Documentation: networking: update PHY error counter diagnostics in twisted pair guide

Replace generic instructions for monitoring error counters with a
procedure using the unified PHY statistics interface (`--all-groups`).

Signed-off-by: Oleksij Rempel <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>

Author: olerem

Documentation/networking/diagnostic/twisted_pair_layer1_diagnostics.rst

@@ -713,17 +713,23 @@ driver supports reporting such events.
 
 - **Monitor Error Counters**:
 
-  - While some NIC drivers and PHYs provide error counters, there is no unified
-    set of PHY-specific counters across all hardware. Additionally, not all
-    PHYs provide useful information related to errors like CRC errors, frame
-    drops, or link flaps. Therefore, this step is dependent on the specific
-    hardware and driver support.
-
-  - **Next Steps**: Use `ethtool -S <interface>` to check if your driver
-    provides useful error counters. In some cases, counters may provide
-    information about errors like link flaps or physical layer problems (e.g.,
-    excessive CRC errors), but results can vary significantly depending on the
-    PHY.
+  - Use `ethtool -S <interface> --all-groups` to retrieve standardized interface
+    statistics if the driver supports the unified interface:
+
+  - **Command:** `ethtool -S <interface> --all-groups`
+
+  - **Example Output (if supported)**:
+
+    .. code-block:: bash
+
+      phydev-RxFrames: 100391
+      phydev-RxErrors: 0
+      phydev-TxFrames: 9
+      phydev-TxErrors: 0
+
+  - If the unified interface is not supported, use `ethtool -S <interface>` to
+    retrieve MAC and PHY counters. Note that non-standardized PHY counter names
+    vary by driver and must be interpreted accordingly:
 
   - **Command:** `ethtool -S <interface>`
 
@@ -740,6 +746,17 @@ driver supports reporting such events.
     condition) or kernel log messages (e.g., link up/down events) to further
     diagnose the issue.
 
+  - **Compare Counters**:
+
+    - Compare the egress and ingress frame counts reported by the PHY and MAC.
+
+    - A small difference may occur due to sampling rate differences between the
+      MAC and PHY drivers, or if the PHY and MAC are not always fully
+      synchronized in their UP or DOWN states.
+
+    - Significant discrepancies indicate potential issues in the data path
+      between the MAC and PHY.
+
 When All Else Fails...
 ~~~~~~~~~~~~~~~~~~~~~~