them to
> >> Root Ports and Downstream Ports.
> > 
> > Can you help me understand this?  The subject says "Limit AER resets"
> > and here you say "limit them to RPs and DPs", but it's not completely
> > obvious how the resets are being limited, i.e., the patch doesn't add
> > anything like:
> > 
> > +  if (type == PCI_EXP_TYPE_ROOT_PORT ||
> > +      type == PCI_EXP_TYPE_DOWNSTREAM)
> >      reset_subordinates(bridge);
> > 
> > It *does* add checks around pcie_clear_device_status(), but that also
> > includes RC_EC.  And that's not a reset, so I don't think that's
> > explicitly mentioned in the commit log.
> 
> The subject should have referred to the clearing of the device status rather than resets.
> It originally came from this simpler patch in which I made use of reset instead of clear:
> 
> https://lore.kernel.org/linux-pci/20201002184735.1229220-8-seanvk.dev@xxxxxxxxxxxxxxxx/
> 
> So a rephrase of clearing in place of resets would be more appropriate.
> 
> Then we added the notion of bridges…below
> 
> > 
> > Also see the question below.
> > 
> >> Link: https://lore.kernel.org/r/20201002184735.1229220-8-seanvk.dev@xxxxxxxxxxxxxxxx
> >> Signed-off-by: Sean V Kelley <sean.v.kelley@xxxxxxxxx>
> >> Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> >> Acked-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> >> ---
> >> drivers/pci/pcie/err.c | 31 +++++++++++++++++++++++++------
> >> 1 file changed, 25 insertions(+), 6 deletions(-)
> >> 
> >> diff --git a/drivers/pci/pcie/err.c b/drivers/pci/pcie/err.c
> >> index 8b53aecdb43d..7883c9791562 100644
> >> --- a/drivers/pci/pcie/err.c
> >> +++ b/drivers/pci/pcie/err.c
> >> @@ -148,13 +148,17 @@ static int report_resume(struct pci_dev *dev, void *data)
> >> 
> >> /**
> >>  * pci_walk_bridge - walk bridges potentially AER affected
> >> - * @bridge:	bridge which may be a Port
> >> + * @bridge:	bridge which may be a Port, an RCEC with associated RCiEPs,
> >> + *		or an RCiEP associated with an RCEC
> >>  * @cb:		callback to be called for each device found
> >>  * @userdata:	arbitrary pointer to be passed to callback
> >>  *
> >>  * If the device provided is a bridge, walk the subordinate bus, including
> >>  * any bridged devices on buses under this bus.  Call the provided callback
> >>  * on each device found.
> >> + *
> >> + * If the device provided has no subordinate bus, call the callback on the
> >> + * device itself.
> >>  */
> >> static void pci_walk_bridge(struct pci_dev *bridge,
> >> 			    int (*cb)(struct pci_dev *, void *),
> >> @@ -162,6 +166,8 @@ static void pci_walk_bridge(struct pci_dev *bridge,
> >> {
> >> 	if (bridge->subordinate)
> >> 		pci_walk_bus(bridge->subordinate, cb, userdata);
> >> +	else
> >> +		cb(bridge, userdata);
> >> }
> >> 
> >> pci_ers_result_t pcie_do_recovery(struct pci_dev *dev,
> >> @@ -174,10 +180,13 @@ pci_ers_result_t pcie_do_recovery(struct pci_dev *dev,
> >> 
> >> 	/*
> >> 	 * Error recovery runs on all subordinates of the bridge.  If the
> >> -	 * bridge detected the error, it is cleared at the end.
> >> +	 * bridge detected the error, it is cleared at the end.  For RCiEPs
> >> +	 * we should reset just the RCiEP itself.
> >> 	 */
> >> 	if (type == PCI_EXP_TYPE_ROOT_PORT ||
> >> -	    type == PCI_EXP_TYPE_DOWNSTREAM)
> >> +	    type == PCI_EXP_TYPE_DOWNSTREAM ||
> >> +	    type == PCI_EXP_TYPE_RC_EC ||
> >> +	    type == PCI_EXP_TYPE_RC_END)
> >> 		bridge = dev;
> >> 	else
> >> 		bridge = pci_upstream_bridge(dev);
> >> @@ -185,6 +194,12 @@ pci_ers_result_t pcie_do_recovery(struct pci_dev *dev,
> >> 	pci_dbg(bridge, "broadcast error_detected message\n");
> >> 	if (state == pci_channel_io_frozen) {
> >> 		pci_walk_bridge(bridge, report_frozen_detected, &status);
> >> +		if (type == PCI_EXP_TYPE_RC_END) {
> >> +			pci_warn(dev, "subordinate device reset not possible for RCiEP\n");
> >> +			status = PCI_ERS_RESULT_NONE;
> >> +			goto failed;
> >> +		}
> >> +
> >> 		status = reset_subordinates(bridge);
> >> 		if (status != PCI_ERS_RESULT_RECOVERED) {
> >> 			pci_warn(bridge, "subordinate device reset failed\n");
> >> @@ -217,9 +232,13 @@ pci_ers_result_t pcie_do_recovery(struct pci_dev *dev,
> >> 	pci_dbg(bridge, "broadcast resume message\n");
> >> 	pci_walk_bridge(bridge, report_resume, &status);
> >> 
> >> -	if (pcie_aer_is_native(bridge))
> >> -		pcie_clear_device_status(bridge);
> >> -	pci_aer_clear_nonfatal_status(bridge);
> >> +	if (type == PCI_EXP_TYPE_ROOT_PORT ||
> >> +	    type == PCI_EXP_TYPE_DOWNSTREAM ||
> >> +	    type == PCI_EXP_TYPE_RC_EC) {
> >> +		if (pcie_aer_is_native(bridge))
> >> +			pcie_clear_device_status(bridge);
> >> +		pci_aer_clear_nonfatal_status(bridge);
> > 
> > This is hard to understand because "type" is from "dev", but "bridge"
> > is not necessarily the same device.  Should it be this?
> > 
> >  type = pci_pcie_type(bridge);
> >  if (type == PCI_EXP_TYPE_ROOT_PORT ||
> >      ...)
> 
> Correct, it would be better if the type was based on the ‘bridge’.

OK.  This is similar to
https://lore.kernel.org/linux-pci/20201002184735.1229220-8-seanvk.dev@xxxxxxxxxxxxxxxx/,
which you cited above except for the bridge/dev question and the
addition here of RC_EC.

I tried to split that back into its own patch and started with the
commit message from that patch.  But I got stuck on the commit
message.  I got as far as:

  In some cases an error may be reported by a device not visible to
  the OS, e.g., if firmware manages the device and passes error
  information to the OS via ACPI APEI.

But I still can't quite connect that to the patch.  "bridge" is
clearly a device visible to Linux.

I guess we're trying to assert that if "bridge" is not a Root Port,
Downstream Port, or RCEC, we shouldn't clear the error status because 
the error came from a device Linux doesn't know about.  But I think
"bridge" is *always* either a Root Port or a Downstream Port:

  if (type == PCI_EXP_TYPE_ROOT_PORT ||
      type == PCI_EXP_TYPE_DOWNSTREAM)
	  bridge = dev;
  else
	  bridge = pci_upstream_bridge(dev);

pci_upstream_bridge() returns either NULL (in which case previous uses
dereference a NULL pointer), or dev->bus->self, which is always a Root
Port, Switch Downstream Port, or Switch Upstream Port (or NULL for the
special case of VFs).

> >> +	}
> >> 	pci_info(bridge, "device recovery successful\n");
> >> 	return status;
> >> 
> >> -- 
> >> 2.29.2
>