net: phy: phy_caps: Don't skip better duplex macth on non-exact match

When performing a non-exact phy_caps lookup, we are looking for a
supported mode that matches as closely as possible the passed speed/duplex.

Blamed patch broke that logic by returning a match too early in case
the caller asks for half-duplex, as a full-duplex linkmode may match
first, and returned as a non-exact match without even trying to mach on
half-duplex modes.

Reported-by: Jijie Shao <shaojijie@huawei.com>
Closes: https://lore.kernel.org/netdev/20250603102500.4ec743cf@fedora/T/#m22ed60ca635c67dc7d9cbb47e8995b2beb5c1576
Tested-by: Jijie Shao <shaojijie@huawei.com>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Fixes: fc81e25 ("net: phy: phy_caps: Allow looking-up link caps based on speed and duplex")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250606094321.483602-1-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/phy_caps.c

@@ -188,14 +188,17 @@ phy_caps_lookup_by_linkmode_rev(const unsigned long *linkmodes, bool fdx_only)
  * When @exact is not set, we return either an exact match, or matching capabilities
  * at lower speed, or the lowest matching speed, or NULL.
  *
+ * Non-exact matches will try to return an exact speed and duplex match, but may
+ * return matching capabilities with same speed but a different duplex.
+ *
  * Returns: a matched link_capabilities according to the above process, NULL
  *	    otherwise.
  */
 const struct link_capabilities *
 phy_caps_lookup(int speed, unsigned int duplex, const unsigned long *supported,
 		bool exact)
 {
-	const struct link_capabilities *lcap, *last = NULL;
+	const struct link_capabilities *lcap, *match = NULL, *last = NULL;
 
 	for_each_link_caps_desc_speed(lcap) {
 		if (linkmode_intersects(lcap->linkmodes, supported)) {
@@ -204,16 +207,19 @@ phy_caps_lookup(int speed, unsigned int duplex, const unsigned long *supported,
 			if (lcap->speed == speed && lcap->duplex == duplex) {
 				return lcap;
 			} else if (!exact) {
-				if (lcap->speed <= speed)
-					return lcap;
+				if (!match && lcap->speed <= speed)
+					match = lcap;
+
+				if (lcap->speed < speed)
+					break;
 			}
 		}
 	}
 
-	if (!exact)
-		return last;
+	if (!match && !exact)
+		match = last;
 
-	return NULL;
+	return match;
 }
 EXPORT_SYMBOL_GPL(phy_caps_lookup);