net: sparx5: Fix invalid timestamps

Bit 270-271 are occasionally unexpectedly set by the hardware. This issue
was observed with 10G SFPs causing huge time errors (> 30ms) in PTP. Only
30 bits are needed for the nanosecond part of the timestamp, clear 2 most
significant bits before extracting timestamp from the internal frame
header.

Fixes: 70dfe25 ("net: sparx5: Update extraction/injection for timestamping")
Signed-off-by: Aakash Menon <aakash.menon@protempis.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/microchip/sparx5/sparx5_packet.c

@@ -45,8 +45,12 @@ void sparx5_ifh_parse(u32 *ifh, struct frame_info *info)
 	fwd = (fwd >> 5);
 	info->src_port = FIELD_GET(GENMASK(7, 1), fwd);
 
+	/*
+	 * Bit 270-271 are occasionally unexpectedly set by the hardware,
+	 * clear bits before extracting timestamp
+	 */
 	info->timestamp =
-		((u64)xtr_hdr[2] << 24) |
+		((u64)(xtr_hdr[2] & GENMASK(5, 0)) << 24) |
 		((u64)xtr_hdr[3] << 16) |
 		((u64)xtr_hdr[4] <<  8) |
 		((u64)xtr_hdr[5] <<  0);