1/*
2 * Smp timebase synchronization for ppc.
3 *
4 * Copyright (C) 2003 Samuel Rydh (samuel@ibrium.se)
5 *
6 */
7
8#include <linux/kernel.h>
9#include <linux/sched.h>
10#include <linux/smp.h>
11#include <linux/unistd.h>
12#include <linux/slab.h>
13#include <linux/atomic.h>
14#include <asm/smp.h>
15#include <asm/time.h>
16
17#define NUM_ITER		300
18
19enum {
20	kExit=0, kSetAndTest, kTest
21};
22
23static struct {
24	volatile u64		tb;
25	volatile u64		mark;
26	volatile int		cmd;
27	volatile int		handshake;
28	int			filler[2];
29
30	volatile int		ack;
31	int			filler2[7];
32
33	volatile int		race_result;
34} *tbsync;
35
36static volatile int		running;
37
38static void enter_contest(u64 mark, long add)
39{
40	while (get_tb() < mark)
41		tbsync->race_result = add;
42}
43
44void smp_generic_take_timebase(void)
45{
46	int cmd;
47	u64 tb;
48	unsigned long flags;
49
50	local_irq_save(flags);
51	while (!running)
52		barrier();
53	rmb();
54
55	for (;;) {
56		tbsync->ack = 1;
57		while (!tbsync->handshake)
58			barrier();
59		rmb();
60
61		cmd = tbsync->cmd;
62		tb = tbsync->tb;
63		mb();
64		tbsync->ack = 0;
65		if (cmd == kExit)
66			break;
67
68		while (tbsync->handshake)
69			barrier();
70		if (cmd == kSetAndTest)
71			set_tb(tb >> 32, tb & 0xfffffffful);
72		enter_contest(tbsync->mark, -1);
73	}
74	local_irq_restore(flags);
75}
76
77static int start_contest(int cmd, long offset, int num)
78{
79	int i, score=0;
80	u64 tb;
81	u64 mark;
82
83	tbsync->cmd = cmd;
84
85	local_irq_disable();
86	for (i = -3; i < num; ) {
87		tb = get_tb() + 400;
88		tbsync->tb = tb + offset;
89		tbsync->mark = mark = tb + 400;
90
91		wmb();
92
93		tbsync->handshake = 1;
94		while (tbsync->ack)
95			barrier();
96
97		while (get_tb() <= tb)
98			barrier();
99		tbsync->handshake = 0;
100		enter_contest(mark, 1);
101
102		while (!tbsync->ack)
103			barrier();
104
105		if (i++ > 0)
106			score += tbsync->race_result;
107	}
108	local_irq_enable();
109	return score;
110}
111
112void smp_generic_give_timebase(void)
113{
114	int i, score, score2, old, min=0, max=5000, offset=1000;
115
116	pr_debug("Software timebase sync\n");
117
118	/* if this fails then this kernel won't work anyway... */
119	tbsync = kzalloc( sizeof(*tbsync), GFP_KERNEL );
120	mb();
121	running = 1;
122
123	while (!tbsync->ack)
124		barrier();
125
126	pr_debug("Got ack\n");
127
128	/* binary search */
129	for (old = -1; old != offset ; offset = (min+max) / 2) {
130		score = start_contest(kSetAndTest, offset, NUM_ITER);
131
132		pr_debug("score %d, offset %d\n", score, offset );
133
134		if( score > 0 )
135			max = offset;
136		else
137			min = offset;
138		old = offset;
139	}
140	score = start_contest(kSetAndTest, min, NUM_ITER);
141	score2 = start_contest(kSetAndTest, max, NUM_ITER);
142
143	pr_debug("Min %d (score %d), Max %d (score %d)\n",
144		 min, score, max, score2);
145	score = abs(score);
146	score2 = abs(score2);
147	offset = (score < score2) ? min : max;
148
149	/* guard against inaccurate mttb */
150	for (i = 0; i < 10; i++) {
151		start_contest(kSetAndTest, offset, NUM_ITER/10);
152
153		if ((score2 = start_contest(kTest, offset, NUM_ITER)) < 0)
154			score2 = -score2;
155		if (score2 <= score || score2 < 20)
156			break;
157	}
158	pr_debug("Final offset: %d (%d/%d)\n", offset, score2, NUM_ITER );
159
160	/* exiting */
161	tbsync->cmd = kExit;
162	wmb();
163	tbsync->handshake = 1;
164	while (tbsync->ack)
165		barrier();
166	tbsync->handshake = 0;
167	kfree(tbsync);
168	tbsync = NULL;
169	running = 0;
170}
171