ice: Document tx_scheduling_layers parameter

New driver specific parameter 'tx_scheduling_layers' was introduced.
Describe parameter in the documentation.

Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Co-developed-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Documentation/networking/devlink/ice.rst

@@ -21,6 +21,53 @@ Parameters
    * - ``enable_iwarp``
      - runtime
      - mutually exclusive with ``enable_roce``
+   * - ``tx_scheduling_layers``
+     - permanent
+     - The ice hardware uses hierarchical scheduling for Tx with a fixed
+       number of layers in the scheduling tree. Each of them are decision
+       points. Root node represents a port, while all the leaves represent
+       the queues. This way of configuring the Tx scheduler allows features
+       like DCB or devlink-rate (documented below) to configure how much
+       bandwidth is given to any given queue or group of queues, enabling
+       fine-grained control because scheduling parameters can be configured
+       at any given layer of the tree.
+
+       The default 9-layer tree topology was deemed best for most workloads,
+       as it gives an optimal ratio of performance to configurability. However,
+       for some specific cases, this 9-layer topology might not be desired.
+       One example would be sending traffic to queues that are not a multiple
+       of 8. Because the maximum radix is limited to 8 in 9-layer topology,
+       the 9th queue has a different parent than the rest, and it's given
+       more bandwidth credits. This causes a problem when the system is
+       sending traffic to 9 queues:
+
+       | tx_queue_0_packets: 24163396
+       | tx_queue_1_packets: 24164623
+       | tx_queue_2_packets: 24163188
+       | tx_queue_3_packets: 24163701
+       | tx_queue_4_packets: 24163683
+       | tx_queue_5_packets: 24164668
+       | tx_queue_6_packets: 23327200
+       | tx_queue_7_packets: 24163853
+       | tx_queue_8_packets: 91101417 < Too much traffic is sent from 9th
+
+       To address this need, you can switch to a 5-layer topology, which
+       changes the maximum topology radix to 512. With this enhancement,
+       the performance characteristic is equal as all queues can be assigned
+       to the same parent in the tree. The obvious drawback of this solution
+       is a lower configuration depth of the tree.
+
+       Use the ``tx_scheduling_layer`` parameter with the devlink command
+       to change the transmit scheduler topology. To use 5-layer topology,
+       use a value of 5. For example:
+       $ devlink dev param set pci/0000:16:00.0 name tx_scheduling_layers
+       value 5 cmode permanent
+       Use a value of 9 to set it back to the default value.
+
+       You must do PCI slot powercycle for the selected topology to take effect.
+
+       To verify that value has been set:
+       $ devlink dev param show pci/0000:16:00.0 name tx_scheduling_layers
 
 Info versions
 =============