The reconstruction procedure for partial timestamps reads the current
PTP time and fills in the low 2 bits of the second portion, as well as
the nanoseconds portion, from the actual hardware packet timestamp.
Critically, the reconstruction procedure works because it assumes that
the current PTP time is strictly larger than the hardware timestamp was:
it detects a 2-bit wraparound of the 'seconds' portion by checking whether
the 'seconds' portion of the partial hardware timestamp is larger than
the 'seconds' portion of the current time. That can only happen if the
hardware timestamp was captured by the PHY during the last phase of a
'modulo 4 seconds' interval, and the current PTP time was read by the
driver during the initial phase of the next 'modulo 4 seconds' interval.

The partial RX timestamps are added to priv->rx_queue in
nxp_c45_rxtstamp() and they are processed potentially in parallel by the
aux worker thread in nxp_c45_do_aux_work(). This means that it is
possible for nxp_c45_do_aux_work() to process more than one RX timestamp
during the same schedule.

There is one premature optimization that will cause issues: for RX
timestamping, the driver reads the current time only once, and it uses
that to reconstruct all PTP RX timestamps in the queue. For the second
and later timestamps, this will be an issue if we are processing two RX
timestamps which are to the left and to the right, respectively, of a
4-bit wraparound of the 'seconds' portion of the PTP time, and the
current PTP time is also pre-wraparound.

 0.000000000        4.000000000        8.000000000        12.000000000
 |..................|..................|..................|............>
                 ^ ^ ^ ^                                            time
                 | | | |
                 | | | process hwts 1 and hwts 2
                 | | |
                 | | hwts 2
                 | |
                 | read current PTP time
                 |
                 hwts 1

What will happen in that case is that hwts 2 (post-wraparound) will use
a stale current PTP time that is pre-wraparound.
But nxp_c45_reconstruct_ts will not detect this condition, because it is
not coded up for it, so it will reconstruct hwts 2 with a current time
from the previous 4 second interval (i.e. 0.something instead of
4.something).

This is solvable by making sure that the full 64-bit current time is
always read after the PHY has taken the partial RX timestamp. We do this
by reading the current PTP time for every timestamp in the RX queue.

Fixes: 514def5d ("phy: nxp-c45-tja11xx: add timestamping support")
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>