Documentation: igb: Add a section about CBS

Add some pointers to the definition of the CBS algorithm, and some
notes about the limits of its implementation in the i210 family of
controllers.
Signed-off-by: NVinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

Documentation/networking/device_drivers/intel/igb.rst

@@ -177,6 +177,25 @@ rate limit using the IProute2 tool. Download the latest version of the
 IProute2 tool from Sourceforge if your version does not have all the features
 you require.
 
+Credit Based Shaper (Qav Mode)
+------------------------------
+When enabling the CBS qdisc in the hardware offload mode, traffic shaping using
+the CBS (described in the IEEE 802.1Q-2018 Section 8.6.8.2 and discussed in the
+Annex L) algorithm will run in the i210 controller, so it's more accurate and
+uses less CPU.
+
+When using offloaded CBS, and the traffic rate obeys the configured rate
+(doesn't go above it), CBS should have little to no effect in the latency.
+
+The offloaded version of the algorithm has some limits, caused by how the idle
+slope is expressed in the adapter's registers. It can only represent idle slopes
+in 16.38431 kbps units, which means that if a idle slope of 2576kbps is
+requested, the controller will be configured to use a idle slope of ~2589 kbps,
+because the driver rounds the value up. For more details, see the comments on
+:c:func:`igb_config_tx_modes()`.
+
+NOTE: This feature is exclusive to i210 models.
+
 
 Support
 =======