[PATCH net-next] net: dsa: mv88e6xxx: Allow dynamic reconfiguration of tag protocol

[Tobias Waldekranz <tobias@waldekranz.com>]

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.

Because there are ethernet controllers that do not handle regular DSA
tags in all cases, it is sometimes preferable to rely on the
undocumented behavior, as the alternative is a very crippled
system. But, in those cases, make sure to log the fact that an
undocumented feature has been enabled.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
---

In a system using an NXP T1023 SoC connected to a 6390X switch, we
noticed that TO_CPU frames where not reaching the CPU. This only
happened on hardware port 8. Looking at the DSA master interface
(dpaa-ethernet) we could see that an Rx error counter was bumped at
the same rate. The logs indicated a parser error.

It just so happens that a TO_CPU coming in on device 0, port 8, will
result in the first two bytes of the DSA tag being one of:

00 40
00 44
00 46

My guess is that since these values look like 802.3 length fields, the
controller's parser will signal an error if the frame length does not
match what is in the header.

As a workaround, switching to EDSA (thereby always having a proper
EtherType in the frame) solves the issue.

 drivers/net/dsa/mv88e6xxx/chip.c | 41 +++++++++++++++++++++++++++++---
 drivers/net/dsa/mv88e6xxx/chip.h |  3 +++
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
index 95f07fcd4f85..e7ec883d5f6b 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.c
+++ b/drivers/net/dsa/mv88e6xxx/chip.c
@@ -2531,10 +2531,10 @@ static int mv88e6xxx_setup_port_mode(struct mv88e6xxx_chip *chip, int port)
 		return mv88e6xxx_set_port_mode_normal(chip, port);
 
 	/* Setup CPU port mode depending on its supported tag format */
-	if (chip->info->tag_protocol == DSA_TAG_PROTO_DSA)
+	if (chip->tag_protocol == DSA_TAG_PROTO_DSA)
 		return mv88e6xxx_set_port_mode_dsa(chip, port);
 
-	if (chip->info->tag_protocol == DSA_TAG_PROTO_EDSA)
+	if (chip->tag_protocol == DSA_TAG_PROTO_EDSA)
 		return mv88e6xxx_set_port_mode_edsa(chip, port);
 
 	return -EINVAL;
@@ -5564,7 +5564,39 @@ static enum dsa_tag_protocol mv88e6xxx_get_tag_protocol(struct dsa_switch *ds,
 {
 	struct mv88e6xxx_chip *chip = ds->priv;
 
-	return chip->info->tag_protocol;
+	return chip->tag_protocol;
+}
+
+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;
+	}
+
+	old_protocol = chip->tag_protocol;
+	chip->tag_protocol = proto;
+
+	mv88e6xxx_reg_lock(chip);
+	err = mv88e6xxx_setup_port_mode(chip, port);
+	mv88e6xxx_reg_unlock(chip);
+
+	if (err)
+		chip->tag_protocol = old_protocol;
+
+	return err;
 }
 
 static int mv88e6xxx_port_mdb_add(struct dsa_switch *ds, int port,
@@ -6029,6 +6061,7 @@ static int mv88e6xxx_crosschip_lag_leave(struct dsa_switch *ds, int sw_index,
 
 static const struct dsa_switch_ops mv88e6xxx_switch_ops = {
 	.get_tag_protocol	= mv88e6xxx_get_tag_protocol,
+	.change_tag_protocol	= mv88e6xxx_change_tag_protocol,
 	.setup			= mv88e6xxx_setup,
 	.teardown		= mv88e6xxx_teardown,
 	.phylink_validate	= mv88e6xxx_validate,
@@ -6209,6 +6242,8 @@ static int mv88e6xxx_probe(struct mdio_device *mdiodev)
 	if (err)
 		goto out;
 
+	chip->tag_protocol = chip->info->tag_protocol;
+
 	mv88e6xxx_phy_init(chip);
 
 	if (chip->info->ops->get_eeprom) {
diff --git a/drivers/net/dsa/mv88e6xxx/chip.h b/drivers/net/dsa/mv88e6xxx/chip.h
index bce6e0dc8535..96b775f3fda2 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.h
+++ b/drivers/net/dsa/mv88e6xxx/chip.h
@@ -261,6 +261,9 @@ struct mv88e6xxx_region_priv {
 struct mv88e6xxx_chip {
 	const struct mv88e6xxx_info *info;
 
+	/* Currently configured tagging protocol */
+	enum dsa_tag_protocol tag_protocol;
+
 	/* The dsa_switch this private structure is related to */
 	struct dsa_switch *ds;
 
-- 
2.25.1