net: phy: nxp-c45-tja11xx: express timestamp wraparound interval in terms of TS_SEC_MASK

nxp_c45_reconstruct_ts() takes a partial hardware timestamp in @hwts,
with 2 bits of the 'seconds' portion, and a full PTP time in @ts.

It patches in the lower bits of @hwts into @ts, and to ensure that the
reconstructed timestamp is correct, it checks whether the lower 2 bits
of @hwts are not in fact higher than the lower 2 bits of @ts. This is
not logically possible because, according to the calling convention, @ts
was collected later in time than @hwts, but due to two's complement
arithmetic it can actually happen, because the current PTP time might
have wrapped around between when @hwts was collected and when @ts was,
yielding the lower 2 bits of @ts smaller than those of @hwts.

To correct for that situation which is expected to happen under normal
conditions, the driver subtracts exactly one wraparound interval from
the reconstructed timestamp, since the upper bits of that need to
correspond to what the upper bits of @hwts were, not to what the upper
bits of @ts were.

Readers might be confused because the driver denotes the amount of bits
that the partial hardware timestamp has to offer as TS_SEC_MASK
(timestamp mask for seconds). But it subtracts a seemingly unrelated
BIT(2), which is in fact more subtle: if the hardware timestamp provides
2 bits of partial 'seconds' timestamp, then the wraparound interval is
2^2 == BIT(2).

But nonetheless, it is better to express the wraparound interval in
terms of a definition we already have, so replace BIT(2) with
1 + GENMASK(1, 0) which produces the same result but is clearer.
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
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>

drivers/net/phy/nxp-c45-tja11xx.c

@@ -325,7 +325,7 @@ static void nxp_c45_reconstruct_ts(struct timespec64 *ts,
 {
 	ts->tv_nsec = hwts->nsec;
 	if ((ts->tv_sec & TS_SEC_MASK) < (hwts->sec & TS_SEC_MASK))
-		ts->tv_sec -= BIT(2);
+		ts->tv_sec -= TS_SEC_MASK + 1;
 	ts->tv_sec &= ~TS_SEC_MASK;
 	ts->tv_sec |= hwts->sec & TS_SEC_MASK;
 }