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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: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
This commit is contained in:
Vinicius Costa Gomes 2018-11-16 16:19:24 -08:00 committed by Jeff Kirsher
parent 6f9ae17530
commit bad87ee82f
1 changed files with 19 additions and 0 deletions
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Documentation/networking/device_drivers/intel/igb.rst
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@ -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.
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