stmmac: update the doc with new info about the driver's debug (v3)

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/stmmac.txt

@@ -235,7 +235,38 @@ reset procedure etc).
  o enh_desc.c: functions for handling enhanced descriptors
  o norm_desc.c: functions for handling normal descriptors
 
-5) TODO:
+5) Debug Information
+
+The driver exports many information i.e. internal statistics,
+debug information, MAC and DMA registers etc.
+
+These can be read in several ways depending on the
+type of the information actually needed.
+
+For example a user can be use the ethtool support
+to get statistics: e.g. using: ethtool -S ethX
+(that shows the Management counters (MMC) if supported)
+or sees the MAC/DMA registers: e.g. using: ethtool -d ethX
+
+Compiling the Kernel with CONFIG_DEBUG_FS and enabling the
+STMMAC_DEBUG_FS option the driver will export the following
+debugfs entries:
+
+/sys/kernel/debug/stmmaceth/descriptors_status
+  To show the DMA TX/RX descriptor rings
+
+Developer can also use the "debug" module parameter to get
+further debug information.
+
+In the end, there are other macros (that cannot be enabled
+via menuconfig) to turn-on the RX/TX DMA debugging,
+specific MAC core debug printk etc. Others to enable the
+debug in the TX and RX processes.
+All these are only useful during the developing stage
+and should never enabled inside the code for general usage.
+In fact, these can generate an huge amount of debug messages.
+
+6) TODO:
  o XGMAC is not supported.
  o Review the timer optimisation code to use an embedded device that will be
   available in new chip generations.