1 /*
2 * Copyright (c) 2013-2015, ARM Limited and Contributors. All rights reserved.
3 *
4 * SPDX-License-Identifier: BSD-3-Clause
5 */
6
7 #include <arch_helpers.h>
8 #include <assert.h>
9 #include <bakery_lock.h>
10 #include <cpu_data.h>
11 #include <platform.h>
12 #include <string.h>
13
14 /*
15 * Functions in this file implement Bakery Algorithm for mutual exclusion with the
16 * bakery lock data structures in coherent memory.
17 *
18 * ARM architecture offers a family of exclusive access instructions to
19 * efficiently implement mutual exclusion with hardware support. However, as
20 * well as depending on external hardware, the these instructions have defined
21 * behavior only on certain memory types (cacheable and Normal memory in
22 * particular; see ARMv8 Architecture Reference Manual section B2.10). Use cases
23 * in trusted firmware are such that mutual exclusion implementation cannot
24 * expect that accesses to the lock have the specific type required by the
25 * architecture for these primitives to function (for example, not all
26 * contenders may have address translation enabled).
27 *
28 * This implementation does not use mutual exclusion primitives. It expects
29 * memory regions where the locks reside to be fully ordered and coherent
30 * (either by disabling address translation, or by assigning proper attributes
31 * when translation is enabled).
32 *
33 * Note that the ARM architecture guarantees single-copy atomicity for aligned
34 * accesses regardless of status of address translation.
35 */
36
37 #define assert_bakery_entry_valid(entry, bakery) do { \
38 assert(bakery); \
39 assert(entry < BAKERY_LOCK_MAX_CPUS); \
40 } while (0)
41
42 /* Obtain a ticket for a given CPU */
bakery_get_ticket(bakery_lock_t * bakery,unsigned int me)43 static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me)
44 {
45 unsigned int my_ticket, their_ticket;
46 unsigned int they;
47
48 /* Prevent recursive acquisition */
49 assert(!bakery_ticket_number(bakery->lock_data[me]));
50
51 /*
52 * Flag that we're busy getting our ticket. All CPUs are iterated in the
53 * order of their ordinal position to decide the maximum ticket value
54 * observed so far. Our priority is set to be greater than the maximum
55 * observed priority
56 *
57 * Note that it's possible that more than one contender gets the same
58 * ticket value. That's OK as the lock is acquired based on the priority
59 * value, not the ticket value alone.
60 */
61 my_ticket = 0;
62 bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket);
63 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) {
64 their_ticket = bakery_ticket_number(bakery->lock_data[they]);
65 if (their_ticket > my_ticket)
66 my_ticket = their_ticket;
67 }
68
69 /*
70 * Compute ticket; then signal to other contenders waiting for us to
71 * finish calculating our ticket value that we're done
72 */
73 ++my_ticket;
74 bakery->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket);
75
76 return my_ticket;
77 }
78
79
80 /*
81 * Acquire bakery lock
82 *
83 * Contending CPUs need first obtain a non-zero ticket and then calculate
84 * priority value. A contending CPU iterate over all other CPUs in the platform,
85 * which may be contending for the same lock, in the order of their ordinal
86 * position (CPU0, CPU1 and so on). A non-contending CPU will have its ticket
87 * (and priority) value as 0. The contending CPU compares its priority with that
88 * of others'. The CPU with the highest priority (lowest numerical value)
89 * acquires the lock
90 */
bakery_lock_get(bakery_lock_t * bakery)91 void bakery_lock_get(bakery_lock_t *bakery)
92 {
93 unsigned int they, me;
94 unsigned int my_ticket, my_prio, their_ticket;
95 unsigned int their_bakery_data;
96
97 me = plat_my_core_pos();
98
99 assert_bakery_entry_valid(me, bakery);
100
101 /* Get a ticket */
102 my_ticket = bakery_get_ticket(bakery, me);
103
104 /*
105 * Now that we got our ticket, compute our priority value, then compare
106 * with that of others, and proceed to acquire the lock
107 */
108 my_prio = PRIORITY(my_ticket, me);
109 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) {
110 if (me == they)
111 continue;
112
113 /* Wait for the contender to get their ticket */
114 do {
115 their_bakery_data = bakery->lock_data[they];
116 } while (bakery_is_choosing(their_bakery_data));
117
118 /*
119 * If the other party is a contender, they'll have non-zero
120 * (valid) ticket value. If they do, compare priorities
121 */
122 their_ticket = bakery_ticket_number(their_bakery_data);
123 if (their_ticket && (PRIORITY(their_ticket, they) < my_prio)) {
124 /*
125 * They have higher priority (lower value). Wait for
126 * their ticket value to change (either release the lock
127 * to have it dropped to 0; or drop and probably content
128 * again for the same lock to have an even higher value)
129 */
130 do {
131 wfe();
132 } while (their_ticket ==
133 bakery_ticket_number(bakery->lock_data[they]));
134 }
135 }
136 /* Lock acquired */
137 }
138
139
140 /* Release the lock and signal contenders */
bakery_lock_release(bakery_lock_t * bakery)141 void bakery_lock_release(bakery_lock_t *bakery)
142 {
143 unsigned int me = plat_my_core_pos();
144
145 assert_bakery_entry_valid(me, bakery);
146 assert(bakery_ticket_number(bakery->lock_data[me]));
147
148 /*
149 * Release lock by resetting ticket. Then signal other
150 * waiting contenders
151 */
152 bakery->lock_data[me] = 0;
153 dsb();
154 sev();
155 }
156