1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Smp timebase synchronization for ppc.
4 *
5 * Copyright (C) 2003 Samuel Rydh (samuel@ibrium.se)
6 *
7 */
8
9#include <linux/kernel.h>
10#include <linux/sched.h>
11#include <linux/smp.h>
12#include <linux/unistd.h>
13#include <linux/slab.h>
14#include <linux/atomic.h>
15#include <asm/smp.h>
16#include <asm/time.h>
17
18#define NUM_ITER		300
19
20enum {
21	kExit=0, kSetAndTest, kTest
22};
23
24static struct {
25	volatile u64		tb;
26	volatile u64		mark;
27	volatile int		cmd;
28	volatile int		handshake;
29	int			filler[2];
30
31	volatile int		ack;
32	int			filler2[7];
33
34	volatile int		race_result;
35} *tbsync;
36
37static volatile int		running;
38
39static void enter_contest(u64 mark, long add)
40{
41	while (get_tb() < mark)
42		tbsync->race_result = add;
43}
44
45void smp_generic_take_timebase(void)
46{
47	int cmd;
48	u64 tb;
49	unsigned long flags;
50
51	local_irq_save(flags);
52	while (!running)
53		barrier();
54	rmb();
55
56	for (;;) {
57		tbsync->ack = 1;
58		while (!tbsync->handshake)
59			barrier();
60		rmb();
61
62		cmd = tbsync->cmd;
63		tb = tbsync->tb;
64		mb();
65		tbsync->ack = 0;
66		if (cmd == kExit)
67			break;
68
69		while (tbsync->handshake)
70			barrier();
71		if (cmd == kSetAndTest)
72			set_tb(tb >> 32, tb & 0xfffffffful);
73		enter_contest(tbsync->mark, -1);
74	}
75	local_irq_restore(flags);
76}
77
78static int start_contest(int cmd, long offset, int num)
79{
80	int i, score=0;
81	u64 tb;
82	u64 mark;
83
84	tbsync->cmd = cmd;
85
86	local_irq_disable();
87	for (i = -3; i < num; ) {
88		tb = get_tb() + 400;
89		tbsync->tb = tb + offset;
90		tbsync->mark = mark = tb + 400;
91
92		wmb();
93
94		tbsync->handshake = 1;
95		while (tbsync->ack)
96			barrier();
97
98		while (get_tb() <= tb)
99			barrier();
100		tbsync->handshake = 0;
101		enter_contest(mark, 1);
102
103		while (!tbsync->ack)
104			barrier();
105
106		if (i++ > 0)
107			score += tbsync->race_result;
108	}
109	local_irq_enable();
110	return score;
111}
112
113void smp_generic_give_timebase(void)
114{
115	int i, score, score2, old, min=0, max=5000, offset=1000;
116
117	pr_debug("Software timebase sync\n");
118
119	/* if this fails then this kernel won't work anyway... */
120	tbsync = kzalloc( sizeof(*tbsync), GFP_KERNEL );
121	mb();
122	running = 1;
123
124	while (!tbsync->ack)
125		barrier();
126
127	pr_debug("Got ack\n");
128
129	/* binary search */
130	for (old = -1; old != offset ; offset = (min+max) / 2) {
131		score = start_contest(kSetAndTest, offset, NUM_ITER);
132
133		pr_debug("score %d, offset %d\n", score, offset );
134
135		if( score > 0 )
136			max = offset;
137		else
138			min = offset;
139		old = offset;
140	}
141	score = start_contest(kSetAndTest, min, NUM_ITER);
142	score2 = start_contest(kSetAndTest, max, NUM_ITER);
143
144	pr_debug("Min %d (score %d), Max %d (score %d)\n",
145		 min, score, max, score2);
146	score = abs(score);
147	score2 = abs(score2);
148	offset = (score < score2) ? min : max;
149
150	/* guard against inaccurate mttb */
151	for (i = 0; i < 10; i++) {
152		start_contest(kSetAndTest, offset, NUM_ITER/10);
153
154		if ((score2 = start_contest(kTest, offset, NUM_ITER)) < 0)
155			score2 = -score2;
156		if (score2 <= score || score2 < 20)
157			break;
158	}
159	pr_debug("Final offset: %d (%d/%d)\n", offset, score2, NUM_ITER );
160
161	/* exiting */
162	tbsync->cmd = kExit;
163	wmb();
164	tbsync->handshake = 1;
165	while (tbsync->ack)
166		barrier();
167	tbsync->handshake = 0;
168	kfree(tbsync);
169	tbsync = NULL;
170	running = 0;
171}
172