sfc: fix timestamp reconstruction at 16-bit rollover points

				    u32 nic_major, u32 nic_minor,

				    s32 correction)

{

	u32 sync_timestamp;

	ktime_t kt = { 0 };

	s16 delta;

	if (!(nic_major & 0x80000000)) {

		WARN_ON_ONCE(nic_major >> 16);

		/* Use the top bits from the latest sync event. */

		nic_major &= 0xffff;

		nic_major |= (last_sync_timestamp_major(efx) & 0xffff0000);

		/* Medford provides 48 bits of timestamp, so we must get the top

		 * 16 bits from the timesync event state.

		 *

		 * We only have the lower 16 bits of the time now, but we do

		 * have a full resolution timestamp at some point in past. As

		 * long as the difference between the (real) now and the sync

		 * is less than 2^15, then we can reconstruct the difference

		 * between those two numbers using only the lower 16 bits of

		 * each.

		 *

		 * Put another way

		 *

		 * a - b = ((a mod k) - b) mod k

		 *

		 * when -k/2 < (a-b) < k/2. In our case k is 2^16. We know

		 * (a mod k) and b, so can calculate the delta, a - b.

		 *

		 */

		sync_timestamp = last_sync_timestamp_major(efx);

		/* Because delta is s16 this does an implicit mask down to

		 * 16 bits which is what we need, assuming

		 * MEDFORD_TX_SECS_EVENT_BITS is 16. delta is signed so that

		 * we can deal with the (unlikely) case of sync timestamps

		 * arriving from the future.

		 */

		delta = nic_major - sync_timestamp;

		/* Recover the fully specified time now, by applying the offset

		 * to the (fully specified) sync time.

		 */

		nic_major = sync_timestamp + delta;

		kt = ptp->nic_to_kernel_time(nic_major, nic_minor,

					     correction);