[Intel-wired-lan] [PATCH iwl-next v4 5/5] ice: Document tx_scheduling_layers parameter

[Mateusz Polchlopek <mateusz.polchlopek@intel.com>]

From: Michal Wilczynski <michal.wilczynski@intel.com>

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

Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Co-developed-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
---
 Documentation/networking/devlink/ice.rst | 41 ++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/Documentation/networking/devlink/ice.rst b/Documentation/networking/devlink/ice.rst
index efc6be109dc3..1ae46dee0fd5 100644
--- a/Documentation/networking/devlink/ice.rst
+++ b/Documentation/networking/devlink/ice.rst
@@ -36,6 +36,47 @@ Parameters
        The latter allows for bandwidth higher than external port speed
        when looping back traffic between VFs. Works with 8x10G and 4x25G
        cards.
+   * - ``tx_scheduling_layers``
+     - permanent
+     - The ice hardware uses hierarchical scheduling for Tx with a fixed
+       number of layers in the scheduling tree. Root node is representing a
+       port, while all the leaves represents the queues. This way of
+       configuring Tx scheduler allows features like DCB or devlink-rate
+       (documented below) for fine-grained configuration how much BW is given
+       to any given queue or group of queues, as scheduling parameters can be
+       configured at any given layer of the tree. By default 9-layer tree
+       topology was deemed best for most workloads, as it gives optimal
+       performance to configurability ratio. However for some specific cases,
+       this might not be the case. A great example would be sending traffic to
+       queues that is not a multiple of 8. Since in 9-layer topology maximum
+       number of children is limited to 8, the 9th queue has a different parent
+       than the rest, and it's given more BW 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 to 9th
+
+       Sometimes this might be a big concern, so the idea is to empower the
+       user to switch to 5-layer topology, enabling performance gains but
+       sacrificing configurability for features like DCB and devlink-rate.
+
+       This parameter gives user flexibility to choose the 5-layer transmit
+       scheduler topology. After switching parameter reboot is required for
+       the feature to start working.
+
+       User could choose 9 (the default) or 5 as a value of parameter, e.g.:
+       $ devlink dev param set pci/0000:16:00.0 name tx_scheduling_layers
+       value 5 cmode permanent
+
+       And verify that value has been set:
+       $ devlink dev param show pci/0000:16:00.0 name tx_scheduling_layers
 
 Info versions
 =============
-- 
2.38.1