xref: /openbmc/linux/arch/powerpc/kernel/smp-tbsync.c (revision b2441318)
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 
20 enum {
21 	kExit=0, kSetAndTest, kTest
22 };
23 
24 static 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 
37 static volatile int		running;
38 
enter_contest(u64 mark,long add)39 static void enter_contest(u64 mark, long add)
40 {
41 	while (get_tb() < mark)
42 		tbsync->race_result = add;
43 }
44 
smp_generic_take_timebase(void)45 void 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 
start_contest(int cmd,long offset,int num)78 static 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 
smp_generic_give_timebase(void)113 void 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