Power Management for USB Alan Stern Last-updated: February 2014 Contents: --------- * What is Power Management? * What is Remote Wakeup? * When is a USB device idle? * Forms of dynamic PM * The user interface for dynamic PM * Changing the default idle-delay time * Warnings * The driver interface for Power Management * The driver interface for autosuspend and autoresume * Other parts of the driver interface * Mutual exclusion * Interaction between dynamic PM and system PM * xHCI hardware link PM * USB Port Power Control * User Interface for Port Power Control * Suggested Userspace Port Power Policy What is Power Management? ------------------------- Power Management (PM) is the practice of saving energy by suspending parts of a computer system when they aren't being used. While a component is "suspended" it is in a nonfunctional low-power state; it might even be turned off completely. A suspended component can be "resumed" (returned to a functional full-power state) when the kernel needs to use it. (There also are forms of PM in which components are placed in a less functional but still usable state instead of being suspended; an example would be reducing the CPU's clock rate. This document will not discuss those other forms.) When the parts being suspended include the CPU and most of the rest of the system, we speak of it as a "system suspend". When a particular device is turned off while the system as a whole remains running, we call it a "dynamic suspend" (also known as a "runtime suspend" or "selective suspend"). This document concentrates mostly on how dynamic PM is implemented in the USB subsystem, although system PM is covered to some extent (see Documentation/power/*.txt for more information about system PM). System PM support is present only if the kernel was built with CONFIG_SUSPEND or CONFIG_HIBERNATION enabled. Dynamic PM support for USB is present whenever the kernel was built with CONFIG_PM enabled. [Historically, dynamic PM support for USB was present only if the kernel had been built with CONFIG_USB_SUSPEND enabled (which depended on CONFIG_PM_RUNTIME). Starting with the 3.10 kernel release, dynamic PM support for USB was present whenever the kernel was built with CONFIG_PM_RUNTIME enabled. The CONFIG_USB_SUSPEND option had been eliminated.] What is Remote Wakeup? ---------------------- When a device has been suspended, it generally doesn't resume until the computer tells it to. Likewise, if the entire computer has been suspended, it generally doesn't resume until the user tells it to, say by pressing a power button or opening the cover. However some devices have the capability of resuming by themselves, or asking the kernel to resume them, or even telling the entire computer to resume. This capability goes by several names such as "Wake On LAN"; we will refer to it generically as "remote wakeup". When a device is enabled for remote wakeup and it is suspended, it may resume itself (or send a request to be resumed) in response to some external event. Examples include a suspended keyboard resuming when a key is pressed, or a suspended USB hub resuming when a device is plugged in. When is a USB device idle? -------------------------- A device is idle whenever the kernel thinks it's not busy doing anything important and thus is a candidate for being suspended. The exact definition depends on the device's driver; drivers are allowed to declare that a device isn't idle even when there's no actual communication taking place. (For example, a hub isn't considered idle unless all the devices plugged into that hub are already suspended.) In addition, a device isn't considered idle so long as a program keeps its usbfs file open, whether or not any I/O is going on. If a USB device has no driver, its usbfs file isn't open, and it isn't being accessed through sysfs, then it definitely is idle. Forms of dynamic PM ------------------- Dynamic suspends occur when the kernel decides to suspend an idle device. This is called "autosuspend" for short. In general, a device won't be autosuspended unless it has been idle for some minimum period of time, the so-called idle-delay time. Of course, nothing the kernel does on its own initiative should prevent the computer or its devices from working properly. If a device has been autosuspended and a program tries to use it, the kernel will automatically resume the device (autoresume). For the same reason, an autosuspended device will usually have remote wakeup enabled, if the device supports remote wakeup. It is worth mentioning that many USB drivers don't support autosuspend. In fact, at the time of this writing (Linux 2.6.23) the only drivers which do support it are the hub driver, kaweth, asix, usblp, usblcd, and usb-skeleton (which doesn't count). If a non-supporting driver is bound to a device, the device won't be autosuspended. In effect, the kernel pretends the device is never idle. We can categorize power management events in two broad classes: external and internal. External events are those triggered by some agent outside the USB stack: system suspend/resume (triggered by userspace), manual dynamic resume (also triggered by userspace), and remote wakeup (triggered by the device). Internal events are those triggered within the USB stack: autosuspend and autoresume. Note that all dynamic suspend events are internal; external agents are not allowed to issue dynamic suspends. The user interface for dynamic PM --------------------------------- The user interface for controlling dynamic PM is located in the power/ subdirectory of each USB device's sysfs directory, that is, in /sys/bus/usb/devices/.../power/ where "..." is the device's ID. The relevant attribute files are: wakeup, control, and autosuspend_delay_ms. (There may also be a file named "level"; this file was deprecated as of the 2.6.35 kernel and replaced by the "control" file. In 2.6.38 the "autosuspend" file will be deprecated and replaced by the "autosuspend_delay_ms" file. The only difference is that the newer file expresses the delay in milliseconds whereas the older file uses seconds. Confusingly, both files are present in 2.6.37 but only "autosuspend" works.) power/wakeup This file is empty if the device does not support remote wakeup. Otherwise the file contains either the word "enabled" or the word "disabled", and you can write those words to the file. The setting determines whether or not remote wakeup will be enabled when the device is next suspended. (If the setting is changed while the device is suspended, the change won't take effect until the following suspend.) power/control This file contains one of two words: "on" or "auto". You can write those words to the file to change the device's setting. "on" means that the device should be resumed and autosuspend is not allowed. (Of course, system suspends are still allowed.) "auto" is the normal state in which the kernel is allowed to autosuspend and autoresume the device. (In kernels up to 2.6.32, you could also specify "suspend", meaning that the device should remain suspended and autoresume was not allowed. This setting is no longer supported.) power/autosuspend_delay_ms This file contains an integer value, which is the number of milliseconds the device should remain idle before the kernel will autosuspend it (the idle-delay time). The default is 2000. 