[DCCP]: Simplify TFRC calculation

In migrating towards using the newer functions scaled_div/scaled_div32
for TFRC computations mapped from floating-point onto integer arithmetic,
this completes the last stage of modifications.

In particular, the overflow case for computing X_calc is circumvented by
 * breaking the computation into two stages
 * the first stage, res = (s*1E6)/R, cannot overflow due to use of u64
 * in the second stage, res = (res*1E6)/f, overflow on u32 is avoided due
   to (i) returning UINT_MAX in this case (which is logically appropriate)
   and (ii) issuing a warning message into the system log (since very likely
   there is a problem somewhere else with the parameters)

Lastly, all such scaling operations are now exported into tfrc.h, since
actually this form of scaled computation is specific to TFRC and not to CCID3.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/net/dccp/ccids/lib/tfrc.h b/net/dccp/ccids/lib/tfrc.h
index 45f30f59ea2a..faf5f7e219e3 100644
--- a/net/dccp/ccids/lib/tfrc.h
+++ b/net/dccp/ccids/lib/tfrc.h
@@ -13,8 +13,29 @@
  *  the Free Software Foundation; either version 2 of the License, or
  *  (at your option) any later version.
  */
-
 #include <linux/types.h>
+#include <asm/div64.h>
+
+/* integer-arithmetic divisions of type (a * 1000000)/b */
+static inline u64 scaled_div(u64 a, u32 b)
+{
+	BUG_ON(b==0);
+	a *= 1000000;
+	do_div(a, b);
+	return a;
+}
+
+static inline u32 scaled_div32(u64 a, u32 b)
+{
+	u64 result = scaled_div(a, b);
+
+	if (result > UINT_MAX) {
+		DCCP_CRIT("Overflow: a(%llu)/b(%u) > ~0U",
+			  (unsigned long long)a, b);
+		return UINT_MAX;
+	}
+	return result;
+}
 
 extern u32 tfrc_calc_x(u16 s, u32 R, u32 p);
 extern u32 tfrc_calc_x_reverse_lookup(u32 fvalue);