diff options
Diffstat (limited to 'lib/locks/bakery')
| -rw-r--r-- | lib/locks/bakery/bakery_lock.c | 193 |
1 files changed, 193 insertions, 0 deletions
diff --git a/lib/locks/bakery/bakery_lock.c b/lib/locks/bakery/bakery_lock.c new file mode 100644 index 0000000..4e148b5 --- /dev/null +++ b/lib/locks/bakery/bakery_lock.c @@ -0,0 +1,193 @@ +/* + * Copyright (c) 2013-2014, ARM Limited and Contributors. All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions are met: + * + * Redistributions of source code must retain the above copyright notice, this + * list of conditions and the following disclaimer. + * + * Redistributions in binary form must reproduce the above copyright notice, + * this list of conditions and the following disclaimer in the documentation + * and/or other materials provided with the distribution. + * + * Neither the name of ARM nor the names of its contributors may be used + * to endorse or promote products derived from this software without specific + * prior written permission. + * + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" + * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE + * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE + * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR + * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF + * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS + * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN + * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) + * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE + * POSSIBILITY OF SUCH DAMAGE. + */ + +#include <arch_helpers.h> +#include <assert.h> +#include <bakery_lock.h> +#include <platform.h> +#include <string.h> + +/* + * Functions in this file implement Bakery Algorithm for mutual exclusion. + * + * ARM architecture offers a family of exclusive access instructions to + * efficiently implement mutual exclusion with hardware support. However, as + * well as depending on external hardware, the these instructions have defined + * behavior only on certain memory types (cacheable and Normal memory in + * particular; see ARMv8 Architecture Reference Manual section B2.10). Use cases + * in trusted firmware are such that mutual exclusion implementation cannot + * expect that accesses to the lock have the specific type required by the + * architecture for these primitives to function (for example, not all + * contenders may have address translation enabled). + * + * This implementation does not use mutual exclusion primitives. It expects + * memory regions where the locks reside to be fully ordered and coherent + * (either by disabling address translation, or by assigning proper attributes + * when translation is enabled). + * + * Note that the ARM architecture guarantees single-copy atomicity for aligned + * accesses regardless of status of address translation. + */ + +#define assert_bakery_entry_valid(entry, bakery) do { \ + assert(bakery); \ + assert(entry < BAKERY_LOCK_MAX_CPUS); \ +} while (0) + +/* Convert a ticket to priority */ +#define PRIORITY(t, pos) (((t) << 8) | (pos)) + + +/* Initialize Bakery Lock to reset ownership and all ticket values */ +void bakery_lock_init(bakery_lock_t *bakery) +{ + assert(bakery); + + /* All ticket values need to be 0 */ + memset(bakery, 0, sizeof(*bakery)); + bakery->owner = NO_OWNER; +} + + +/* Obtain a ticket for a given CPU */ +static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me) +{ + unsigned int my_ticket, their_ticket; + unsigned int they; + + /* + * Flag that we're busy getting our ticket. All CPUs are iterated in the + * order of their ordinal position to decide the maximum ticket value + * observed so far. Our priority is set to be greater than the maximum + * observed priority + * + * Note that it's possible that more than one contender gets the same + * ticket value. That's OK as the lock is acquired based on the priority + * value, not the ticket value alone. + */ + my_ticket = 0; + bakery->entering[me] = 1; + for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { + their_ticket = bakery->number[they]; + if (their_ticket > my_ticket) + my_ticket = their_ticket; + } + + /* + * Compute ticket; then signal to other contenders waiting for us to + * finish calculating our ticket value that we're done + */ + ++my_ticket; + bakery->number[me] = my_ticket; + bakery->entering[me] = 0; + sev(); + + return my_ticket; +} + + +/* + * Acquire bakery lock + * + * Contending CPUs need first obtain a non-zero ticket and then calculate + * priority value. A contending CPU iterate over all other CPUs in the platform, + * which may be contending for the same lock, in the order of their ordinal + * position (CPU0, CPU1 and so on). A non-contending CPU will have its ticket + * (and priority) value as 0. The contending CPU compares its priority with that + * of others'. The CPU with the highest priority (lowest numerical value) + * acquires the lock + */ +void bakery_lock_get(unsigned long mpidr, bakery_lock_t *bakery) +{ + unsigned int they, me; + unsigned int my_ticket, my_prio, their_ticket; + + me = platform_get_core_pos(mpidr); + + assert_bakery_entry_valid(me, bakery); + + /* Prevent recursive acquisition */ + assert(bakery->owner != me); + + /* Get a ticket */ + my_ticket = bakery_get_ticket(bakery, me); + + /* + * Now that we got our ticket, compute our priority value, then compare + * with that of others, and proceed to acquire the lock + */ + my_prio = PRIORITY(my_ticket, me); + for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { + if (me == they) + continue; + + /* Wait for the contender to get their ticket */ + while (bakery->entering[they]) + wfe(); + + /* + * If the other party is a contender, they'll have non-zero + * (valid) ticket value. If they do, compare priorities + */ + their_ticket = bakery->number[they]; + if (their_ticket && (PRIORITY(their_ticket, they) < my_prio)) { + /* + * They have higher priority (lower value). Wait for + * their ticket value to change (either release the lock + * to have it dropped to 0; or drop and probably content + * again for the same lock to have an even higher value) + */ + do { + wfe(); + } while (their_ticket == bakery->number[they]); + } + } + + /* Lock acquired */ + bakery->owner = me; +} + + +/* Release the lock and signal contenders */ +void bakery_lock_release(unsigned long mpidr, bakery_lock_t *bakery) +{ + unsigned int me = platform_get_core_pos(mpidr); + + assert_bakery_entry_valid(me, bakery); + assert(bakery->owner == me); + + /* + * Release lock by resetting ownership and ticket. Then signal other + * waiting contenders + */ + bakery->owner = NO_OWNER; + bakery->number[me] = 0; + sev(); +} |