0 means to autosuspend as soon as the device becomes idle, and negative values mean never to autosuspend. You can write a number to the file to change the autosuspend idle-delay time. Writing "-1" to power/autosuspend_delay_ms and writing "on" to power/control do essentially the same thing -- they both prevent the device from being autosuspended. Yes, this is a redundancy in the API. (In 2.6.21 writing "0" to power/autosuspend would prevent the device from being autosuspended; the behavior was changed in 2.6.22. The power/autosuspend attribute did not exist prior to 2.6.21, and the power/level attribute did not exist prior to 2.6.22. power/control was added in 2.6.34, and power/autosuspend_delay_ms was added in 2.6.37 but did not become functional until 2.6.38.) Changing the default idle-delay time ------------------------------------ The default autosuspend idle-delay time (in seconds) is controlled by a module parameter in usbcore. You can specify the value when usbcore is loaded. For example, to set it to 5 seconds instead of 2 you would do: modprobe usbcore autosuspend=5 Equivalently, you could add to a configuration file in /etc/modprobe.d a line saying: options usbcore autosuspend=5 Some distributions load the usbcore module very early during the boot process, by means of a program or script running from an initramfs image. To alter the parameter value you would have to rebuild that image. If usbcore is compiled into the kernel rather than built as a loadable module, you can add usbcore.autosuspend=5 to the kernel's boot command line. Finally, the parameter value can be changed while the system is running. If you do: echo 5 >/sys/module/usbcore/parameters/autosuspend then each new USB device will have its autosuspend idle-delay initialized to 5. (The idle-delay values for already existing devices will not be affected.) Setting the initial default idle-delay to -1 will prevent any autosuspend of any USB device. This has the benefit of allowing you then to enable autosuspend for selected devices. Warnings -------- The USB specification states that all USB devices must support power management. Nevertheless, the sad fact is that many devices do not support it very well. You can suspend them all right, but when you try to resume them they disconnect themselves from the USB bus or they stop working entirely. This seems to be especially prevalent among printers and scanners, but plenty of other types of device have the same deficiency. For this reason, by default the kernel disables autosuspend (the power/control attribute is initialized to "on") for all devices other than hubs. Hubs, at least, appear to be reasonably well-behaved in this regard. (In 2.6.21 and 2.6.22 this wasn't the case. Autosuspend was enabled by default for almost all USB devices. A number of people experienced problems as a result.) This means that non-hub devices won't be autosuspended unless the user or a program explicitly enables it. As of this writing there aren't any widespread programs which will do this; we hope that in the near future device managers such as HAL will take on this added responsibility. In the meantime you can always carry out the necessary operations by hand or add them to a udev script. You can also change the idle-delay time; 2 seconds is not the best choice for every device. If a driver knows that its device has proper suspend/resume support, it can enable autosuspend all by itself. For example, the video driver for a laptop's webcam might do this (in recent kernels they do), since these devices are rarely used and so should normally be autosuspended. Sometimes it turns out that even when a device does work okay with autosuspend there are still problems. For example, the usbhid driver, which manages keyboards and mice, has autosuspend support. Tests with a number of keyboards show that typing on a suspended keyboard, while causing the keyboard to do a remote wakeup all right, will nonetheless frequently result in lost keystrokes. Tests with mice show that some of them will issue a remote-wakeup request in response to button presses but not to motion, and some in response to neither. The kernel will not prevent you from enabling autosuspend on devices that can't handle it. It is even possible in theory to damage a device by suspending it at the wrong time. (Highly unlikely, but possible.) Take care. The driver interface for Power Management ----------------------------------------- The requirements for a USB driver to support external power management are pretty modest; the driver need only define .suspend .resume .reset_resume methods in its usb_driver structure, and the reset_resume method is optional. The methods' jobs are quite simple: The suspend method is called to warn the driver that the device is going to be suspended. If the driver returns a negative error code, the suspend will be aborted. Normally the driver will return 0, in which case it must cancel all outstanding URBs (usb_kill_urb()) and not submit any more. The resume method is called to tell the driver that the device has been resumed and the driver can return to normal operation. URBs may once more be submitted. The reset_resume method is called to tell the driver that the device has been resumed and it also has been reset. The driver should redo any necessary device initialization, since the device has probably lost most or all of its state (although the interfaces will be in the same altsettings as before the suspend). If the device is disconnected or powered down while it is suspended, the disconnect method will be called instead of the resume or reset_resume method. This is also quite likely to happen when waking up from hibernation, as many systems do not maintain suspend current to the USB host controllers during hibernation. (It's possible to work around the hibernation-forces-disconnect problem by using the USB Persist facility.) The reset_resume method is used by the USB Persist facility (see Documentation/usb/persist.txt) and it can also be used under certain circumstances when CONFIG_USB_PERSIST is not enabled. Currently, if a device is reset during a resume and the driver does not have a reset_resume method, the driver won't receive any notification about the resume. Later kernels will call the driver's disconnect method; 2.6.23 doesn't do this. USB drivers are bound to interfaces, so their suspend and resume methods get called when the interfaces are suspended or resumed. In principle one might want to suspend some interfaces on a device (i.e., force the drivers for those interface to stop all activity) without suspending the other interfaces. The USB core doesn't allow this; all interfaces are suspended when the device itself is suspended and all interfaces are resumed when the device is resumed. It isn't possible to suspend or resume some but not all of a device's interfaces. The closest you can come is to unbind the interfaces' drivers. The driver interface for autosuspend and autoresume --------------------------------------------------- To support autosuspend and autoresume, a driver should implement all three of the methods listed above. In addition, a driver indicates that it supports autosuspend by setting the .supports_autosuspend flag in its usb_driver structure. It is then responsible for informing the USB core w
#ifndef __M68K_MMU_CONTEXT_H
#define __M68K_MMU_CONTEXT_H

