USB: power management documentation update (baf67741) · Commits · e / devices / android_kernel_xiaomi_markw

Documentation/ABI/testing/sysfs-bus-usb

+10 −8

Original line number	Diff line number	Diff line
		@@ -21,25 +21,27 @@ Contact: Alan Stern <stern@rowland.harvard.edu>
		Description:
		Each USB device directory will contain a file named
		power/level. This file holds a power-level setting for
		the device, one of "on", "auto", or "suspend".
		the device, either "on" or "auto".

		"on" means that the device is not allowed to autosuspend,
		although normal suspends for system sleep will still
		be honored. "auto" means the device will autosuspend
		and autoresume in the usual manner, according to the
		capabilities of its driver. "suspend" means the device
		is forced into a suspended state and it will not autoresume
		in response to I/O requests. However remote-wakeup requests
		from the device may still be enabled (the remote-wakeup
		setting is controlled separately by the power/wakeup
		attribute).
		capabilities of its driver.

		During normal use, devices should be left in the "auto"
		level. The other levels are meant for administrative uses.
		level. The "on" level is meant for administrative uses.
		If you want to suspend a device immediately but leave it
		free to wake up in response to I/O requests, you should
		write "0" to power/autosuspend.

		Device not capable of proper suspend and resume should be
		left in the "on" level. Although the USB spec requires
		devices to support suspend/resume, many of them do not.
		In fact so many don't that by default, the USB core
		initializes all non-hub devices in the "on" level. Some
		drivers may change this setting when they are bound.

		What: /sys/bus/usb/devices/.../power/persist
		Date: May 2007
		KernelVersion: 2.6.23

Documentation/usb/power-management.txt

+15 −26

Original line number	Diff line number	Diff line
		@@ -71,12 +71,10 @@ being accessed through sysfs, then it definitely is idle.
		Forms of dynamic PM
		-------------------

		Dynamic suspends can occur in two ways: manual and automatic.
		"Manual" means that the user has told the kernel to suspend a device,
		whereas "automatic" means that the kernel has decided all by itself to
		suspend a device. Automatic suspend 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.
		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
		@@ -96,10 +94,11 @@ 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 suspend/resume (also triggered by
		userspace), and remote wakeup (triggered by the device). Internal
		events are those triggered within the USB stack: autosuspend and
		autoresume.
		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
		@@ -145,9 +144,9 @@ relevant attribute files are: wakeup, level, and autosuspend.
		number of seconds the device should remain idle before
		the kernel will autosuspend it (the idle-delay time).
		The default is 2. 0 means to autosuspend as soon as
		the device becomes idle, and -1 means never to
		autosuspend. You can write a number to the file to
		change the autosuspend idle-delay time.
		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 and writing "on" to power/level do
		essentially the same thing -- they both prevent the device from being
		@@ -377,9 +376,9 @@ the device hasn't been idle for long enough, a delayed workqueue
		routine is automatically set up to carry out the operation when the
		autosuspend idle-delay has expired.

		Autoresume attempts also can fail. This will happen if power/level is
		set to "suspend" or if the device doesn't manage to resume properly.
		Unlike autosuspend, there's no delay for an autoresume.
		Autoresume attempts also can fail, although failure would mean that
		the device is no longer present or operating properly. Unlike
		autosuspend, there's no delay for an autoresume.


		Other parts of the driver interface
		@@ -527,13 +526,3 @@ succeed, it may still remain active and thus cause the system to
		resume as soon as the system suspend is complete. Or the remote
		wakeup may fail and get lost. Which outcome occurs depends on timing
		and on the hardware and firmware design.

		More interestingly, a device might undergo a manual resume or
		autoresume during system suspend. With current kernels this shouldn't
		happen, because manual resumes must be initiated by userspace and
		autoresumes happen in response to I/O requests, but all user processes
		and I/O should be quiescent during a system suspend -- thanks to the
		freezer. However there are plans to do away with the freezer, which
		would mean these things would become possible. If and when this comes
		about, the USB core will carefully arrange matters so that either type
		of resume will block until the entire system has resumed.