The MMCSR register contains 2 fields with overlapping meaning:

- LPA (Local preemption active):
This read-only status bit indicates whether preemption is active for
this port. This bit will be set if preemption is both enabled and has
completed the verification process.
- TXSTS (Merge status):
This read-only status field provides the state of the MAC Merge sublayer
transmit status as defined in IEEE Std 802.3-2018 Clause 99.
00 Transmit preemption is inactive
01 Transmit preemption is active
10 Reserved
11 Reserved

However none of these 2 fields offer reliable reporting to software.

When connecting ENETC to a link partner which is not capable of Frame
Preemption, the expectation is that ENETC's verification should fail
(VSTS=4) and its MM TX direction should be inactive (LPA=0, TXSTS=00)
even though the MM TX is enabled (ME=1). But surprise, the LPA bit of
MMCSR stays set even if VSTS=4 and ME=1.

OTOH, the TXSTS field has the opposite problem. I cannot get its value
to change from 0, even when connecting to a link partner capable of
frame preemption, which does respond to its verification frames (ME=1
and VSTS=3, "SUCCEEDED").

The only option with such buggy hardware seems to be to reimplement the
formula for calculating tx-active in software, which is for tx-enabled
to be true, and for the verify-status to be either SUCCEEDED, or
DISABLED.

Without reliable tx-active reporting, we have no good indication when
to commit the preemptible traffic classes to hardware, which makes it
possible (but not desirable) to send preemptible traffic to a link
partner incapable of receiving it. However, currently we do not have the
logic to wait for TX to be active yet, so the impact is limited.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>