net: phy: sfp: handle cases where neither BR, min nor BR, max is given

When computing the bitrate using values read from an SFP module EEPROM,
we use the nominal BR plus BR,min and BR,max to determine the
boundaries. But in some cases BR,min and BR,max aren't provided, which
led the SFP code to end up having the nominal value for both the minimum
and maximum bitrate values. When using a passive cable, the nominal
value should be used as the maximum one, and there is no minimum one
so we should use 0.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/sfp-bus.c

@@ -132,6 +132,13 @@ void sfp_parse_support(struct sfp_bus *bus, const struct sfp_eeprom_id *id,
 			br_max = br_nom + br_nom * id->ext.br_min / 100;
 			br_min = br_nom - br_nom * id->ext.br_min / 100;
 		}
+
+		/* When using passive cables, in case neither BR,min nor BR,max
+		 * are specified, set br_min to 0 as the nominal value is then
+		 * used as the maximum.
+		 */
+		if (br_min == br_max && id->base.sfp_ct_passive)
+			br_min = 0;
 	}
 
 	/* Set ethtool support from the compliance fields. */