[PATCH for-stable] sch_cake: Simplify logic in cake_select_tin()

[Toke Høiland-Jørgensen]

commit 4b454433221de445f6d3d73b0ac27b4f7da25b83 upstream.

The logic in cake_select_tin() was getting a bit hairy, and it turns out we
can simplify it quite a bit. This also allows us to get rid of one of the
two diffserv parsing functions, which has the added benefit that
already-zeroed DSCP fields won't get re-written.

Suggested-by: Kevin Darbyshire-Bryant <l...@darbyshire-bryant.me.uk>
Signed-off-by: Toke Høiland-Jørgensen <t...@redhat.com>
---

This is a backported version of the upstream patch, that should apply to 5.0
and 4.19 and make the other two sch_cake patches queued up for stable apply
as-is on top.

 net/sched/sch_cake.c | 44 ++++++++++++++++----------------------------
 1 file changed, 16 insertions(+), 28 deletions(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 793016d722ec..640f00e9f665 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1508,20 +1508,6 @@ static unsigned int cake_drop(struct Qdisc *sch, struct 
sk_buff **to_free)
        return idx + (tin << 16);
 }
 
-static void cake_wash_diffserv(struct sk_buff *skb)
-{
-       switch (skb->protocol) {
-       case htons(ETH_P_IP):
-               ipv4_change_dsfield(ip_hdr(skb), INET_ECN_MASK, 0);
-               break;
-       case htons(ETH_P_IPV6):
-               ipv6_change_dsfield(ipv6_hdr(skb), INET_ECN_MASK, 0);
-               break;
-       default:
-               break;
-       }
-}
-
 static u8 cake_handle_diffserv(struct sk_buff *skb, u16 wash)
 {
        u8 dscp;
@@ -1553,25 +1539,27 @@ static struct cake_tin_data *cake_select_tin(struct 
Qdisc *sch,
 {
        struct cake_sched_data *q = qdisc_priv(sch);
        u32 tin;
+       u8 dscp;
 
-       if (TC_H_MAJ(skb->priority) == sch->handle &&
-           TC_H_MIN(skb->priority) > 0 &&
-           TC_H_MIN(skb->priority) <= q->tin_cnt) {
+       /* Tin selection: Default to diffserv-based selection, allow overriding
+        * using firewall marks or skb->priority.
+        */
+       dscp = cake_handle_diffserv(skb,
+                                   q->rate_flags & CAKE_FLAG_WASH);
+
+       if (q->tin_mode == CAKE_DIFFSERV_BESTEFFORT)
+               tin = 0;
+
+       else if (TC_H_MAJ(skb->priority) == sch->handle &&
+                TC_H_MIN(skb->priority) > 0 &&
+                TC_H_MIN(skb->priority) <= q->tin_cnt)
                tin = q->tin_order[TC_H_MIN(skb->priority) - 1];
 
-               if (q->rate_flags & CAKE_FLAG_WASH)
-                       cake_wash_diffserv(skb);
-       } else if (q->tin_mode != CAKE_DIFFSERV_BESTEFFORT) {
-               /* extract the Diffserv Precedence field, if it exists */
-               /* and clear DSCP bits if washing */
-               tin = q->tin_index[cake_handle_diffserv(skb,
-                               q->rate_flags & CAKE_FLAG_WASH)];
+       else {
+               tin = q->tin_index[dscp];
+
                if (unlikely(tin >= q->tin_cnt))
                        tin = 0;
-       } else {
-               tin = 0;
-               if (q->rate_flags & CAKE_FLAG_WASH)
-                       cake_wash_diffserv(skb);
        }
 
        return &q->tins[tin];
-- 
2.21.0