On Fri, Mar 26, 2021 at 11:56:46AM +0100, Tobias Waldekranz wrote:

All devices are capable of using regular DSA tags. Support for
Ethertyped DSA tags sort into three categories:

1. No support. Older chips fall into this category.

2. Full support. Datasheet explicitly supports configuring the CPU
   port to receive FORWARDs with a DSA tag.

3. Undocumented support. Datasheet lists the configuration from
   category 2 as "reserved for future use", but does empirically
   behave like a category 2 device.

+static int mv88e6xxx_change_tag_protocol(struct dsa_switch *ds, int port,
+					 enum dsa_tag_protocol proto)
+{
+	struct mv88e6xxx_chip *chip = ds->priv;
+	enum dsa_tag_protocol old_protocol;
+	int err;
+
+	switch (proto) {
+	case DSA_TAG_PROTO_EDSA:
+		if (chip->info->tag_protocol != DSA_TAG_PROTO_EDSA)
+			dev_warn(chip->dev, "Relying on undocumented EDSA tagging behavior\n");
+
+		break;
+	case DSA_TAG_PROTO_DSA:
+		break;
+	default:
+		return -EPROTONOSUPPORT;
+	}

You are handling cases 2 and 3 here, but not 1. Which makes it a bit
of a foot cannon for older devices.

Now that we have chip->tag_protocol, maybe we should change
chip->info->tag_protocol to mean supported protocols?

BIT(0) DSA
BIT(1) EDSA
BIT(2) Undocumented EDSA

Andrew