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Date:	Fri, 17 Jan 2014 01:28:38 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	netdev@...r.kernel.org
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	Jakub Zawadzki <darkjames-ws@...kjames.pl>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH net-next v2 2/3] net: add trim helper and convert users

From: Daniel Borkmann <dborkman@...hat.com>

As David Laight suggests, we shouldn't necessarily call this
reciprocal_divide() when users didn't requested a reciprocal_value().
Lets make the name short and nifty, put this where we have other
generic helpers of similar kind and convert users.

Joint work with Hannes Frederic Sowa.

Cc: Jakub Zawadzki <darkjames-ws@...kjames.pl>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
---
 include/linux/kernel.h | 19 +++++++++++++++++++
 include/net/codel.h    |  4 +---
 net/packet/af_packet.c |  3 +--
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index ecb8754..62ecbfa 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -193,6 +193,25 @@ extern int _cond_resched(void);
 		(__x < 0) ? -__x : __x;		\
 	})
 
+/**
+ * trim - perform a reciprocal multiplication in order to "clamp" a
+ *        value into range [0, ep_ro), where the upper interval
+ *        endpoint is right-open. This is useful, e.g. for accessing
+ *        a index of an array containing ep_ro elements, for example.
+ *        Think of it as sort of modulus, only that the result isn't
+ *        that of modulo. ;)
+ *        More: http://homepage.cs.uiowa.edu/~jones/bcd/divide.html
+ *
+ * @val: value to 'trim'
+ * @ep_ro: right open interval endpoint
+ *
+ * Returns a result based on val in interval [0, ep_ro).
+ */
+static inline u32 trim(u32 val, u32 ep_ro)
+{
+	return (u32)(((u64) val * ep_ro) >> 32);
+}
+
 #if defined(CONFIG_MMU) && \
 	(defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP))
 void might_fault(void);
diff --git a/include/net/codel.h b/include/net/codel.h
index 3b04ff5..d302b4c 100644
--- a/include/net/codel.h
+++ b/include/net/codel.h
@@ -46,7 +46,6 @@
 #include <linux/skbuff.h>
 #include <net/pkt_sched.h>
 #include <net/inet_ecn.h>
-#include <linux/reciprocal_div.h>
 
 /* Controlling Queue Delay (CoDel) algorithm
  * =========================================
@@ -211,10 +210,9 @@ static codel_time_t codel_control_law(codel_time_t t,
 				      codel_time_t interval,
 				      u32 rec_inv_sqrt)
 {
-	return t + reciprocal_divide(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT);
+	return t + trim(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT);
 }
 
-
 static bool codel_should_drop(const struct sk_buff *skb,
 			      struct Qdisc *sch,
 			      struct codel_vars *vars,
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 91daa01..d327774 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -88,7 +88,6 @@
 #include <linux/virtio_net.h>
 #include <linux/errqueue.h>
 #include <linux/net_tstamp.h>
-#include <linux/reciprocal_div.h>
 #ifdef CONFIG_INET
 #include <net/inet_common.h>
 #endif
@@ -1220,7 +1219,7 @@ static unsigned int fanout_demux_hash(struct packet_fanout *f,
 				      struct sk_buff *skb,
 				      unsigned int num)
 {
-	return reciprocal_divide(skb->rxhash, num);
+	return trim(skb->rxhash, num);
 }
 
 static unsigned int fanout_demux_lb(struct packet_fanout *f,
-- 
1.8.4.2

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