1 /* 2 * Copyright (c) 2013-2018, Arm Limited and Contributors. All rights reserved. 3 * 4 * SPDX-License-Identifier: BSD-3-Clause 5 */ 6 7 #include <assert.h> 8 #include <string.h> 9 10 #include <arch_helpers.h> 11 #include <lib/bakery_lock.h> 12 #include <lib/el3_runtime/cpu_data.h> 13 #include <plat/common/platform.h> 14 15 /* 16 * Functions in this file implement Bakery Algorithm for mutual exclusion with the 17 * bakery lock data structures in coherent memory. 18 * 19 * ARM architecture offers a family of exclusive access instructions to 20 * efficiently implement mutual exclusion with hardware support. However, as 21 * well as depending on external hardware, the these instructions have defined 22 * behavior only on certain memory types (cacheable and Normal memory in 23 * particular; see ARMv8 Architecture Reference Manual section B2.10). Use cases 24 * in trusted firmware are such that mutual exclusion implementation cannot 25 * expect that accesses to the lock have the specific type required by the 26 * architecture for these primitives to function (for example, not all 27 * contenders may have address translation enabled). 28 * 29 * This implementation does not use mutual exclusion primitives. It expects 30 * memory regions where the locks reside to be fully ordered and coherent 31 * (either by disabling address translation, or by assigning proper attributes 32 * when translation is enabled). 33 * 34 * Note that the ARM architecture guarantees single-copy atomicity for aligned 35 * accesses regardless of status of address translation. 36 */ 37 38 #define assert_bakery_entry_valid(_entry, _bakery) do { \ 39 assert((_bakery) != NULL); \ 40 assert((_entry) < BAKERY_LOCK_MAX_CPUS); \ 41 } while (false) 42 43 /* Obtain a ticket for a given CPU */ 44 static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me) 45 { 46 unsigned int my_ticket, their_ticket; 47 unsigned int they; 48 49 /* Prevent recursive acquisition */ 50 assert(bakery_ticket_number(bakery->lock_data[me]) == 0U); 51 52 /* 53 * Flag that we're busy getting our ticket. All CPUs are iterated in the 54 * order of their ordinal position to decide the maximum ticket value 55 * observed so far. Our priority is set to be greater than the maximum 56 * observed priority 57 * 58 * Note that it's possible that more than one contender gets the same 59 * ticket value. That's OK as the lock is acquired based on the priority 60 * value, not the ticket value alone. 61 */ 62 my_ticket = 0U; 63 bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket); 64 for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) { 65 their_ticket = bakery_ticket_number(bakery->lock_data[they]); 66 if (their_ticket > my_ticket) { 67 my_ticket = their_ticket; 68 } 69 } 70 71 /* 72 * Compute ticket; then signal to other contenders waiting for us to 73 * finish calculating our ticket value that we're done 74 */ 75 ++my_ticket; 76 bakery->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket); 77 78 return my_ticket; 79 } 80 81 82 /* 83 * Acquire bakery lock 84 * 85 * Contending CPUs need first obtain a non-zero ticket and then calculate 86 * priority value. A contending CPU iterate over all other CPUs in the platform, 87 * which may be contending for the same lock, in the order of their ordinal 88 * position (CPU0, CPU1 and so on). A non-contending CPU will have its ticket 89 * (and priority) value as 0. The contending CPU compares its priority with that 90 * of others'. The CPU with the highest priority (lowest numerical value) 91 * acquires the lock 92 */ 93 void bakery_lock_get(bakery_lock_t *bakery) 94 { 95 unsigned int they, me; 96 unsigned int my_ticket, my_prio, their_ticket; 97 unsigned int their_bakery_data; 98 99 me = plat_my_core_pos(); 100 101 assert_bakery_entry_valid(me, bakery); 102 103 /* Get a ticket */ 104 my_ticket = bakery_get_ticket(bakery, me); 105 106 /* 107 * Now that we got our ticket, compute our priority value, then compare 108 * with that of others, and proceed to acquire the lock 109 */ 110 my_prio = bakery_get_priority(my_ticket, me); 111 for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) { 112 if (me == they) { 113 continue; 114 } 115 116 /* Wait for the contender to get their ticket */ 117 do { 118 their_bakery_data = bakery->lock_data[they]; 119 } while (bakery_is_choosing(their_bakery_data)); 120 121 /* 122 * If the other party is a contender, they'll have non-zero 123 * (valid) ticket value. If they do, compare priorities 124 */ 125 their_ticket = bakery_ticket_number(their_bakery_data); 126 if ((their_ticket != 0U) && 127 (bakery_get_priority(their_ticket, they) < my_prio)) { 128 /* 129 * They have higher priority (lower value). Wait for 130 * their ticket value to change (either release the lock 131 * to have it dropped to 0; or drop and probably content 132 * again for the same lock to have an even higher value) 133 */ 134 do { 135 wfe(); 136 } while (their_ticket == 137 bakery_ticket_number(bakery->lock_data[they])); 138 } 139 } 140 141 /* 142 * Lock acquired. Ensure that any reads and writes from a shared 143 * resource in the critical section read/write values after the lock is 144 * acquired. 145 */ 146 dmbish(); 147 } 148 149 150 /* Release the lock and signal contenders */ 151 void bakery_lock_release(bakery_lock_t *bakery) 152 { 153 unsigned int me = plat_my_core_pos(); 154 155 assert_bakery_entry_valid(me, bakery); 156 assert(bakery_ticket_number(bakery->lock_data[me]) != 0U); 157 158 /* 159 * Ensure that other observers see any stores in the critical section 160 * before releasing the lock. Also ensure all loads in the critical 161 * section are complete before releasing the lock. Release the lock by 162 * resetting ticket. Then signal other waiting contenders. 163 */ 164 dmbish(); 165 bakery->lock_data[me] = 0U; 166 167 /* Required to ensure ordering of the following sev */ 168 dsb(); 169 sev(); 170 } 171