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 /* Initialize Bakery Lock to reset ownership and all ticket values */ 67 void bakery_lock_init(bakery_lock_t *bakery) 68 { 69 assert(bakery); 70 71 /* All ticket values need to be 0 */ 72 memset(bakery, 0, sizeof(*bakery)); 73 bakery->owner = NO_OWNER; 74 } 75 76 77 /* Obtain a ticket for a given CPU */ 78 static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me) 79 { 80 unsigned int my_ticket, their_ticket; 81 unsigned int they; 82 83 /* 84 * Flag that we're busy getting our ticket. All CPUs are iterated in the 85 * order of their ordinal position to decide the maximum ticket value 86 * observed so far. Our priority is set to be greater than the maximum 87 * observed priority 88 * 89 * Note that it's possible that more than one contender gets the same 90 * ticket value. That's OK as the lock is acquired based on the priority 91 * value, not the ticket value alone. 92 */ 93 my_ticket = 0; 94 bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket); 95 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { 96 their_ticket = bakery_ticket_number(bakery->lock_data[they]); 97 if (their_ticket > my_ticket) 98 my_ticket = their_ticket; 99 } 100 101 /* 102 * Compute ticket; then signal to other contenders waiting for us to 103 * finish calculating our ticket value that we're done 104 */ 105 ++my_ticket; 106 bakery->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket); 107 108 return my_ticket; 109 } 110 111 112 /* 113 * Acquire bakery lock 114 * 115 * Contending CPUs need first obtain a non-zero ticket and then calculate 116 * priority value. A contending CPU iterate over all other CPUs in the platform, 117 * which may be contending for the same lock, in the order of their ordinal 118 * position (CPU0, CPU1 and so on). A non-contending CPU will have its ticket 119 * (and priority) value as 0. The contending CPU compares its priority with that 120 * of others'. The CPU with the highest priority (lowest numerical value) 121 * acquires the lock 122 */ 123 void bakery_lock_get(bakery_lock_t *bakery) 124 { 125 unsigned int they, me; 126 unsigned int my_ticket, my_prio, their_ticket; 127 unsigned int their_bakery_data; 128 129 me = platform_get_core_pos(read_mpidr_el1()); 130 131 assert_bakery_entry_valid(me, bakery); 132 133 /* Prevent recursive acquisition */ 134 assert(bakery->owner != me); 135 136 /* Get a ticket */ 137 my_ticket = bakery_get_ticket(bakery, me); 138 139 /* 140 * Now that we got our ticket, compute our priority value, then compare 141 * with that of others, and proceed to acquire the lock 142 */ 143 my_prio = PRIORITY(my_ticket, me); 144 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { 145 if (me == they) 146 continue; 147 148 /* Wait for the contender to get their ticket */ 149 do { 150 their_bakery_data = bakery->lock_data[they]; 151 } while (bakery_is_choosing(their_bakery_data)); 152 153 /* 154 * If the other party is a contender, they'll have non-zero 155 * (valid) ticket value. If they do, compare priorities 156 */ 157 their_ticket = bakery_ticket_number(their_bakery_data); 158 if (their_ticket && (PRIORITY(their_ticket, they) < my_prio)) { 159 /* 160 * They have higher priority (lower value). Wait for 161 * their ticket value to change (either release the lock 162 * to have it dropped to 0; or drop and probably content 163 * again for the same lock to have an even higher value) 164 */ 165 do { 166 wfe(); 167 } while (their_ticket == 168 bakery_ticket_number(bakery->lock_data[they])); 169 } 170 } 171 172 /* Lock acquired */ 173 bakery->owner = me; 174 } 175 176 177 /* Release the lock and signal contenders */ 178 void bakery_lock_release(bakery_lock_t *bakery) 179 { 180 unsigned int me = platform_get_core_pos(read_mpidr_el1()); 181 182 assert_bakery_entry_valid(me, bakery); 183 assert(bakery->owner == me); 184 185 /* 186 * Release lock by resetting ownership and ticket. Then signal other 187 * waiting contenders 188 */ 189 bakery->owner = NO_OWNER; 190 bakery->lock_data[me] = 0; 191 dsb(); 192 sev(); 193 } 194