1 files changed, 88 insertions, 56 deletions

diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index 2601012383fb..57665e979308 100644
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -18,10 +18,76 @@
 #include "tfrc.h"
 
 #define TFRC_CALC_X_ARRSIZE 500
+#define TFRC_CALC_X_SPLIT   50000       /* 0.05 * 1000000, details below */
 
-#define TFRC_CALC_X_SPLIT 50000
-/* equivalent to 0.05 */
-
+/*
+  TFRC TCP Reno Throughput Equation Lookup Table for f(p)
+
+  The following two-column lookup table implements a part of the TCP throughput
+  equation from [RFC 3448, sec. 3.1]:
+
+                                     s
+  X_calc  =  --------------------------------------------------------------
+             R * sqrt(2*b*p/3) + (3 * t_RTO * sqrt(3*b*p/8) * (p + 32*p^3))
+
+  Where:
+        X      is the transmit rate in bytes/second
+        s      is the packet size in bytes
+        R      is the round trip time in seconds
+        p      is the loss event rate, between 0 and 1.0, of the number of loss
+                      events as a fraction of the number of packets transmitted
+        t_RTO  is the TCP retransmission timeout value in seconds
+        b      is the number of packets acknowledged by a single TCP ACK
+
+  We can assume that b = 1 and t_RTO is 4 * R. The equation now becomes:
+
+                                     s
+  X_calc  =  -------------------------------------------------------
+             R * sqrt(p*2/3) + (12 * R * sqrt(p*3/8) * (p + 32*p^3))
+
+  which we can break down into:
+
+                      s
+        X_calc  =  ---------
+                    R * f(p)
+
+  where f(p) is given for 0 < p <= 1 by:
+
+        f(p)  =  sqrt(2*p/3) + 12 * sqrt(3*p/8) *  (p + 32*p^3)
+
+  Since this is kernel code, floating-point arithmetic is avoided in favour of
+  integer arithmetic. This means that nearly all fractional parameters are
+  scaled by 1000000:
+    * the parameters p and R
+    * the return result f(p)
+  The lookup table therefore actually tabulates the following function g(q):
+
+        g(q)  =  1000000 * f(q/1000000)
+
+  Hence, when p <= 1, q must be less than or equal to 1000000. To achieve finer
+  granularity for the practically more relevant case of small values of p (up to
+  5%), the second column is used; the first one ranges up to 100%.  This split
+  corresponds to the value of q = TFRC_CALC_X_SPLIT. At the same time this also
+  determines the smallest resolution.
+
+  The entire table is generated by:
+    for(i=0; i < TFRC_CALC_X_ARRSIZE; i++) {
+        lookup[i][0]  =  g((i+1) * 1000000/TFRC_CALC_X_ARRSIZE);
+        lookup[i][1]  =  g((i+1) * TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE);
+    }
+
+  With the given configuration, we have, with M = TFRC_CALC_X_ARRSIZE-1,
+    lookup[0][0]  =  g(1000000/(M+1))            =  1000000 * f(0.2%)
+    lookup[M][0]  =  g(1000000)                  =  1000000 * f(100%)
+    lookup[0][1]  =  g(TFRC_CALC_X_SPLIT/(M+1))  =  1000000 * f(0.01%)
+    lookup[M][1]  =  g(TFRC_CALC_X_SPLIT)        =  1000000 * f(5%)
+
+  In summary, the two columns represent f(p) for the following ranges:
+    * The first column is for   0.002  <= p <= 1.0
+    * The second column is for  0.0001 <= p <= 0.05
+  Where the columns overlap, the second (finer-grained) is given preference,
+  i.e. the first column is used only for p >= 0.05.
+ */
 static const u32 tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE][2] = {
         {     37172,   8172 },
         {     53499,  11567 },
@@ -525,62 +591,26 @@ static const u32 tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE][2] = {
         { 243315981, 271305 }
 };
 
-/* Calculate the send rate as per section 3.1 of RFC3448
- 
-Returns send rate in bytes per second
-
-Integer maths and lookups are used as not allowed floating point in kernel
-
-The function for Xcalc as per section 3.1 of RFC3448 is:
-
-X =                            s
-     -------------------------------------------------------------
-     R*sqrt(2*b*p/3) + (t_RTO * (3*sqrt(3*b*p/8) * p * (1+32*p^2)))
-
-where 
-X is the trasmit rate in bytes/second
-s is the packet size in bytes
-R is the round trip time in seconds
-p is the loss event rate, between 0 and 1.0, of the number of loss events 
-  as a fraction of the number of packets transmitted
-t_RTO is the TCP retransmission timeout value in seconds
-b is the number of packets acknowledged by a single TCP acknowledgement
-
-we can assume that b = 1 and t_RTO is 4 * R. With this the equation becomes:
-
-X =                            s
-     -----------------------------------------------------------------------
-     R * sqrt(2 * p / 3) + (12 * R * (sqrt(3 * p / 8) * p * (1 + 32 * p^2)))
-
-
-which we can break down into:
-
-X =     s
-     --------
-     R * f(p)
-
-where f(p) = sqrt(2 * p / 3) + (12 * sqrt(3 * p / 8) * p * (1 + 32 * p * p))
-
-Function parameters:
-s - bytes
-R - RTT in usecs
-p - loss rate (decimal fraction multiplied by 1,000,000)
-
-Returns Xcalc in bytes per second
-
-DON'T alter this code unless you run test cases against it as the code
-has been manipulated to stop underflow/overlow.
-
-*/
+/**
+ * tfrc_calc_x - Calculate the send rate as per section 3.1 of RFC3448
+ *
+ *  @s: packet size          in bytes
+ *  @R: RTT                  scaled by 1000000   (i.e., microseconds)
+ *  @p: loss ratio estimate  scaled by 1000000
+ *  Returns X_calc           in bytes per second (not scaled).
+ *
+ * Note: DO NOT alter this code unless you run test cases against it,
+ *       as the code has been optimized to stop underflow/overflow.
+ */
 u32 tfrc_calc_x(u16 s, u32 R, u32 p)
 {
         int index;
         u32 f;
         u64 tmp1, tmp2;
 
-        if (p < TFRC_CALC_X_SPLIT)
+        if (p < TFRC_CALC_X_SPLIT)                    /* 0      <= p <  0.05  */
                 index = (p / (TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE)) - 1;
-        else
+        else                                          /* 0.05   <= p <= 1.00  */
                 index = (p / (1000000 / TFRC_CALC_X_ARRSIZE)) - 1;
 
         if (index < 0)
@@ -599,11 +629,13 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
         else
                 f = tfrc_calc_x_lookup[index][1];
 
+        /* The following computes X = s/(R*f(p)) in bytes per second. Since f(p)
+         * and R are both scaled by 1000000, we need to multiply by 1000000^2.
+         * ==> DO NOT alter this unless you test against overflow on 32 bit   */
         tmp1 = ((u64)s * 100000000);
         tmp2 = ((u64)R * (u64)f);
         do_div(tmp2, 10000);
         do_div(tmp1, tmp2); 
-        /* Don't alter above math unless you test due to overflow on 32 bit */
 
         return (u32)tmp1; 
 }
@@ -611,10 +643,10 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
 EXPORT_SYMBOL_GPL(tfrc_calc_x);
 
 /*
- * args: fvalue - function value to match
- * returns: p closest to that value
+ *  tfrc_calc_x_reverse_lookup  -  try to find p given f(p)
  *
- * both fvalue and p are multiplied by 1,000,000 to use ints
+ *  @fvalue: function value to match, scaled by 1000000
+ *  Returns closest match for p, also scaled by 1000000
  */
 u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
 {