sch_red: generalize accurate MAX_P support to RED/GRED/CHOKE

Now RED uses a Q0.32 number to store max_p (max probability), allow
RED/GRED/CHOKE to use/report full resolution at config/dump time.

Old tc binaries are non aware of new attributes, and still set/get Plog.

New tc binary set/get both Plog and max_p for backward compatibility,
they display "probability value" if they get max_p from new kernels.

# tc -d  qdisc show dev ...
...
qdisc red 10: parent 1:1 limit 360Kb min 30Kb max 90Kb ecn ewma 5
probability 0.09 Scell_log 15

Make sure we avoid potential divides by 0 in reciprocal_value(), if
(max_th - min_th) is big.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 11 insertions, 5 deletions

diff --git a/include/net/red.h b/include/net/red.h
index 24606b22d01e..ef715a16cce4 100644
--- a/include/net/red.h
+++ b/include/net/red.h
@@ -155,9 +155,10 @@ static inline u32 red_maxp(u8 Plog)
 
 static inline void red_set_parms(struct red_parms *p,
                                  u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
-                                 u8 Scell_log, u8 *stab)
+                                 u8 Scell_log, u8 *stab, u32 max_P)
 {
         int delta = qth_max - qth_min;
+        u32 max_p_delta;
 
         /* Reset average queue length, the value is strictly bound
          * to the parameters below, reseting hurts a bit but leaving
@@ -173,10 +174,14 @@ static inline void red_set_parms(struct red_parms *p,
         if (delta < 0)
                 delta = 1;
         p->qth_delta    = delta;
-        p->max_P        = red_maxp(Plog);
-        p->max_P        *= delta; /* max_P = (qth_max-qth_min)/2^Plog */
-
-        p->max_P_reciprocal  = reciprocal_value(p->max_P / delta);
+        if (!max_P) {
+                max_P = red_maxp(Plog);
+                max_P *= delta; /* max_P = (qth_max - qth_min)/2^Plog */
+        }
+        p->max_P = max_P;
+        max_p_delta = max_P / delta;
+        max_p_delta = max(max_p_delta, 1U);
+        p->max_P_reciprocal  = reciprocal_value(max_p_delta);
 
         /* RED Adaptative target :
          * [min_th + 0.4*(min_th - max_th),
@@ -380,6 +385,7 @@ static inline void red_adaptative_algo(struct red_parms *p)
                 p->max_P = (p->max_P/10)*9; /* maxp = maxp * Beta */
 
         max_p_delta = DIV_ROUND_CLOSEST(p->max_P, p->qth_delta);
+        max_p_delta = max(max_p_delta, 1U);
         p->max_P_reciprocal = reciprocal_value(max_p_delta);
 }
 #endif