[RFC 6/6] docs: net: dsa: sja1105: document the best_effort_vlan_filtering option

[Vladimir Oltean <olteanv@gmail.com>]

From: Vladimir Oltean <vladimir.oltean@nxp.com>

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 .../networking/devlink-params-sja1105.txt     | 24 ++++++++++
 Documentation/networking/dsa/sja1105.rst      | 46 +++++++++++++++++++
 2 files changed, 70 insertions(+)

diff --git a/Documentation/networking/devlink-params-sja1105.txt b/Documentation/networking/devlink-params-sja1105.txt
index 5096a4cf923c..576dcc6e2d96 100644
--- a/Documentation/networking/devlink-params-sja1105.txt
+++ b/Documentation/networking/devlink-params-sja1105.txt
@@ -7,3 +7,27 @@ hostprio		[DEVICE, DRIVER-SPECIFIC]
 			your PTP frames.
 			Configuration mode: runtime
 			Type: u8. 0-7 valid.
+
+best_effort_vlan_filtering
+			[DEVICE, DRIVER-SPECIFIC]
+			Allow plain ETH_P_8021Q headers to be used as DSA tags.
+			Benefits:
+			- Can terminate untagged traffic over switch net
+			  devices even when enslaved to a bridge with
+			  vlan_filtering=1.
+			- Can terminate VLAN-tagged traffic over switch net
+			  devices even when enslaved to a bridge with
+			  vlan_filtering=1, with some constraints (no more than
+			  7 VLANs per user port).
+			- Can do QoS based on VLAN PCP and VLAN membership
+			  admission control for autonomously forwarded frames
+			  (regardless of whether they can be terminated on the
+			  CPU or not).
+			Drawbacks:
+			- User cannot use VLANs in range 1024-3071. If the
+			  switch receives frames with such VIDs, it will
+			  misinterpret them as DSA tags.
+			- Switch uses Shared VLAN Learning (FDB lookup uses
+			  only DMAC as key).
+			Configuration mode: runtime
+			Type: bool.
diff --git a/Documentation/networking/dsa/sja1105.rst b/Documentation/networking/dsa/sja1105.rst
index 4a8639cba1f3..d963ff2ac1c9 100644
--- a/Documentation/networking/dsa/sja1105.rst
+++ b/Documentation/networking/dsa/sja1105.rst
@@ -77,6 +77,52 @@ change.
 The TPID is restored when ``vlan_filtering`` is requested by the user through
 the bridge layer, and general IP termination becomes no longer possible through
 the switch netdevices in this mode.
+There exists a third configuration option, via ``best_effort_vlan_filtering``.
+This permits termination of some traffic on switch net devices, at the expense
+of losing some VLAN filtering abilities: reduced range of usable VIDs and
+shared VLAN learning.
+The frames which can be terminated on the CPU in this mode are:
+- All untagged frames
+- VLAN-tagged frames, up to 7 different VLANs per user port
+This operating mode is slightly insane to be collated with the default
+``vlan_filtering``, so it is an opt-in that needs to be enabled using a devlink
+parameter. To enable it::
+
+  ip link set dev br0 type bridge vlan_filtering 1
+  [   61.204770] sja1105 spi0.1: Reset switch and programmed static config. Reason: VLAN filtering
+  [   61.239944] sja1105 spi0.1: Disabled switch tagging
+  devlink dev param set spi/spi0.1 name best_effort_vlan_filtering value true cmode runtime
+  [   64.682927] sja1105 spi0.1: Reset switch and programmed static config. Reason: VLAN filtering
+  [   64.711925] sja1105 spi0.1: Enabled switch tagging
+  bridge vlan add dev swp2 vid 1025 untagged pvid
+  RTNETLINK answers: Operation not permitted
+  bridge vlan add dev swp2 vid 100
+  bridge vlan add dev swp2 vid 101 untagged
+  bridge vlan
+  port    vlan ids
+  swp5     1 PVID Egress Untagged
+
+  swp2     1 PVID Egress Untagged
+           100
+           101 Egress Untagged
+
+  swp3     1 PVID Egress Untagged
+
+  swp4     1 PVID Egress Untagged
+
+  br0      1 PVID Egress Untagged
+  bridge vlan add dev swp2 vid 102
+  bridge vlan add dev swp2 vid 103
+  bridge vlan add dev swp2 vid 104
+  bridge vlan add dev swp2 vid 105
+  bridge vlan add dev swp2 vid 106
+  bridge vlan add dev swp2 vid 107
+  [ 3885.216832] sja1105 spi0.1: No more free subvlans
+
+The "No more free subvlans" warning message means that once the capacity is
+exceeded, frames tagged with newly added VLANs (in this case 107) are not able
+to be terminated on the CPU. They are still accepted and forwarded
+autonomously.
 
 The switches have two programmable filters for link-local destination MACs.
 These are used to trap BPDUs and PTP traffic to the master netdevice, and are
-- 
2.17.1