/*
* 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);
