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/*
* 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;
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(bakery_lock_t *bakery)
{
unsigned int they, me;
unsigned int my_ticket, my_prio, their_ticket;
me = platform_get_core_pos(read_mpidr_el1());
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])
;
/*
* 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(bakery_lock_t *bakery)
{
unsigned int me = platform_get_core_pos(read_mpidr_el1());
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;
dsb();
sev();
}