/* * 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(¤t_max_latency)) atomic_set(¤t_max_latency, usecs); spin_unlock_irqrestore(&latency_lock, flags); blocking_notifier_call_chain(&latency_notifier, atomic_read(¤t_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(¤t_max_latency)) atomic_set(¤t_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(¤t_max_latency)) { newmax = __find_max_latency(); atomic_set(¤t_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(¤t_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(¤t_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(¤t_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);