ethtool: Expand documentation of ethtool_ops::{get,set}_rxfh()

Some corner-cases are not explained properly.
Signed-off-by: NBen Hutchings <ben@decadent.org.uk>

include/linux/ethtool.h

@@ -162,15 +162,16 @@ static inline u32 ethtool_rxfh_indir_default(u32 index, u32 n_rx_rings)
  *	Will not be called if @get_rxfh_indir_size returns zero.
  * @get_rxfh: Get the contents of the RX flow hash indirection table and hash
  *	key.
- *	Will not be called if @get_rxfh_indir_size and @get_rxfh_key_size
- *	returns zero.
+ *	Will only be called if one or both of @get_rxfh_indir_size and
+ *	@get_rxfh_key_size are implemented and return non-zero.
  *	Returns a negative error code or zero.
  * @set_rxfh_indir: Set the contents of the RX flow hash indirection table.
  *	Will not be called if @get_rxfh_indir_size returns zero.
- * @set_rxfh: Set the contents of the RX flow hash indirection table and
- *	hash key.
- *	Will not be called if @get_rxfh_indir_size and @get_rxfh_key_size
- *	returns zero.
+ * @set_rxfh: Set the contents of the RX flow hash indirection table and/or
+ *	hash key.  Either or both arguments may be %NULL if that attribute
+ *	is not to be changed.
+ *	Will only be called if one or both of @get_rxfh_indir_size and
+ *	@get_rxfh_key_size are implemented and return non-zero.
  *	Returns a negative error code or zero.
  * @get_channels: Get number of channels.
  * @set_channels: Set number of channels.  Returns a negative error code or
@@ -244,8 +245,8 @@ struct ethtool_ops {
 	int	(*reset)(struct net_device *, u32 *);
 	u32	(*get_rxfh_key_size)(struct net_device *);
 	u32	(*get_rxfh_indir_size)(struct net_device *);
-	int	(*get_rxfh)(struct net_device *, u32 *, u8 *);
-	int	(*set_rxfh)(struct net_device *, u32 *, u8 *);
+	int	(*get_rxfh)(struct net_device *, u32 *indir, u8 *key);
+	int	(*set_rxfh)(struct net_device *, u32 *indir, u8 *key);
 	int	(*get_rxfh_indir)(struct net_device *, u32 *);
 	int	(*set_rxfh_indir)(struct net_device *, const u32 *);
 	void	(*get_channels)(struct net_device *, struct ethtool_channels *);