At present, there is a problem when user space bombards a port with PTP
event frames which have TX timestamping requests (or when a tc-taprio
offload is installed on a port, which delays the TX timestamps by a
significant amount of time). The driver will happily roll over the 2-bit
timestamp ID and this will cause incorrect matches between an skb and
the TX timestamp collected from the FIFO.

The Ocelot switches have a 6-bit PTP timestamp identifier, and the value
63 is reserved, so that leaves identifiers 0-62 to be used.

The timestamp identifiers are selected by the REW_OP packet field, and
are actually shared between CPU-injected frames and frames which match a
VCAP IS2 rule that modifies the REW_OP. The hardware supports
partitioning between the two uses of the REW_OP field through the
PTP_ID_LOW and PTP_ID_HIGH registers, and by default reserves the PTP
IDs 0-3 for CPU-injected traffic and the rest for VCAP IS2.

The driver does not use VCAP IS2 to set REW_OP for 2-step timestamping,
and it also writes 0xffffffff to both PTP_ID_HIGH and PTP_ID_LOW in
ocelot_init_timestamp() which makes all timestamp identifiers available
to CPU injection.

Therefore, we can make use of all 63 timestamp identifiers, which should
allow more timestampable packets to be in flight on each port. This is
only part of the solution, more issues will be addressed in future changes.

Fixes: 4e3b0468 ("net: mscc: PTP Hardware Clock (PHC) support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>