[PATCH v7 net-next 07/11] net: dsa: keep a copy of the tagging protocol in the DSA switch tree

[Vladimir Oltean <olteanv@gmail.com>]

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

Cascading DSA switches can be done multiple ways. There is the brute
force approach / tag stacking, where one upstream switch, located
between leaf switches and the host Ethernet controller, will just
happily transport the DSA header of those leaf switches as payload.
For this kind of setups, DSA works without any special kind of treatment
compared to a single switch - they just aren't aware of each other.
Then there's the approach where the upstream switch understands the tags
it transports from its leaves below, as it doesn't push a tag of its own,
but it routes based on the source port & switch id information present
in that tag (as opposed to DMAC & VID) and it strips the tag when
egressing a front-facing port. Currently only Marvell implements the
latter, and Marvell DSA trees contain only Marvell switches.

So it is safe to say that DSA trees already have a single tag protocol
shared by all switches, and in fact this is what makes the switches able
to understand each other.

It's time to make this official and enforce it (yes, this also means we
won't have any "switch understands tag to some extent but is not able to
speak it" hardware oddities that we'll support in the future).

This is needed due to the imminent introduction of the dsa_switch_ops::
{set,del}_tag_protocol driver API. When that is introduced, we'll have
to notify switches of the tagging protocol that they're configured to
use, even at probe and remove time. Currently the tag_ops structure
pointer is held only for CPU ports. But there are switches which don't
have CPU ports and nonetheless still need to be configured. These would
be Marvell leaf switches whose upstream port is just a DSA link. How do
we inform these of their tagging protocol setup/deletion?

One answer to the above would be: iterate through the DSA switch tree's
ports once, list the CPU ports, get their tag_ops, then iterate again
now that we have it, and notify everybody of that tag_ops. But what to
do if conflicts appear between one cpu_dp->tag_ops and another? There's
no escaping the fact that it makes no sense to have multiple tag_ops in
the same dst.

Ease our work and just keep the master copy of the tag_ops inside the
struct dsa_switch_tree. Note that reference counting the tagger module
driver still happens for each CPU port that uses that tagging protocol.

There are many places in the data path that access master->dsa_ptr->tag_ops
and we would introduce unnecessary performance penalty going through yet
another indirection, so keep those right where they are.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
Changes in v7:
Patch is new.

 include/net/dsa.h | 7 ++++++-
 net/dsa/dsa2.c    | 6 ++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/net/dsa.h b/include/net/dsa.h
index 2f5435d3d1db..b8af1d6c879a 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -140,6 +140,9 @@ struct dsa_switch_tree {
 	/* Has this tree been applied to the hardware? */
 	bool setup;
 
+	/* Tagging protocol operations */
+	const struct dsa_device_ops *tag_ops;
+
 	/*
 	 * Configuration data for the platform device that owns
 	 * this dsa switch tree instance.
@@ -225,7 +228,9 @@ struct dsa_port {
 		struct net_device *slave;
 	};
 
-	/* CPU port tagging operations used by master or slave devices */
+	/* Copy of the tagging protocol operations, for quicker access
+	 * in the data path. Valid only for the CPU ports.
+	 */
 	const struct dsa_device_ops *tag_ops;
 
 	/* Copies for faster access in master receive hot path */
diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
index 2953d0c1c7bc..42f22955e111 100644
--- a/net/dsa/dsa2.c
+++ b/net/dsa/dsa2.c
@@ -1026,6 +1026,12 @@ static int dsa_port_parse_cpu(struct dsa_port *dp, struct net_device *master)
 
 	dp->master = master;
 	dp->type = DSA_PORT_TYPE_CPU;
+	if (dst->tag_ops && dst->tag_ops != tag_ops) {
+		dev_err(ds->dev,
+			"A DSA switch tree can have only one tagging protocol\n");
+		return -EINVAL;
+	}
+	dst->tag_ops = tag_ops;
 	dp->filter = tag_ops->filter;
 	dp->rcv = tag_ops->rcv;
 	dp->tag_ops = tag_ops;
-- 
2.25.1