tcp: Replace constants with #define macros

to record the state of SACK/FACK and DSACK for better readability and maintenance.

Signed-off-by: Vijay Subramanian <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/tcp.h

···
 	u32	end_seq;
 };
 
+/*These are used to set the sack_ok field in struct tcp_options_received */
+#define TCP_SACK_SEEN     (1 << 0)   /*1 = peer is SACK capable, */
+#define TCP_FACK_ENABLED  (1 << 1)   /*1 = FACK is enabled locally*/
+#define TCP_DSACK_SEEN    (1 << 2)   /*1 = DSACK was received from peer*/
+
 struct tcp_options_received {
 /*	PAWS/RTTM data	*/
 	long	ts_recent_stamp;/* Time we stored ts_recent (for aging) */

include/net/tcp.h

···
 
 static inline int tcp_is_fack(const struct tcp_sock *tp)
 {
-	return tp->rx_opt.sack_ok & 2;
+	return tp->rx_opt.sack_ok & TCP_FACK_ENABLED;
 }
 
 static inline void tcp_enable_fack(struct tcp_sock *tp)
 {
-	tp->rx_opt.sack_ok |= 2;
+	tp->rx_opt.sack_ok |= TCP_FACK_ENABLED;
 }
 
 static inline unsigned int tcp_left_out(const struct tcp_sock *tp)

net/ipv4/syncookies.c

···
 	if (!sysctl_tcp_timestamps)
 		return false;
 
-	tcp_opt->sack_ok = (options >> 4) & 0x1;
+	tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
 	*ecn_ok = (options >> 5) & 1;
 	if (*ecn_ok && !sysctl_tcp_ecn)
 		return false;

net/ipv4/tcp_input.c

···
 	/* RFC3517 uses different metric in lost marker => reset on change */
 	if (tcp_is_fack(tp))
 		tp->lost_skb_hint = NULL;
-	tp->rx_opt.sack_ok &= ~2;
+	tp->rx_opt.sack_ok &= ~TCP_FACK_ENABLED;
 }
 
 /* Take a notice that peer is sending D-SACKs */
 static void tcp_dsack_seen(struct tcp_sock *tp)
 {
-	tp->rx_opt.sack_ok |= 4;
+	tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;
 }
 
 /* Initialize metrics on socket. */
···
 			case TCPOPT_SACK_PERM:
 				if (opsize == TCPOLEN_SACK_PERM && th->syn &&
 				    !estab && sysctl_tcp_sack) {
-					opt_rx->sack_ok = 1;
+					opt_rx->sack_ok = TCP_SACK_SEEN;
 					tcp_sack_reset(opt_rx);
 				}
 				break;