Date:   Fri, 25 Feb 2022 11:22:18 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        UNGLinuxDriver@...rochip.com, Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        DENG Qingfang <dqfext@...il.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alvin Šipraga <alsi@...g-olufsen.dk>,
        George McCollister <george.mccollister@...il.com>
Subject: [PATCH v2 net-next 03/10] docs: net: dsa: sja1105: document limitations of tc-flower rule VLAN awareness

After change "net: dsa: tag_8021q: replace the SVL bridging with
VLAN-unaware IVL bridging", tag_8021q enforces two different pvids on a
port, depending on whether it is standalone or in a VLAN-unaware bridge.

Up until now, there was a single pvid, represented by
dsa_tag_8021q_rx_vid(), and that was used as the VLAN for VLAN-unaware
virtual link rules, regardless of whether the port was bridged or
standalone.

To keep VLAN-unaware virtual links working, we need to follow whether
the port is in a bridge or not, and update the VLAN ID from those rules.

In fact we can't fully do that. Depending on whether the switch is
VLAN-aware or not, we can accept Virtual Link rules with just the MAC
DA, or with a MAC DA and a VID. So we already deny changes to the VLAN
awareness of the switch. But the VLAN awareness may also change as a
result of joining or leaving a bridge.

One might say we could just allow the following: a port may leave a
VLAN-unaware bridge while it has VLAN-unaware VL (tc-flower) rules, and
the driver will update those with the new tag_8021q pvid for standalone
mode, but the driver won't accept joining a bridge at all while VL rules
were installed in standalone mode. This is sort of a compromise made
because leaving a bridge is an operation that cannot be vetoed.
But this sort of setup change is not fully supported, either: as
mentioned, VLAN filtering changes can also be triggered by leaving a
bridge, therefore, the existing veto we have in place for turning VLAN
filtering off with VLAN-aware VL rules active still isn't fully
effective.

I really don't know how to deal with this in a way that produces
predictable behavior for user space. Since at the moment, keeping this
feature fully functional on constellation changes (not changing the
tag_8021q port pvid when joining a bridge) is blocking progress for the
DSA FDB isolation, I'd rather document it as a (potentially temporary)
limitation and go on without it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
---
 Documentation/networking/dsa/sja1105.rst | 27 ++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/Documentation/networking/dsa/sja1105.rst b/Documentation/networking/dsa/sja1105.rst
index 29b1bae0cf00..e0219c1452ab 100644
--- a/Documentation/networking/dsa/sja1105.rst
+++ b/Documentation/networking/dsa/sja1105.rst
@@ -293,6 +293,33 @@ of dropped frames, which is a sum of frames dropped due to timing violations,
 lack of destination ports and MTU enforcement checks). Byte-level counters are
 not available.
 
+Limitations
+===========
+
+The SJA1105 switch family always performs VLAN processing. When configured as
+VLAN-unaware, frames carry a different VLAN tag internally, depending on
+whether the port is standalone or under a VLAN-unaware bridge.
+
+The virtual link keys are always fixed at {MAC DA, VLAN ID, VLAN PCP}, but the
+driver asks for the VLAN ID and VLAN PCP when the port is under a VLAN-aware
+bridge. Otherwise, it fills in the VLAN ID and PCP automatically, based on
+whether the port is standalone or in a VLAN-unaware bridge, and accepts only
+"VLAN-unaware" tc-flower keys (MAC DA).
+
+The existing tc-flower keys that are offloaded using virtual links are no
+longer operational after one of the following happens:
+
+- port was standalone and joins a bridge (VLAN-aware or VLAN-unaware)
+- port is part of a bridge whose VLAN awareness state changes
+- port was part of a bridge and becomes standalone
+- port was standalone, but another port joins a VLAN-aware bridge and this
+  changes the global VLAN awareness state of the bridge
+
+The driver cannot veto all these operations, and it cannot update/remove the
+existing tc-flower filters either. So for proper operation, the tc-flower
+filters should be installed only after the forwarding configuration of the port
+has been made, and removed by user space before making any changes to it.
+
 Device Tree bindings and board design
 =====================================
 
-- 
2.25.1

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