#include <asm-generic/mm_hooks.h>

static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
{
}

#ifdef CONFIG_MMU

#if defined(CONFIG_COLDFIRE)

#include <asm/atomic.h>
#include <asm/bitops.h>
#include <asm/mcfmmu.h>
#include <asm/mmu.h>

#define NO_CONTEXT		256
#define LAST_CONTEXT		255
#define FIRST_CONTEXT		1

extern unsigned long context_map[];
extern mm_context_t next_mmu_context;

extern atomic_t nr_free_contexts;
extern struct mm_struct *context_mm[LAST_CONTEXT+1];
extern void steal_context(void);

static inline void get_mmu_context(struct mm_struct *mm)
{
	mm_context_t ctx;

	if (mm->context != NO_CONTEXT)
		return;
	while (atomic_dec_and_test_lt(&nr_free_contexts)) {
		atomic_inc(&nr_free_contexts);
		steal_context();
	}
	ctx = next_mmu_context;
	while (test_and_set_bit(ctx, context_map)) {
		ctx = find_next_zero_bit(context_map, LAST_CONTEXT+1, ctx);
		if (ctx > LAST_CONTEXT)
			ctx = 0;
	}
	next_mmu_context = (ctx + 1) & LAST_CONTEXT;
	mm->context = ctx;
	context_mm[ctx] = mm;
}

/*
 * Set up the context for a new address space.
 */
#define init_new_context(tsk, mm)	(((mm)->context = NO_CONTEXT), 0)

/*
 * We're finished using the context for an address space.
 */
static inline void destroy_context(struct mm_struct *mm)
{
	if (mm->context != NO_CONTEXT) {
		clear_bit(mm->context, context_map);
		mm->context = NO_CONTEXT;
		atomic_inc(&nr_free_contexts);
	}
}

static inline void set_context(mm_context_t context, pgd_t *pgd)
{
	__asm__ __volatile__ ("movec %0,%%asid" : : "d" (context));
}

static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
	struct task_struct *tsk)
{
	get_mmu_context(tsk->mm);
	set_context(tsk->mm->context, next->pgd);
}

/*
 * After we have set current->mm to a new value, this activates
 * the context for the new mm so we see the new mappings.
 */
static inline void activate_mm(struct mm_struct *active_mm,
	struct mm_struct *mm)
{
	get_mmu_context(mm);
	set_context(mm->context, mm->pgd);
}

#define deactivate_mm(tsk, mm) do { } while (0)

extern void mmu_context_init(void);
#define prepare_arch_switch(next) load_ksp_mmu(next)

static inline void load_ksp_mmu(struct task_struct *task