mac80211: Extend filter flag documentation about unsupported flags

This extends the filter flags documentation to make it clear
what clearing a flag really means.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -793,8 +793,18 @@ static inline void SET_IEEE80211_PERM_ADDR(struct ieee80211_hw *hw, u8 *addr)
  * parameter to see whether multicast frames should be accepted
  * or dropped.
  *
- * All unsupported flags in @total_flags must be cleared, i.e. you
- * should clear all bits except those you honoured.
+ * All unsupported flags in @total_flags must be cleared.
+ * Hardware does not support a flag if it is incapable of _passing_
+ * the frame to the stack. Otherwise the driver must ignore
+ * the flag, but not clear it.
+ * You must _only_ clear the flag (announce no support for the
+ * flag to mac80211) if you are not able to pass the packet type
+ * to the stack (so the hardware always filters it).
+ * So for example, you should clear @FIF_CONTROL, if your hardware
+ * always filters control frames. If your hardware always passes
+ * control frames to the kernel and is incapable of filtering them,
+ * you do _not_ clear the @FIF_CONTROL flag.
+ * This rule applies to all other FIF flags as well.
  */
 
 /**