docs: networking: extend the statistics documentation

Make the lack of expectations for switching NICs explicit,
describe the new stats.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/statistics.rst

@@ -44,8 +44,27 @@ If `-s` is specified once the detailed errors won't be shown.
 Protocol-specific statistics
 ----------------------------
 
-Some of the interfaces used for configuring devices are also able
-to report related statistics. For example ethtool interface used
+Protocol-specific statistics are exposed via relevant interfaces,
+the same interfaces as are used to configure them.
+
+ethtool
+~~~~~~~
+
+Ethtool exposes common low-level statistics.
+All the standard statistics are expected to be maintained
+by the device, not the driver (as opposed to driver-defined stats
+described in the next section which mix software and hardware stats).
+For devices which contain unmanaged
+switches (e.g. legacy SR-IOV or multi-host NICs) the events counted
+may not pertain exclusively to the packets destined to
+the local host interface. In other words the events may
+be counted at the network port (MAC/PHY blocks) without separation
+for different host side (PCIe) devices. Such ambiguity must not
+be present when internal switch is managed by Linux (so called
+switchdev mode for NICs).
+
+Standard ethtool statistics can be accessed via the interfaces used
+for configuration. For example ethtool interface used
 to configure pause frames can report corresponding hardware counters::
 
   $ ethtool --include-statistics -a eth0
@@ -57,6 +76,27 @@ to configure pause frames can report corresponding hardware counters::
     tx_pause_frames: 1
     rx_pause_frames: 1
 
+General Ethernet statistics not associated with any particular
+functionality are exposed via ``ethtool -S $ifc`` by specifying
+the ``--groups`` parameter::
+
+  $ ethtool -S eth0 --groups eth-phy eth-mac eth-ctrl rmon
+  Stats for eth0:
+  eth-phy-SymbolErrorDuringCarrier: 0
+  eth-mac-FramesTransmittedOK: 1
+  eth-mac-FrameTooLongErrors: 1
+  eth-ctrl-MACControlFramesTransmitted: 1
+  eth-ctrl-MACControlFramesReceived: 0
+  eth-ctrl-UnsupportedOpcodesReceived: 1
+  rmon-etherStatsUndersizePkts: 1
+  rmon-etherStatsJabbers: 0
+  rmon-rx-etherStatsPkts64Octets: 1
+  rmon-rx-etherStatsPkts65to127Octets: 0
+  rmon-rx-etherStatsPkts128to255Octets: 0
+  rmon-tx-etherStatsPkts64Octets: 2
+  rmon-tx-etherStatsPkts65to127Octets: 3
+  rmon-tx-etherStatsPkts128to255Octets: 0
+
 Driver-defined statistics
 -------------------------