1 files changed, 279 insertions, 0 deletions
diff --git a/kernel/latency.c b/kernel/latency.c
new file mode 100644
index 00000000000..258f2555abb
--- /dev/null
+++ b/kernel/latency.c
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+/*
+ * latency.c: Explicit system-wide latency-expectation infrastructure
+ *
+ * The purpose of this infrastructure is to allow device drivers to set
+ * latency constraint they have and to collect and summarize these
+ * expectations globally. The cummulated result can then be used by
+ * power management and similar users to make decisions that have
+ * tradoffs with a latency component.
+ *
+ * An example user of this are the x86 C-states; each higher C state saves
+ * more power, but has a higher exit latency. For the idle loop power
+ * code to make a good decision which C-state to use, information about
+ * acceptable latencies is required.
+ *
+ * An example announcer of latency is an audio driver that knowns it
+ * will get an interrupt when the hardware has 200 usec of samples
+ * left in the DMA buffer; in that case the driver can set a latency
+ * constraint of, say, 150 usec.
+ *
+ * Multiple drivers can each announce their maximum accepted latency,
+ * to keep these appart, a string based identifier is used.
+ *
+ *
+ * (C) Copyright 2006 Intel Corporation
+ * Author: Arjan van de Ven <arjan@linux.intel.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; version 2
+ * of the License.
+ */
+
+#include <linux/latency.h>
+#include <linux/list.h>
+#include <linux/spinlock.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/notifier.h>
+#include <asm/atomic.h>
+
+struct latency_info {
+	struct list_head list;
+	int usecs;
+	char *identifier;
+};
+
+/*
+ * locking rule: all modifications to current_max_latency and
+ * latency_list need to be done while holding the latency_lock.
+ * latency_lock needs to be taken _irqsave.
+ */
+static atomic_t current_max_latency;
+static DEFINE_SPINLOCK(latency_lock);
+
+static LIST_HEAD(latency_list);
+static BLOCKING_NOTIFIER_HEAD(latency_notifier);
+
+/*
+ * This function returns the maximum latency allowed, which
+ * happens to be the minimum of all maximum latencies on the
+ * list.
+ */
+static int __find_max_latency(void)
+{
+	int min = INFINITE_LATENCY;
+	struct latency_info *info;
+
+	list_for_each_entry(info, &latency_list, list) {
+		if (info->usecs < min)
+			min = info->usecs;
+	}
+	return min;
+}
+
+/**
+ * set_acceptable_latency - sets the maximum latency acceptable
+ * @identifier: string that identifies this driver
+ * @usecs: maximum acceptable latency for this driver
+ *
+ * This function informs the kernel that this device(driver)
+ * can accept at most usecs latency. This setting is used for
+ * power management and similar tradeoffs.
+ *
+ * This function sleeps and can only be called from process
+ * context.
+ * Calling this function with an existing identifier is valid
+ * and will cause the existing latency setting to be changed.
+ */
+void set_acceptable_latency(char *identifier, int usecs)
+{
+	struct latency_info *info, *iter;
+	unsigned long flags;
+	int found_old = 0;
+
+	info = kzalloc(sizeof(struct latency_info), GFP_KERNEL);
+	if (!info)
+		return;
+	info->usecs = usecs;
+	info->identifier = kstrdup(identifier, GFP_KERNEL);
+	if (!info->identifier)
+		goto free_info;
+
+	spin_lock_irqsave(&latency_lock, flags);
+	list_for_each_entry(iter, &latency_list, list) {
+		if (strcmp(iter->identifier, identifier)==0) {
+			found_old = 1;
+			iter->usecs = usecs;
+			break;
+		}
+	}
+	if (!found_old)
+		list_add(&info->list, &latency_list);
+
+	if (usecs < atomic_read(&current_max_latency))
+		atomic_set(&current_max_latency, usecs);
+
+	spin_unlock_irqrestore(&latency_lock, flags);
+
+	blocking_notifier_call_chain(&latency_notifier,
+		atomic_read(&current_max_latency), NULL);
+
+	/*
+	 * if we inserted the new one, we're done; otherwise there was
+	 * an existing one so we need to free the redundant data
+	 */
+	if (!found_old)
+		return;
+
+	kfree(info->identifier);
+free_info:
+	kfree(info);
+}
+EXPORT_SYMBOL_GPL(set_acceptable_latency);
+
+/**
+ * modify_acceptable_latency - changes the maximum latency acceptable
+ * @identifier: string that identifies this driver
+ * @usecs: maximum acceptable latency for this driver
+ *
+ * This function informs the kernel that this device(driver)
+ * can accept at most usecs latency. This setting is used for
+ * power management and similar tradeoffs.
+ *
+ * This function does not sleep and can be called in any context.
