net: ethtool: add helpers for MM fragment size translation

We deliberately make the Linux UAPI pass the minimum fragment size in
octets, even though IEEE 802.3 defines it as discrete values, and
addFragSize is just the multiplier. This is because there is nothing
impossible in operating with an in-between value for the fragment size
of non-final preempted fragments, and there may even appear hardware
which supports the in-between sizes.

For the hardware which just understands the addFragSize multiplier,
create two helpers which translate back and forth the values passed in
octets.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ethtool.h

@@ -15,6 +15,7 @@
 
 #include <linux/bitmap.h>
 #include <linux/compat.h>
+#include <linux/if_ether.h>
 #include <linux/netlink.h>
 #include <uapi/linux/ethtool.h>
 
@@ -1001,6 +1002,47 @@ void ethtool_aggregate_pause_stats(struct net_device *dev,
 void ethtool_aggregate_rmon_stats(struct net_device *dev,
 				  struct ethtool_rmon_stats *rmon_stats);
 
+/**
+ * ethtool_mm_frag_size_add_to_min - Translate (standard) additional fragment
+ *	size expressed as multiplier into (absolute) minimum fragment size
+ *	value expressed in octets
+ * @val_add: Value of addFragSize multiplier
+ */
+static inline u32 ethtool_mm_frag_size_add_to_min(u32 val_add)
+{
+	return (ETH_ZLEN + ETH_FCS_LEN) * (1 + val_add) - ETH_FCS_LEN;
+}
+
+/**
+ * ethtool_mm_frag_size_min_to_add - Translate (absolute) minimum fragment size
+ *	expressed in octets into (standard) additional fragment size expressed
+ *	as multiplier
+ * @val_min: Value of addFragSize variable in octets
+ * @val_add: Pointer where the standard addFragSize value is to be returned
+ * @extack: Netlink extended ack
+ *
+ * Translate a value in octets to one of 0, 1, 2, 3 according to the reverse
+ * application of the 802.3 formula 64 * (1 + addFragSize) - 4. To be called
+ * by drivers which do not support programming the minimum fragment size to a
+ * continuous range. Returns error on other fragment length values.
+ */
+static inline int ethtool_mm_frag_size_min_to_add(u32 val_min, u32 *val_add,
+						  struct netlink_ext_ack *extack)
+{
+	u32 add_frag_size;
+
+	for (add_frag_size = 0; add_frag_size < 4; add_frag_size++) {
+		if (ethtool_mm_frag_size_add_to_min(add_frag_size) == val_min) {
+			*val_add = add_frag_size;
+			return 0;
+		}
+	}
+
+	NL_SET_ERR_MSG_MOD(extack,
+			   "minFragSize required to be one of 60, 124, 188 or 252");
+	return -EINVAL;
+}
+
 /**
  * ethtool_sprintf - Write formatted string to ethtool string data
  * @data: Pointer to start of string to update