net: add linkmode helper for setting flow control advertisement

Add a linkmode helper to set the flow control advertisement in an
ethtool linkmode mask according to the tx/rx capabilities. This
implementation is moved from phylib, and documented with an
analysis of its shortcomings.

Signed-off-by: Russell King <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/phy/linkmode.c

···
 	}
 }
 EXPORT_SYMBOL_GPL(linkmode_resolve_pause);
+
+/**
+ * linkmode_set_pause - set the pause mode advertisement
+ * @advertisement: advertisement in ethtool format
+ * @tx: boolean from ethtool struct ethtool_pauseparam tx_pause member
+ * @rx: boolean from ethtool struct ethtool_pauseparam rx_pause member
+ *
+ * Configure the advertised Pause and Asym_Pause bits according to the
+ * capabilities of provided in @tx and @rx.
+ *
+ * We convert as follows:
+ *  tx rx  Pause AsymDir
+ *  0  0   0     0
+ *  0  1   1     1
+ *  1  0   0     1
+ *  1  1   1     0
+ *
+ * Note: this translation from ethtool tx/rx notation to the advertisement
+ * is actually very problematical. Here are some examples:
+ *
+ * For tx=0 rx=1, meaning transmit is unsupported, receive is supported:
+ *
+ *  Local device  Link partner
+ *  Pause AsymDir Pause AsymDir Result
+ *    1     1       1     0     TX + RX - but we have no TX support.
+ *    1     1       0     1	Only this gives RX only
+ *
+ * For tx=1 rx=1, meaning we have the capability to transmit and receive
+ * pause frames:
+ *
+ *  Local device  Link partner
+ *  Pause AsymDir Pause AsymDir Result
+ *    1     0       0     1     Disabled - but since we do support tx and rx,
+ *				this should resolve to RX only.
+ *
+ * Hence, asking for:
+ *  rx=1 tx=0 gives Pause+AsymDir advertisement, but we may end up
+ *            resolving to tx+rx pause or only rx pause depending on
+ *            the partners advertisement.
+ *  rx=0 tx=1 gives AsymDir only, which will only give tx pause if
+ *            the partners advertisement allows it.
+ *  rx=1 tx=1 gives Pause only, which will only allow tx+rx pause
+ *            if the other end also advertises Pause.
+ */
+void linkmode_set_pause(unsigned long *advertisement, bool tx, bool rx)
+{
+	linkmode_mod_bit(ETHTOOL_LINK_MODE_Pause_BIT, advertisement, rx);
+	linkmode_mod_bit(ETHTOOL_LINK_MODE_Asym_Pause_BIT, advertisement,
+			 rx ^ tx);
+}
+EXPORT_SYMBOL_GPL(linkmode_set_pause);

drivers/net/phy/phy_device.c

···
 	__ETHTOOL_DECLARE_LINK_MODE_MASK(oldadv);
 
 	linkmode_copy(oldadv, phydev->advertising);
-
-	linkmode_clear_bit(ETHTOOL_LINK_MODE_Pause_BIT,
-			   phydev->advertising);
-	linkmode_clear_bit(ETHTOOL_LINK_MODE_Asym_Pause_BIT,
-			   phydev->advertising);
-
-	if (rx) {
-		linkmode_set_bit(ETHTOOL_LINK_MODE_Pause_BIT,
-				 phydev->advertising);
-		linkmode_set_bit(ETHTOOL_LINK_MODE_Asym_Pause_BIT,
-				 phydev->advertising);
-	}
-
-	if (tx)
-		linkmode_change_bit(ETHTOOL_LINK_MODE_Asym_Pause_BIT,
-				    phydev->advertising);
+	linkmode_set_pause(phydev->advertising, tx, rx);
 
 	if (!linkmode_equal(oldadv, phydev->advertising) &&
 	    phydev->autoneg)

include/linux/linkmode.h

···
 			    const unsigned long *partner_adv,
 			    bool *tx_pause, bool *rx_pause);
 
+void linkmode_set_pause(unsigned long *advertisement, bool tx, bool rx);
+
 #endif /* __LINKMODE_H */