|author||Alan Stern <email@example.com>||2009-12-08 15:49:48 -0500|
|committer||Greg Kroah-Hartman <firstname.lastname@example.org>||2009-12-23 11:34:09 -0800|
USB: power management documentation update
This patch (as1313) updates the documentation concerning USB power management. Signed-off-by: Alan Stern <email@example.com> Signed-off-by: Greg Kroah-Hartman <firstname.lastname@example.org>
Diffstat (limited to 'Documentation')
2 files changed, 25 insertions, 34 deletions
diff --git a/Documentation/ABI/testing/sysfs-bus-usb b/Documentation/ABI/testing/sysfs-bus-usb
index deb6b489e4e..a07c0f366f9 100644
@@ -21,25 +21,27 @@ Contact: Alan Stern <email@example.com>
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
+ 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.
Date: May 2007
diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt
index c7c1dc2f801..3bf6818c8cf 100644
@@ -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
+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.