net: dsa: tag_ocelot_8021q: break circular dependency with ocelot switch lib

Michael reported that when using the "ocelot-8021q" tagging protocol,
the switch driver module must be manually loaded before the tagging
protocol can be loaded/is available.

This appears to be the same problem described here:
https://lore.kernel.org/netdev/20210908220834.d7gmtnwrorhharna@skbuf/
where due to the fact that DSA tagging protocols make use of symbols
exported by the switch drivers, circular dependencies appear and this
breaks module autoloading.

The ocelot_8021q driver needs the ocelot_can_inject() and
ocelot_port_inject_frame() functions from the switch library. Previously
the wrong approach was taken to solve that dependency: shims were
provided for the case where the ocelot switch library was compiled out,
but that turns out to be insufficient, because the dependency when the
switch lib _is_ compiled is problematic too.

We cannot declare ocelot_can_inject() and ocelot_port_inject_frame() as
static inline functions, because these access I/O functions like
__ocelot_write_ix() which is called by ocelot_write_rix(). Making those
static inline basically means exposing the whole guts of the ocelot
switch library, not ideal...

We already have one tagging protocol driver which calls into the switch
driver during xmit but not using any exported symbol: sja1105_defer_xmit.
We can do the same thing here: create a kthread worker and one work item
per skb, and let the switch driver itself do the register accesses to
send the skb, and then consume it.

Fixes: 0a6f17c6ae21 ("net: dsa: tag_ocelot_8021q: add support for PTP timestamping")
Reported-by: Michael Walle <michael@walle.cc>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/linux/dsa/ocelot.h b/include/linux/dsa/ocelot.h
index 50641a7529ad..8ae999f587c4 100644
--- a/include/linux/dsa/ocelot.h
+++ b/include/linux/dsa/ocelot.h
@@ -5,6 +5,7 @@
 #ifndef _NET_DSA_TAG_OCELOT_H
 #define _NET_DSA_TAG_OCELOT_H
 
+#include <linux/kthread.h>
 #include <linux/packing.h>
 #include <linux/skbuff.h>
 
@@ -160,6 +161,17 @@ struct ocelot_skb_cb {
  *         +------+------+------+------+------+------+------+------+
  */
 
+struct felix_deferred_xmit_work {
+	struct dsa_port *dp;
+	struct sk_buff *skb;
+	struct kthread_work work;
+};
+
+struct felix_port {
+	void (*xmit_work_fn)(struct kthread_work *work);
+	struct kthread_worker *xmit_worker;
+};
+
 static inline void ocelot_xfh_get_rew_val(void *extraction, u64 *rew_val)
 {
 	packing(extraction, rew_val, 116, 85, OCELOT_TAG_LEN, UNPACK, 0);