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diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
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+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2005 The University of Waikato, Hamilton, New Zealand.
+ * Copyright (c) 2005 Ian McDonald <ian.mcdonald@jandi.co.nz>
+ * Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@conectiva.com.br>
+ * Copyright (c) 2003 Nils-Erik Mattsson, Joacim Haggmark, Magnus Erixzon
+ */
+
+#include <linux/module.h>
+#include "../../dccp.h"
+#include "tfrc.h"
+
+#define TFRC_CALC_X_ARRSIZE 500
+#define TFRC_CALC_X_SPLIT 50000 /* 0.05 * 1000000, details below */
+#define TFRC_SMALLEST_P (TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE)
+
+/*
+ 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 possible with this lookup table:
+
+ TFRC_SMALLEST_P = TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE
+
+ 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_SMALLEST_P) = 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 },
+ { 66664, 14180 },
+ { 78298, 16388 },
+ { 89021, 18339 },
+ { 99147, 20108 },
+ { 108858, 21738 },
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+};
+
+/* return largest index i such that fval <= lookup[i][small] */
+static inline u32 tfrc_binsearch(u32 fval, u8 small)
+{
+ u32 try, low = 0, high = TFRC_CALC_X_ARRSIZE - 1;
+
+ while (low < high) {
+ try = (low + high) / 2;
+ if (fval <= tfrc_calc_x_lookup[try][small])
+ high = try;
+ else
+ low = try + 1;
+ }
+ return high;
+}
+
+/**
+ * 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).
+ */
+u32 tfrc_calc_x(u16 s, u32 R, u32 p)
+{
+ u16 index;
+ u32 f;
+ u64 result;
+
+ /* check against invalid parameters and divide-by-zero */
+ BUG_ON(p > 1000000); /* p must not exceed 100% */
+ BUG_ON(p == 0); /* f(0) = 0, divide by zero */
+ if (R == 0) { /* possible divide by zero */
+ DCCP_CRIT("WARNING: RTT is 0, returning maximum X_calc.");
+ return ~0U;
+ }
+
+ if (p <= TFRC_CALC_X_SPLIT) { /* 0.0000 < p <= 0.05 */
+ if (p < TFRC_SMALLEST_P) { /* 0.0000 < p < 0.0001 */
+ DCCP_WARN("Value of p (%d) below resolution. "
+ "Substituting %d\n", p, TFRC_SMALLEST_P);
+ index = 0;
+ } else /* 0.0001 <= p <= 0.05 */
+ index = p/TFRC_SMALLEST_P - 1;
+
+ f = tfrc_calc_x_lookup[index][1];
+
+ } else { /* 0.05 < p <= 1.00 */
+ index = p/(1000000/TFRC_CALC_X_ARRSIZE) - 1;
+
+ f = tfrc_calc_x_lookup[index][0];
+ }
+
+ /*
+ * Compute 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. To avoid overflow, the result is computed in two stages.
+ * This works under almost all reasonable operational conditions, for a
+ * wide range of parameters. Yet, should some strange combination of
+ * parameters result in overflow, the use of scaled_div32 will catch
+ * this and return UINT_MAX - which is a logically adequate consequence.
+ */
+ result = scaled_div(s, R);
+ return scaled_div32(result, f);
+}
+
+/**
+ * tfrc_calc_x_reverse_lookup - try to find p given f(p)
+ * @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)
+{
+ int index;
+
+ if (fvalue == 0) /* f(p) = 0 whenever p = 0 */
+ return 0;
+
+ /* Error cases. */
+ if (fvalue < tfrc_calc_x_lookup[0][1]) {
+ DCCP_WARN("fvalue %u smaller than resolution\n", fvalue);
+ return TFRC_SMALLEST_P;
+ }
+ if (fvalue > tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][0]) {
+ DCCP_WARN("fvalue %u exceeds bounds!\n", fvalue);
+ return 1000000;
+ }
+
+ if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1]) {
+ index = tfrc_binsearch(fvalue, 1);
+ return (index + 1) * TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE;
+ }
+
+ /* else ... it must be in the coarse-grained column */
+ index = tfrc_binsearch(fvalue, 0);
+ return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE;
+}
+
+/**
+ * tfrc_invert_loss_event_rate - Compute p so that 10^6 corresponds to 100%
+ * @loss_event_rate: loss event rate to invert
+ * When @loss_event_rate is large, there is a chance that p is truncated to 0.
+ * To avoid re-entering slow-start in that case, we set p = TFRC_SMALLEST_P > 0.
+ */
+u32 tfrc_invert_loss_event_rate(u32 loss_event_rate)
+{
+ if (loss_event_rate == UINT_MAX) /* see RFC 4342, 8.5 */
+ return 0;
+ if (unlikely(loss_event_rate == 0)) /* map 1/0 into 100% */
+ return 1000000;
+ return max_t(u32, scaled_div(1, loss_event_rate), TFRC_SMALLEST_P);
+}
generated by cgit v1.2.3 (git 2.39.1) at 2024-12-24 14:08:20 +0000