[PATCH v8 3/6] PCI: pciehp: check and wait port status out of DPC before handling DLLSC and PDC

[Ethan Zhao <haifeng.zhao@intel.com>]

When root port has DPC capability and it is enabled, then triggered by
errors, DPC DLLSC and PDC etc interrupts will be sent to DPC driver, 
pciehp drivers almost at the same time.
Thus will cause following messed and confused errors handling/recovery/
removal/plugin procedure.

1. Port and device are in error recovery resetting initiated by DPC
   hardware, pciehp driver treats them as device is doing hot-remove or 
   hot-plugin the same time.

2. While DPC handler calling device driver->err_handler callback(
   error_detected/resume etc), but the slot may be powered off by

   pciehp
   -> remove_board()
      -> pciehp_power_off_slot().

3. While DPC handler -> pci_do_recovery is doing different action to
   detect error and recover based on device->error_state, pciehp driver
   could change it on the fly by:

   pciehp_unconfigure_device()
   ->pci_walk_bus()
     -> pci_dev_set_disconnected()

4. While DPC handler is calling device driver err_handler callback to 
   detect error and recover, pciehp driver could is doing device unbind
   and release its driver.

   ...

While NON-FATAL/FATAL errors happen while hotplug is(is not)doing, result
is not determinate.

So we need some kind of synchronization between pciehp DLLSC/PDC handling
and DPC driver error recover handling.  we need a determinate result
of DPC error containment, link is recovered, link isn't recovered, device
is still there, device is removed, then do pciehp hot-remove and
hot-plugin procudure, don't mix them together.

Per our test on ICS platform, DPC error containment and software handler
will take 10ms up to 50ms till clean the DPC triggered status. it is quick
enough for pciehp compared with its 1000ms waiting to ignore DLLSC/PDC
after doing power off.

With this patch, the handling flow of DPC containment and hotplug is
partly ordered and serialized, let hardware DPC do the controller reset
etc recovery action first, then DPC driver handling the call-back from
device drivers, clear the DPC status, at the end, pciehp handle the DLLSC
and PDC etc.

After tens of PCIe Gen4 NVMe SSD brute force hot-remove and hot-plugin with
any time internval between the two actions, also stressed with the DPC
injection test. system recovered to normal working state from
NON-FATAL/FATAL errors as expected. hotplug works well without any random
undeterminate errors or malfunction.

Brute DPC error injection script:

for i in {0..100}
do
        setpci -s 64:02.0 0x196.w=000a
        setpci -s 65:00.0 0x04.w=0544
        mount /dev/nvme0n1p1 /root/nvme
        sleep 1
done

Signed-off-by: Ethan Zhao <haifeng.zhao@intel.com>
Tested-by: Wen Jin <wen.jin@intel.com>
Tested-by: Shanshan Zhang <ShanshanX.Zhang@intel.com>
---
Changes:
 v2: revise doc according to Andy's suggestion.
 v3: no change.
 v4: no change.
 v5: no change.
 v6: moved to [3/5] from [2/5] and re-wrote description.
 v7: no change.
 v8: no change.

 drivers/pci/hotplug/pciehp_hpc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
index 53433b37e181..6f271160f18d 100644
--- a/drivers/pci/hotplug/pciehp_hpc.c
+++ b/drivers/pci/hotplug/pciehp_hpc.c
@@ -710,8 +710,10 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
 	down_read(&ctrl->reset_lock);
 	if (events & DISABLE_SLOT)
 		pciehp_handle_disable_request(ctrl);
-	else if (events & (PCI_EXP_SLTSTA_PDC | PCI_EXP_SLTSTA_DLLSC))
+	else if (events & (PCI_EXP_SLTSTA_PDC | PCI_EXP_SLTSTA_DLLSC)) {
+		pci_wait_port_outdpc(pdev);
 		pciehp_handle_presence_or_link_change(ctrl, events);
+	}
 	up_read(&ctrl->reset_lock);
 
 	ret = IRQ_HANDLED;
-- 
2.18.4