+ * Trying to use a non-existing identifier silently gets ignored.
+ *
+ * Due to the atomic nature of this function, the modified latency
+ * value will only be used for future decisions; past decisions
+ * can still lead to longer latencies in the near future.
+ */
+void modify_acceptable_latency(char *identifier, int usecs)
+{
+	struct latency_info *iter;
+	unsigned long flags;
+
+	spin_lock_irqsave(&latency_lock, flags);
+	list_for_each_entry(iter, &latency_list, list) {
+		if (strcmp(iter->identifier, identifier) == 0) {
+			iter->usecs = usecs;
+			break;
+		}
+	}
+	if (usecs < atomic_read(&current_max_latency))
+		atomic_set(&current_max_latency, usecs);
+	spin_unlock_irqrestore(&latency_lock, flags);
+}
+EXPORT_SYMBOL_GPL(modify_acceptable_latency);
+
+/**
+ * remove_acceptable_latency - removes the maximum latency acceptable
+ * @identifier: string that identifies this driver
+ *
+ * This function removes a previously set maximum latency setting
+ * for the driver and frees up any resources associated with the
+ * bookkeeping needed for this.
+ *
+ * This function does not sleep and can be called in any context.
+ * Trying to use a non-existing identifier silently gets ignored.
+ */
+void remove_acceptable_latency(char *identifier)
+{
+	unsigned long flags;
+	int newmax = 0;
+	struct latency_info *iter, *temp;
+
+	spin_lock_irqsave(&latency_lock, flags);
+
+	list_for_each_entry_safe(iter,  temp, &latency_list, list) {
+		if (strcmp(iter->identifier, identifier) == 0) {
+			list_del(&iter->list);
+			newmax = iter->usecs;
+			kfree(iter->identifier);
+			kfree(iter);
+			break;
+		}
+	}
+
+	/* If we just deleted the system wide value, we need to
+	 * recalculate with a full search
+	 */
+	if (newmax == atomic_read(&current_max_latency)) {
+		newmax = __find_max_latency();
+		atomic_set(&current_max_latency, newmax);
+	}
+	spin_unlock_irqrestore(&latency_lock, flags);
+}
+EXPORT_SYMBOL_GPL(remove_acceptable_latency);
+
+/**
+ * system_latency_constraint - queries the system wide latency maximum
+ *
+ * This function returns the system wide maximum latency in
+ * microseconds.
+ *
+ * This function does not sleep and can be called in any context.
+ */
+int system_latency_constraint(void)
+{
+	return atomic_read(&current_max_latency);
+}
+EXPORT_SYMBOL_GPL(system_latency_constraint);
+
+/**
+ * synchronize_acceptable_latency - recalculates all latency decisions
+ *
+ * This function will cause a callback to various kernel pieces that
+ * will make those pieces rethink their latency decisions. This implies
+ * that if there are overlong latencies in hardware state already, those
+ * latencies get taken right now. When this call completes no overlong
+ * latency decisions should be active anymore.
+ *
+ * Typical usecase of this is after a modify_acceptable_latency() call,
+ * which in itself is non-blocking and non-synchronizing.
+ *
+ * This function blocks and should not be called with locks held.
+ */
+
+void synchronize_acceptable_latency(void)
+{
+	blocking_notifier_call_chain(&latency_notifier,
+		atomic_read(&current_max_latency), NULL);
+}
+EXPORT_SYMBOL_GPL(synchronize_acceptable_latency);
+
+/*
+ * Latency notifier: this notifier gets called when a non-atomic new
+ * latency value gets set. The expectation nof the caller of the
+ * non-atomic set is that when the call returns, future latencies
+ * are within bounds, so the functions on the notifier list are
+ * expected to take the overlong latencies immediately, inside the
+ * callback, and not make a overlong latency decision anymore.
+ *
+ * The callback gets called when the new latency value is made
+ * active so system_latency_constraint() returns the new latency.
+ */
+int register_latency_notifier(struct notifier_block * nb)
+{
+	return blocking_notifier_chain_register(&latency_notifier, nb);
+}
+EXPORT_SYMBOL_GPL(register_latency_notifier);
+
+int unregister_latency_notifier(struct notifier_block * nb)
+{
+	return blocking_notifier_chain_unregister(&latency_notifier, nb);
+}
+EXPORT_SYMBOL_GPL(unregister_latency_notifier);
+
+static __init int latency_init(void)
+{
+	atomic_set(&current_max_latency, INFINITE_LATENCY);
+	/*
+	 * we don't want by default to have longer latencies than 2 ticks,
+	 * since that would cause lost ticks
+	 */
+	set_acceptable_latency("kernel", 2*1000000/HZ);
+	return 0;
+}
+
+module_init(latency_init);