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 <arch_helpers.h> 8 #include <assert.h> 9 #include <bakery_lock.h> 10 #include <cpu_data.h> 11 #include <platform.h> 12 #include <string.h> 13 14 /* 15 * Functions in this file implement Bakery Algorithm for mutual exclusion with the 16 * bakery lock data structures in coherent memory. 17 * 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 */ 36 37 #define assert_bakery_entry_valid(_entry, _bakery) do { \ 38 assert((_bakery) != NULL); \ 39 assert((_entry) < BAKERY_LOCK_MAX_CPUS); \ 40 } while (false) 41 42 /* Obtain a ticket for a given CPU */ 43 static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me) 44 { 45 unsigned int my_ticket, their_ticket; 46 unsigned int they; 47 48 /* Prevent recursive acquisition */ 49 assert(bakery_ticket_number(bakery->lock_data[me]) == 0U); 50 51 /* 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 = 0U; 62 bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket); 63 for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) { 64 their_ticket = bakery_ticket_number(bakery->lock_data[they]); 65 if (their_ticket > my_ticket) 66 my_ticket = their_ticket; 67 } 68 69 /* 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; 74 bakery->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket); 75 76 return my_ticket; 77 } 78 79 80 /* 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 */ 91 void bakery_lock_get(bakery_lock_t *bakery) 92 { 93 unsigned int they, me; 94 unsigned int my_ticket, my_prio, their_ticket; 95 unsigned int their_bakery_data; 96 97 me = plat_my_core_pos(); 98 99 assert_bakery_entry_valid(me, bakery); 100 101 /* 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 = bakery_get_priority(my_ticket, me); 109 for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) { 110 if (me == they) 111 continue; 112 113 /* Wait for the contender to get their ticket */ 114 do { 115 their_bakery_data = bakery->lock_data[they]; 116 } while (bakery_is_choosing(their_bakery_data)); 117 118 /* 119 * If the other party is a contender, they'll have non-zero 120 * (valid) ticket value. If they do, compare priorities 121 */ 122 their_ticket = bakery_ticket_number(their_bakery_data); 123 if ((their_ticket != 0U) && 124 (bakery_get_priority(their_ticket, they) < my_prio)) { 125 /* 126 * They have higher priority (lower value). Wait for 127 * their ticket value to change (either release the lock 128 * to have it dropped to 0; or drop and probably content 129 * again for the same lock to have an even higher value) 130 */ 131 do { 132 wfe(); 133 } while (their_ticket == 134 bakery_ticket_number(bakery->lock_data[they])); 135 } 136 } 137 138 /* 139 * Lock acquired. Ensure that any reads from a shared resource in the 140 * critical section read values after the lock is acquired. 141 */ 142 dmbld(); 143 } 144 145 146 /* Release the lock and signal contenders */ 147 void bakery_lock_release(bakery_lock_t *bakery) 148 { 149 unsigned int me = plat_my_core_pos(); 150 151 assert_bakery_entry_valid(me, bakery); 152 assert(bakery_ticket_number(bakery->lock_data[me]) != 0U); 153 154 /* 155 * Ensure that other observers see any stores in the critical section 156 * before releasing the lock. Release the lock by resetting ticket. 157 * Then signal other waiting contenders. 158 */ 159 dmbst(); 160 bakery->lock_data[me] = 0U; 161 dsb(); 162 sev(); 163 } 164