GK SDK 源码库: XMIPCLinuxV100R005C00SPC030 (kernel/tools/open_source excluded)

source/trusted-firmware/trusted-firmware-a-2.8/lib/locks/bakery/bakery_lock_coherent.c

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+/*
+ * Copyright (c) 2013-2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <assert.h>
+#include <string.h>
+
+#include <arch_helpers.h>
+#include <lib/bakery_lock.h>
+#include <lib/el3_runtime/cpu_data.h>
+#include <plat/common/platform.h>
+
+/*
+ * Functions in this file implement Bakery Algorithm for mutual exclusion with the
+ * bakery lock data structures in coherent memory.
+ *
+ * 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) != NULL);			\
+	assert((_entry) < BAKERY_LOCK_MAX_CPUS);	\
+} while (false)
+
+/* 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;
+
+	/* Prevent recursive acquisition */
+	assert(bakery_ticket_number(bakery->lock_data[me]) == 0U);
+
+	/*
+	 * 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 = 0U;
+	bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket);
+	for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) {
+		their_ticket = bakery_ticket_number(bakery->lock_data[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->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket);
+
+	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;
+	unsigned int their_bakery_data;
+
+	me = plat_my_core_pos();
+
+	assert_bakery_entry_valid(me, bakery);
+
+	/* 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 = bakery_get_priority(my_ticket, me);
+	for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) {
+		if (me == they)
+			continue;
+
+		/* Wait for the contender to get their ticket */
+		do {
+			their_bakery_data = bakery->lock_data[they];
+		} while (bakery_is_choosing(their_bakery_data));
+
+		/*
+		 * If the other party is a contender, they'll have non-zero
+		 * (valid) ticket value. If they do, compare priorities
+		 */
+		their_ticket = bakery_ticket_number(their_bakery_data);
+		if ((their_ticket != 0U) &&
+		    (bakery_get_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_ticket_number(bakery->lock_data[they]));
+		}
+	}
+
+	/*
+	 * Lock acquired. Ensure that any reads and writes from a shared
+	 * resource in the critical section read/write values after the lock is
+	 * acquired.
+	 */
+	dmbish();
+}
+
+
+/* Release the lock and signal contenders */
+void bakery_lock_release(bakery_lock_t *bakery)
+{
+	unsigned int me = plat_my_core_pos();
+
+	assert_bakery_entry_valid(me, bakery);
+	assert(bakery_ticket_number(bakery->lock_data[me]) != 0U);
+
+	/*
+	 * Ensure that other observers see any stores in the critical section
+	 * before releasing the lock. Also ensure all loads in the critical
+	 * section are complete before releasing the lock. Release the lock by
+	 * resetting ticket. Then signal other waiting contenders.
+	 */
+	dmbish();
+	bakery->lock_data[me] = 0U;
+
+	/* Required to ensure ordering of the following sev */
+	dsb();
+	sev();
+}