The problem is that DSA tagging protocols really must not depend on the
switch driver, because this creates a circular dependency at insmod
time, and the switch driver will effectively not load when the tagging
protocol driver is missing.

The code was structured in the way it was for a reason, though. The DSA
driver-facing API for PTP timestamping relies on the assumption that
two-step TX timestamps are provided by the hardware in an out-of-band
manner, typically by raising an interrupt and making that timestamp
available inside some sort of FIFO which is to be accessed over
SPI/MDIO/etc.

So the API puts .port_txtstamp into dsa_switch_ops, because it is
expected that the switch driver needs to save some state (like put the
skb into a queue until its TX timestamp arrives).

On SJA1110, TX timestamps are provided by the switch as Ethernet
packets, so this makes them be received and processed by the tagging
protocol driver. This in itself is great, because the timestamps are
full 64-bit and do not require reconstruction, and since Ethernet is the
fastest I/O method available to/from the switch, PTP timestamps arrive
very quickly, no matter how bottlenecked the SPI connection is, because
SPI interaction is not needed at all.

DSA's code structure and strict isolation between the tagging protocol
driver and the switch driver break the natural code organization.

When the tagging protocol driver receives a packet which is classified
as a metadata packet containing timestamps, it passes those timestamps
one by one to the switch driver, which then proceeds to compare them
based on the recorded timestamp ID that was generated in .port_txtstamp.

The communication between the tagging protocol and the switch driver is
done through a method exported by the switch driver, sja1110_process_meta_tstamp.
To satisfy build requirements, we force a dependency to build the
tagging protocol driver as a module when the switch driver is a module.
However, as explained in the first paragraph, that causes the circular
dependency.

To solve this, move the skb queue from struct sja1105_private :: struct
sja1105_ptp_data to struct sja1105_private :: struct sja1105_tagger_data.
The latter is a data structure for which hacks have already been put
into place to be able to create persistent storage per switch that is
accessible from the tagging protocol driver (see sja1105_setup_ports).

With the skb queue directly accessible from the tagging protocol driver,
we can now move sja1110_process_meta_tstamp into the tagging driver
itself, and avoid exporting a symbol.

Fixes: 566b18c8 ("net: dsa: sja1105: implement TX timestamping for SJA1110")
Link: https://lore.kernel.org/netdev/20210908220834.d7gmtnwrorhharna@skbuf/Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>