net: sh_eth: fix incorrect RX length error if R8A7740

This patch fixes an issue that the driver increments the "RX length error"
on every buffer in sh_eth_rx() if the R8A7740.
This patch also adds a description about the Receive Frame Status bits.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/sh_eth.c

@@ -1401,16 +1401,23 @@ static int sh_eth_rx(struct net_device *ndev, u32 intr_status)
 		desc_status = edmac_to_cpu(mdp, rxdesc->status);
 		pkt_len = rxdesc->frame_length;
 
-#if defined(CONFIG_ARCH_R8A7740)
-		desc_status >>= 16;
-#endif
-
 		if (--boguscnt < 0)
 			break;
 
 		if (!(desc_status & RDFEND))
 			ndev->stats.rx_length_errors++;
 
+#if defined(CONFIG_ARCH_R8A7740)
+		/*
+		 * In case of almost all GETHER/ETHERs, the Receive Frame State
+		 * (RFS) bits in the Receive Descriptor 0 are from bit 9 to
+		 * bit 0. However, in case of the R8A7740's GETHER, the RFS
+		 * bits are from bit 25 to bit 16. So, the driver needs right
+		 * shifting by 16.
+		 */
+		desc_status >>= 16;
+#endif
+
 		if (desc_status & (RD_RFS1 | RD_RFS2 | RD_RFS3 | RD_RFS4 |
 				   RD_RFS5 | RD_RFS6 | RD_RFS10)) {
 			ndev->stats.rx_errors++;