net: mscc: ocelot: fix backwards compatibility with single-chain tc-flower offload

ACL rules can be offloaded to VCAP IS2 either through chain 0, or, since
the blamed commit, through a chain index whose number encodes a specific
PAG (Policy Action Group) and lookup number.

The chain number is translated through ocelot_chain_to_pag() into a PAG,
and through ocelot_chain_to_lookup() into a lookup number.

The problem with the blamed commit is that the above 2 functions don't
have special treatment for chain 0. So ocelot_chain_to_pag(0) returns
filter->pag = 224, which is in fact -32, but the "pag" field is an u8.

So we end up programming the hardware with VCAP IS2 entries having a PAG
of 224. But the way in which the PAG works is that it defines a subset
of VCAP IS2 filters which should match on a packet. The default PAG is
0, and previous VCAP IS1 rules (which we offload using 'goto') can
modify it. So basically, we are installing filters with a PAG on which
no packet will ever match. This is the hardware equivalent of adding
filters to a chain which has no 'goto' to it.

Restore the previous functionality by making ACL filters offloaded to
chain 0 go to PAG 0 and lookup number 0. The choice of PAG is clearly
correct, but the choice of lookup number isn't "as before" (which was to
leave the lookup a "don't care"). However, lookup 0 should be fine,
since even though there are ACL actions (policers) which have a
requirement to be used in a specific lookup, that lookup is 0.

Fixes: 226e9cd8 ("net: mscc: ocelot: only install TCAM entries into a specific lookup and PAG")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220316192117.2568261-1-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mscc/ocelot_flower.c

@@ -60,6 +60,12 @@ static int ocelot_chain_to_block(int chain, bool ingress)
  */
 static int ocelot_chain_to_lookup(int chain)
 {
+	/* Backwards compatibility with older, single-chain tc-flower
+	 * offload support in Ocelot
+	 */
+	if (chain == 0)
+		return 0;
+
 	return (chain / VCAP_LOOKUP) % 10;
 }
 
@@ -68,7 +74,15 @@ static int ocelot_chain_to_lookup(int chain)
  */
 static int ocelot_chain_to_pag(int chain)
 {
-	int lookup = ocelot_chain_to_lookup(chain);
+	int lookup;
+
+	/* Backwards compatibility with older, single-chain tc-flower
+	 * offload support in Ocelot
+	 */
+	if (chain == 0)
+		return 0;
+
+	lookup = ocelot_chain_to_lookup(chain);
 
 	/* calculate PAG value as chain index relative to the first PAG */
 	return chain - VCAP_IS2_CHAIN(lookup, 0);