1 /* 2 * Copyright (c) 2013-2014, 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 /* Convert a ticket to priority */ 67 #define PRIORITY(t, pos) (((t) << 8) | (pos)) 68 69 70 /* Initialize Bakery Lock to reset ownership and all ticket values */ 71 void bakery_lock_init(bakery_lock_t *bakery) 72 { 73 assert(bakery); 74 75 /* All ticket values need to be 0 */ 76 memset(bakery, 0, sizeof(*bakery)); 77 bakery->owner = NO_OWNER; 78 } 79 80 81 /* Obtain a ticket for a given CPU */ 82 static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me) 83 { 84 unsigned int my_ticket, their_ticket; 85 unsigned int they; 86 87 /* 88 * Flag that we're busy getting our ticket. All CPUs are iterated in the 89 * order of their ordinal position to decide the maximum ticket value 90 * observed so far. Our priority is set to be greater than the maximum 91 * observed priority 92 * 93 * Note that it's possible that more than one contender gets the same 94 * ticket value. That's OK as the lock is acquired based on the priority 95 * value, not the ticket value alone. 96 */ 97 my_ticket = 0; 98 bakery->entering[me] = 1; 99 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { 100 their_ticket = bakery->number[they]; 101 if (their_ticket > my_ticket) 102 my_ticket = their_ticket; 103 } 104 105 /* 106 * Compute ticket; then signal to other contenders waiting for us to 107 * finish calculating our ticket value that we're done 108 */ 109 ++my_ticket; 110 bakery->number[me] = my_ticket; 111 bakery->entering[me] = 0; 112 113 return my_ticket; 114 } 115 116 117 /* 118 * Acquire bakery lock 119 * 120 * Contending CPUs need first obtain a non-zero ticket and then calculate 121 * priority value. A contending CPU iterate over all other CPUs in the platform, 122 * which may be contending for the same lock, in the order of their ordinal 123 * position (CPU0, CPU1 and so on). A non-contending CPU will have its ticket 124 * (and priority) value as 0. The contending CPU compares its priority with that 125 * of others'. The CPU with the highest priority (lowest numerical value) 126 * acquires the lock 127 */ 128 void bakery_lock_get(bakery_lock_t *bakery) 129 { 130 unsigned int they, me; 131 unsigned int my_ticket, my_prio, their_ticket; 132 133 me = platform_get_core_pos(read_mpidr_el1()); 134 135 assert_bakery_entry_valid(me, bakery); 136 137 /* Prevent recursive acquisition */ 138 assert(bakery->owner != me); 139 140 /* Get a ticket */ 141 my_ticket = bakery_get_ticket(bakery, me); 142 143 /* 144 * Now that we got our ticket, compute our priority value, then compare 145 * with that of others, and proceed to acquire the lock 146 */ 147 my_prio = PRIORITY(my_ticket, me); 148 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { 149 if (me == they) 150 continue; 151 152 /* Wait for the contender to get their ticket */ 153 while (bakery->entering[they]) 154 ; 155 156 /* 157 * If the other party is a contender, they'll have non-zero 158 * (valid) ticket value. If they do, compare priorities 159 */ 160 their_ticket = bakery->number[they]; 161 if (their_ticket && (PRIORITY(their_ticket, they) < my_prio)) { 162 /* 163 * They have higher priority (lower value). Wait for 164 * their ticket value to change (either release the lock 165 * to have it dropped to 0; or drop and probably content 166 * again for the same lock to have an even higher value) 167 */ 168 do { 169 wfe(); 170 } while (their_ticket == bakery->number[they]); 171 } 172 } 173 174 /* Lock acquired */ 175 bakery->owner = me; 176 } 177 178 179 /* Release the lock and signal contenders */ 180 void bakery_lock_release(bakery_lock_t *bakery) 181 { 182 unsigned int me = platform_get_core_pos(read_mpidr_el1()); 183 184 assert_bakery_entry_valid(me, bakery); 185 assert(bakery->owner == me); 186 187 /* 188 * Release lock by resetting ownership and ticket. Then signal other 189 * waiting contenders 190 */ 191 bakery->owner = NO_OWNER; 192 bakery->number[me] = 0; 193 dsb(); 194 sev(); 195 } 196
