Platform API for PCIe AER stats collection (#702)

Update the PCIed hld to add the platform API definition for PCIe AER stats collection

doc/pcie-mon/pcie-monitoring-services-hld.md

@@ -1,6 +1,6 @@
 # SONiC PCIe Monitoring services HLD #
 
-### Rev 0.3 ###
+### Rev 0.4 ###
 
 ### Revision
  | Rev | Date | Author | Change Description |
@@ -10,6 +10,7 @@
  | | | | Add pcied to PMON for runtime monitoring |
  | 0.3 | | Arun Saravanan Balachandran | Add AER stats update support in pcied |
  | | | | Add command to display AER stats |
+ | 0.4 | | Arun Saravanan Balachandran | Add platform API to collect AER stats |
 
 ## About This Manual ##
 
@@ -127,7 +128,38 @@ For AER supported PCIe device, the AER stats belonging to severities `correctabl
 
 ### 2.2 PCIe AER stats collection in pcied ###
 
-For PCIe devices that pass PcieUtil `get_pcie_check`, the AER stats if available will be retrieved and updated in the STATE_DB periodically every minute by pcied.
+A common platform API `get_pcie_aer_stats` is defined in class `PcieBase` for retrieving AER stats of a PCIe device:
+
+```
+ @abc.abstractmethod
+ def get_pcie_aer_stats(self, domain, bus, dev, fn):
+ """
+ Returns a nested dictionary containing the AER stats belonging to a
+ PCIe device
+
+ Args:
+ domain, bus, dev, fn: Domain, bus, device, function of the PCIe
+ device respectively
+
+ Returns:
+ A nested dictionary where key is severity 'correctable', 'fatal' or
+ 'non_fatal', value is a dictionary of key, value pairs in the format:
+ {'AER Error type': Error count}
+
+ Ex. {'correctable': {'BadDLLP': 0, 'BadTLP': 0},
+ 'fatal': {'RxOF': 0, 'MalfTLP': 0},
+ 'non_fatal': {'RxOF': 0, 'MalfTLP': 0}}
+
+ For PCIe devices that do not support AER, the value for each severity
+ key is an empty dictionary.
+ """
+ return {}
+```
+
+Default `get_pcie_aer_stats`is implemented in PcieUtil class at sonic_platform_base/sonic_pcie/pcie_common.py.
+It returns the AER stats for a given PCIe device obtained from the AER sysfs under `/sys/bus/pci/devices/<Domain>:<Bus>:<Dev>.<Fn>`
+
+For PCIe devices that pass PcieUtil `get_pcie_check`, AER stats will be retrieved using `get_pcie_aer_stats` and updated in the STATE_DB periodically every minute by pcied.
 
 ### 2.3 STATE_DB keys and value ###