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Achin Gupta4f6ad662013-10-25 09:08:21 +01001/*
Soby Mathewa0a897d2015-02-19 16:23:51 +00002 * Copyright (c) 2013-2015, ARM Limited and Contributors. All rights reserved.
Achin Gupta4f6ad662013-10-25 09:08:21 +01003 *
dp-armfa3cf0b2017-05-03 09:38:09 +01004 * SPDX-License-Identifier: BSD-3-Clause
Achin Gupta4f6ad662013-10-25 09:08:21 +01005 */
6
Dan Handley2bd4ef22014-04-09 13:14:54 +01007#include <arch_helpers.h>
Achin Gupta4f6ad662013-10-25 09:08:21 +01008#include <assert.h>
Achin Gupta4f6ad662013-10-25 09:08:21 +01009#include <bakery_lock.h>
Soby Mathew523d6332015-01-08 18:02:19 +000010#include <cpu_data.h>
Dan Handleyed6ff952014-05-14 17:44:19 +010011#include <platform.h>
Dan Handley2bd4ef22014-04-09 13:14:54 +010012#include <string.h>
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000013
14/*
Soby Mathew523d6332015-01-08 18:02:19 +000015 * Functions in this file implement Bakery Algorithm for mutual exclusion with the
16 * bakery lock data structures in coherent memory.
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000017 *
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 */
Achin Gupta4f6ad662013-10-25 09:08:21 +010036
Daniel Boulbyfef5d2d2018-05-04 14:04:07 +010037#define assert_bakery_entry_valid(_entry, _bakery) do { \
38 assert(_bakery); \
39 assert(_entry < BAKERY_LOCK_MAX_CPUS); \
Dan Handleya70615f2014-04-09 12:48:25 +010040} while (0)
Achin Gupta4f6ad662013-10-25 09:08:21 +010041
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000042/* Obtain a ticket for a given CPU */
Dan Handleye2712bc2014-04-10 15:37:22 +010043static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me)
Achin Gupta4f6ad662013-10-25 09:08:21 +010044{
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000045 unsigned int my_ticket, their_ticket;
46 unsigned int they;
Achin Gupta4f6ad662013-10-25 09:08:21 +010047
Soby Mathew156280c2015-02-20 16:04:17 +000048 /* Prevent recursive acquisition */
49 assert(!bakery_ticket_number(bakery->lock_data[me]));
50
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000051 /*
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;
Soby Mathewa0a897d2015-02-19 16:23:51 +000062 bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket);
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000063 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) {
Soby Mathewa0a897d2015-02-19 16:23:51 +000064 their_ticket = bakery_ticket_number(bakery->lock_data[they]);
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000065 if (their_ticket > my_ticket)
66 my_ticket = their_ticket;
67 }
Achin Gupta4f6ad662013-10-25 09:08:21 +010068
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000069 /*
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;
Soby Mathewa0a897d2015-02-19 16:23:51 +000074 bakery->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket);
Achin Gupta4f6ad662013-10-25 09:08:21 +010075
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000076 return my_ticket;
77}
Achin Gupta4f6ad662013-10-25 09:08:21 +010078
Achin Gupta4f6ad662013-10-25 09:08:21 +010079
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000080/*
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 */
Andrew Thoelke958cc022014-06-09 12:54:15 +010091void bakery_lock_get(bakery_lock_t *bakery)
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000092{
93 unsigned int they, me;
94 unsigned int my_ticket, my_prio, their_ticket;
Soby Mathewa0a897d2015-02-19 16:23:51 +000095 unsigned int their_bakery_data;
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000096
Soby Mathew3700a922015-07-13 11:21:11 +010097 me = plat_my_core_pos();
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +000098
99 assert_bakery_entry_valid(me, bakery);
100
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000101 /* 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 */
Soby Mathewa0a897d2015-02-19 16:23:51 +0000114 do {
115 their_bakery_data = bakery->lock_data[they];
116 } while (bakery_is_choosing(their_bakery_data));
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000117
118 /*
119 * If the other party is a contender, they'll have non-zero
120 * (valid) ticket value. If they do, compare priorities
121 */
Soby Mathewa0a897d2015-02-19 16:23:51 +0000122 their_ticket = bakery_ticket_number(their_bakery_data);
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000123 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();
Soby Mathewa0a897d2015-02-19 16:23:51 +0000132 } while (their_ticket ==
133 bakery_ticket_number(bakery->lock_data[they]));
Achin Gupta4f6ad662013-10-25 09:08:21 +0100134 }
Achin Gupta4f6ad662013-10-25 09:08:21 +0100135 }
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000136 /* Lock acquired */
Achin Gupta4f6ad662013-10-25 09:08:21 +0100137}
138
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000139
140/* Release the lock and signal contenders */
Andrew Thoelke958cc022014-06-09 12:54:15 +0100141void bakery_lock_release(bakery_lock_t *bakery)
Achin Gupta4f6ad662013-10-25 09:08:21 +0100142{
Soby Mathew3700a922015-07-13 11:21:11 +0100143 unsigned int me = plat_my_core_pos();
Achin Gupta4f6ad662013-10-25 09:08:21 +0100144
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000145 assert_bakery_entry_valid(me, bakery);
Soby Mathew156280c2015-02-20 16:04:17 +0000146 assert(bakery_ticket_number(bakery->lock_data[me]));
Achin Gupta4f6ad662013-10-25 09:08:21 +0100147
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000148 /*
Soby Mathew156280c2015-02-20 16:04:17 +0000149 * Release lock by resetting ticket. Then signal other
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000150 * waiting contenders
151 */
Soby Mathewa0a897d2015-02-19 16:23:51 +0000152 bakery->lock_data[me] = 0;
Achin Guptaed04c4a2014-11-10 11:50:30 +0000153 dsb();
Jeenu Viswambharanbe87cfc2014-03-13 11:16:25 +0000154 sev();
Achin Gupta4f6ad662013-10-25 09:08:21 +0100155}