1 /* 2 * Copyright (c) 2013-2015, ARM Limited and Contributors. All rights reserved. 3 * 4 * Redistribution and use in source and binary forms, with or without 5 * modification, are permitted provided that the following conditions are met: 6 * 7 * Redistributions of source code must retain the above copyright notice, this 8 * list of conditions and the following disclaimer. 9 * 10 * Redistributions in binary form must reproduce the above copyright notice, 11 * this list of conditions and the following disclaimer in the documentation 12 * and/or other materials provided with the distribution. 13 * 14 * Neither the name of ARM nor the names of its contributors may be used 15 * to endorse or promote products derived from this software without specific 16 * prior written permission. 17 * 18 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 19 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE 22 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28 * POSSIBILITY OF SUCH DAMAGE. 29 */ 30 31 #include <arch_helpers.h> 32 #include <assert.h> 33 #include <bakery_lock.h> 34 #include <cpu_data.h> 35 #include <platform.h> 36 #include <string.h> 37 38 /* 39 * Functions in this file implement Bakery Algorithm for mutual exclusion with the 40 * bakery lock data structures in coherent memory. 41 * 42 * ARM architecture offers a family of exclusive access instructions to 43 * efficiently implement mutual exclusion with hardware support. However, as 44 * well as depending on external hardware, the these instructions have defined 45 * behavior only on certain memory types (cacheable and Normal memory in 46 * particular; see ARMv8 Architecture Reference Manual section B2.10). Use cases 47 * in trusted firmware are such that mutual exclusion implementation cannot 48 * expect that accesses to the lock have the specific type required by the 49 * architecture for these primitives to function (for example, not all 50 * contenders may have address translation enabled). 51 * 52 * This implementation does not use mutual exclusion primitives. It expects 53 * memory regions where the locks reside to be fully ordered and coherent 54 * (either by disabling address translation, or by assigning proper attributes 55 * when translation is enabled). 56 * 57 * Note that the ARM architecture guarantees single-copy atomicity for aligned 58 * accesses regardless of status of address translation. 59 */ 60 61 #define assert_bakery_entry_valid(entry, bakery) do { \ 62 assert(bakery); \ 63 assert(entry < BAKERY_LOCK_MAX_CPUS); \ 64 } while (0) 65 66 /* Obtain a ticket for a given CPU */ 67 static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me) 68 { 69 unsigned int my_ticket, their_ticket; 70 unsigned int they; 71 72 /* Prevent recursive acquisition */ 73 assert(!bakery_ticket_number(bakery->lock_data[me])); 74 75 /* 76 * Flag that we're busy getting our ticket. All CPUs are iterated in the 77 * order of their ordinal position to decide the maximum ticket value 78 * observed so far. Our priority is set to be greater than the maximum 79 * observed priority 80 * 81 * Note that it's possible that more than one contender gets the same 82 * ticket value. That's OK as the lock is acquired based on the priority 83 * value, not the ticket value alone. 84 */ 85 my_ticket = 0; 86 bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket); 87 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { 88 their_ticket = bakery_ticket_number(bakery->lock_data[they]); 89 if (their_ticket > my_ticket) 90 my_ticket = their_ticket; 91 } 92 93 /* 94 * Compute ticket; then signal to other contenders waiting for us to 95 * finish calculating our ticket value that we're done 96 */ 97 ++my_ticket; 98 bakery->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket); 99 100 return my_ticket; 101 } 102 103 104 /* 105 * Acquire bakery lock 106 * 107 * Contending CPUs need first obtain a non-zero ticket and then calculate 108 * priority value. A contending CPU iterate over all other CPUs in the platform, 109 * which may be contending for the same lock, in the order of their ordinal 110 * position (CPU0, CPU1 and so on). A non-contending CPU will have its ticket 111 * (and priority) value as 0. The contending CPU compares its priority with that 112 * of others'. The CPU with the highest priority (lowest numerical value) 113 * acquires the lock 114 */ 115 void bakery_lock_get(bakery_lock_t *bakery) 116 { 117 unsigned int they, me; 118 unsigned int my_ticket, my_prio, their_ticket; 119 unsigned int their_bakery_data; 120 121 me = plat_my_core_pos(); 122 123 assert_bakery_entry_valid(me, bakery); 124 125 /* Get a ticket */ 126 my_ticket = bakery_get_ticket(bakery, me); 127 128 /* 129 * Now that we got our ticket, compute our priority value, then compare 130 * with that of others, and proceed to acquire the lock 131 */ 132 my_prio = PRIORITY(my_ticket, me); 133 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { 134 if (me == they) 135 continue; 136 137 /* Wait for the contender to get their ticket */ 138 do { 139 their_bakery_data = bakery->lock_data[they]; 140 } while (bakery_is_choosing(their_bakery_data)); 141 142 /* 143 * If the other party is a contender, they'll have non-zero 144 * (valid) ticket value. If they do, compare priorities 145 */ 146 their_ticket = bakery_ticket_number(their_bakery_data); 147 if (their_ticket && (PRIORITY(their_ticket, they) < my_prio)) { 148 /* 149 * They have higher priority (lower value). Wait for 150 * their ticket value to change (either release the lock 151 * to have it dropped to 0; or drop and probably content 152 * again for the same lock to have an even higher value) 153 */ 154 do { 155 wfe(); 156 } while (their_ticket == 157 bakery_ticket_number(bakery->lock_data[they])); 158 } 159 } 160 /* Lock acquired */ 161 } 162 163 164 /* Release the lock and signal contenders */ 165 void bakery_lock_release(bakery_lock_t *bakery) 166 { 167 unsigned int me = plat_my_core_pos(); 168 169 assert_bakery_entry_valid(me, bakery); 170 assert(bakery_ticket_number(bakery->lock_data[me])); 171 172 /* 173 * Release lock by resetting ticket. Then signal other 174 * waiting contenders 175 */ 176 bakery->lock_data[me] = 0; 177 dsb(); 178 sev(); 179 } 180