1 /* 2 * Copyright (c) 2015-2018, 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 #include <utils_def.h> 14 15 /* 16 * Functions in this file implement Bakery Algorithm for mutual exclusion with the 17 * bakery lock data structures in cacheable and Normal 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, 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 cacheable and Normal. 31 * 32 * Note that the ARM architecture guarantees single-copy atomicity for aligned 33 * accesses regardless of status of address translation. 34 */ 35 36 #ifdef PLAT_PERCPU_BAKERY_LOCK_SIZE 37 /* 38 * Verify that the platform defined value for the per-cpu space for bakery locks is 39 * a multiple of the cache line size, to prevent multiple CPUs writing to the same 40 * bakery lock cache line 41 * 42 * Using this value, if provided, rather than the linker generated value results in 43 * more efficient code 44 */ 45 CASSERT((PLAT_PERCPU_BAKERY_LOCK_SIZE & (CACHE_WRITEBACK_GRANULE - 1)) == 0, \ 46 PLAT_PERCPU_BAKERY_LOCK_SIZE_not_cacheline_multiple); 47 #define PERCPU_BAKERY_LOCK_SIZE (PLAT_PERCPU_BAKERY_LOCK_SIZE) 48 #else 49 /* 50 * Use the linker defined symbol which has evaluated the size reqiurement. 51 * This is not as efficient as using a platform defined constant 52 */ 53 IMPORT_SYM(uintptr_t, __PERCPU_BAKERY_LOCK_SIZE__, PERCPU_BAKERY_LOCK_SIZE); 54 #endif 55 56 #define get_bakery_info(cpu_ix, lock) \ 57 (bakery_info_t *)((uintptr_t)lock + cpu_ix * PERCPU_BAKERY_LOCK_SIZE) 58 59 #define write_cache_op(addr, cached) \ 60 do { \ 61 (cached ? dccvac((uintptr_t)addr) :\ 62 dcivac((uintptr_t)addr));\ 63 dsbish();\ 64 } while (0) 65 66 #define read_cache_op(addr, cached) if (cached) \ 67 dccivac((uintptr_t)addr) 68 69 /* Helper function to check if the lock is acquired */ 70 static inline int is_lock_acquired(const bakery_info_t *my_bakery_info, 71 int is_cached) 72 { 73 /* 74 * Even though lock data is updated only by the owning cpu and 75 * appropriate cache maintenance operations are performed, 76 * if the previous update was done when the cpu was not participating 77 * in coherency, then there is a chance that cache maintenance 78 * operations were not propagated to all the caches in the system. 79 * Hence do a `read_cache_op()` prior to read. 80 */ 81 read_cache_op(my_bakery_info, is_cached); 82 return !!(bakery_ticket_number(my_bakery_info->lock_data)); 83 } 84 85 static unsigned int bakery_get_ticket(bakery_lock_t *lock, 86 unsigned int me, int is_cached) 87 { 88 unsigned int my_ticket, their_ticket; 89 unsigned int they; 90 bakery_info_t *my_bakery_info, *their_bakery_info; 91 92 /* 93 * Obtain a reference to the bakery information for this cpu and ensure 94 * it is not NULL. 95 */ 96 my_bakery_info = get_bakery_info(me, lock); 97 assert(my_bakery_info); 98 99 /* Prevent recursive acquisition.*/ 100 assert(!is_lock_acquired(my_bakery_info, is_cached)); 101 102 /* 103 * Tell other contenders that we are through the bakery doorway i.e. 104 * going to allocate a ticket for this cpu. 105 */ 106 my_ticket = 0; 107 my_bakery_info->lock_data = make_bakery_data(CHOOSING_TICKET, my_ticket); 108 109 write_cache_op(my_bakery_info, is_cached); 110 111 /* 112 * Iterate through the bakery information of each contender to allocate 113 * the highest ticket number for this cpu. 114 */ 115 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { 116 if (me == they) 117 continue; 118 119 /* 120 * Get a reference to the other contender's bakery info and 121 * ensure that a stale copy is not read. 122 */ 123 their_bakery_info = get_bakery_info(they, lock); 124 assert(their_bakery_info); 125 126 read_cache_op(their_bakery_info, is_cached); 127 128 /* 129 * Update this cpu's ticket number if a higher ticket number is 130 * seen 131 */ 132 their_ticket = bakery_ticket_number(their_bakery_info->lock_data); 133 if (their_ticket > my_ticket) 134 my_ticket = their_ticket; 135 } 136 137 /* 138 * Compute ticket; then signal to other contenders waiting for us to 139 * finish calculating our ticket value that we're done 140 */ 141 ++my_ticket; 142 my_bakery_info->lock_data = make_bakery_data(CHOSEN_TICKET, my_ticket); 143 144 write_cache_op(my_bakery_info, is_cached); 145 146 return my_ticket; 147 } 148 149 void bakery_lock_get(bakery_lock_t *lock) 150 { 151 unsigned int they, me, is_cached; 152 unsigned int my_ticket, my_prio, their_ticket; 153 bakery_info_t *their_bakery_info; 154 unsigned int their_bakery_data; 155 156 me = plat_my_core_pos(); 157 #ifdef AARCH32 158 is_cached = read_sctlr() & SCTLR_C_BIT; 159 #else 160 is_cached = read_sctlr_el3() & SCTLR_C_BIT; 161 #endif 162 163 /* Get a ticket */ 164 my_ticket = bakery_get_ticket(lock, me, is_cached); 165 166 /* 167 * Now that we got our ticket, compute our priority value, then compare 168 * with that of others, and proceed to acquire the lock 169 */ 170 my_prio = PRIORITY(my_ticket, me); 171 for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { 172 if (me == they) 173 continue; 174 175 /* 176 * Get a reference to the other contender's bakery info and 177 * ensure that a stale copy is not read. 178 */ 179 their_bakery_info = get_bakery_info(they, lock); 180 assert(their_bakery_info); 181 182 /* Wait for the contender to get their ticket */ 183 do { 184 read_cache_op(their_bakery_info, is_cached); 185 their_bakery_data = their_bakery_info->lock_data; 186 } while (bakery_is_choosing(their_bakery_data)); 187 188 /* 189 * If the other party is a contender, they'll have non-zero 190 * (valid) ticket value. If they do, compare priorities 191 */ 192 their_ticket = bakery_ticket_number(their_bakery_data); 193 if (their_ticket && (PRIORITY(their_ticket, they) < my_prio)) { 194 /* 195 * They have higher priority (lower value). Wait for 196 * their ticket value to change (either release the lock 197 * to have it dropped to 0; or drop and probably content 198 * again for the same lock to have an even higher value) 199 */ 200 do { 201 wfe(); 202 read_cache_op(their_bakery_info, is_cached); 203 } while (their_ticket 204 == bakery_ticket_number(their_bakery_info->lock_data)); 205 } 206 } 207 /* Lock acquired */ 208 } 209 210 void bakery_lock_release(bakery_lock_t *lock) 211 { 212 bakery_info_t *my_bakery_info; 213 #ifdef AARCH32 214 unsigned int is_cached = read_sctlr() & SCTLR_C_BIT; 215 #else 216 unsigned int is_cached = read_sctlr_el3() & SCTLR_C_BIT; 217 #endif 218 219 my_bakery_info = get_bakery_info(plat_my_core_pos(), lock); 220 221 assert(is_lock_acquired(my_bakery_info, is_cached)); 222 223 my_bakery_info->lock_data = 0; 224 write_cache_op(my_bakery_info, is_cached); 225 sev(); 226 } 227