power-management.txt

linux 内核源代码

	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 whenever one of its interfaces becomes busy or idle.  The
driver does so by calling these three functions:

	int  usb_autopm_get_interface(struct usb_interface *intf);
	void usb_autopm_put_interface(struct usb_interface *intf);
	int  usb_autopm_set_interface(struct usb_interface *intf);

The functions work by maintaining a counter in the usb_interface
structure.  When intf->pm_usage_count is > 0 then the interface is
deemed to be busy, and the kernel will not autosuspend the interface's
device.  When intf->pm_usage_count is <= 0 then the interface is
considered to be idle, and the kernel may autosuspend the device.

(There is a similar pm_usage_count field in struct usb_device,
associated with the device itself rather than any of its interfaces.
This field is used only by the USB core.)

The driver owns intf->pm_usage_count; it can modify the value however
and whenever it likes.  A nice aspect of the usb_autopm_* routines is
that the changes they make are protected by the usb_device structure's
PM mutex (udev->pm_mutex); however drivers may change pm_usage_count
without holding the mutex.

	usb_autopm_get_interface() increments pm_usage_count and
	attempts an autoresume if the new value is > 0 and the
	device is suspended.

	usb_autopm_put_interface() decrements pm_usage_count and
	attempts an autosuspend if the new value is <= 0 and the
	device isn't suspended.

	usb_autopm_set_interface() leaves pm_usage_count alone.
	It attempts an autoresume if the value is > 0 and the device
	is suspended, and it attempts an autosuspend if the value is
	<= 0 and the device isn't suspended.

There also are a couple of utility routines drivers can use:

	usb_autopm_enable() sets pm_usage_cnt to 1 and then calls
	usb_autopm_set_interface(), which will attempt an autoresume.

	usb_autopm_disable() sets pm_usage_cnt to 0 and then calls
	usb_autopm_set_interface(), which will attempt an autosuspend.

The conventional usage pattern is that a driver calls
usb_autopm_get_interface() in its open routine and
usb_autopm_put_interface() in its close or release routine.  But
other patterns are possible.

The autosuspend attempts mentioned above will often fail for one
reason or another.  For example, the power/level attribute might be
set to "on", or another interface in the same device might not be
idle.  This is perfectly normal.  If the reason for failure was that
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.


	Other parts of the driver interface
	-----------------------------------

Sometimes a driver needs to make sure that remote wakeup is enabled
during autosuspend.  For example, there's not much point
autosuspending a keyboard if the user can't cause the keyboard to do a
remote wakeup by typing on it.  If the driver sets
intf->needs_remote_wakeup to 1, the kernel won't autosuspend the
device if remote wakeup isn't available or has been disabled through
the power/wakeup attribute.  (If the device is already autosuspended,
though, setting this flag won't cause the kernel to autoresume it.
Normally a driver would set this flag in its probe method, at which
time the device is guaranteed not to be autosuspended.)

The usb_autopm_* routines have to run in a sleepable process context;
they must not be called from an interrupt handler or while holding a
spinlock.  In fact, the entire autosuspend mechanism is not well geared
toward interrupt-driven operation.  However there is one thing a
driver can do in an interrupt handler:

	usb_mark_last_busy(struct usb_device *udev);

This sets udev->last_busy to the current time.  udev->last_busy is the
field used for idle-delay calculations; updating it will cause any
pending autosuspend to be moved back.  The usb_autopm_* routines will
also set the last_busy field to the current time.

Calling urb_mark_last_busy() from within an URB completion handler is
subject to races: The kernel may have just finished deciding the
device has been idle for long enough but not yet gotten around to
calling the driver's suspend method.  The driver would have to be
responsible for synchronizing its suspend method with its URB
completion handler and causing the autosuspend to fail with -EBUSY if
an URB had completed too recently.

External suspend calls should never be allowed to fail in this way,
only autosuspend calls.  The driver can tell them apart by checking
udev->auto_pm; this flag will be set to 1 for internal PM events
(autosuspend or autoresume) and 0 for external PM events.

Many of the ingredients in the autosuspend framework are oriented
towards interfaces: The usb_interface structure contains the
pm_usage_cnt field, and the usb_autopm_* routines take an interface
pointer as their argument.  But somewhat confusingly, a few of the
pieces (usb_mark_last_busy() and udev->auto_pm) use the usb_device
structure instead.  Drivers need to keep this straight; they can call
interface_to_usbdev() to find the device structure for a given
interface.


	Locking requirements
	--------------------

All three suspend/resume methods are always called while holding the
usb_device's PM mutex.  For external events -- but not necessarily for
autosuspend or autoresume -- the device semaphore (udev->dev.sem) will
also be held.  This implies that external suspend/resume events are
mutually exclusive with calls to probe, disconnect, pre_reset, and
post_reset; the USB core guarantees that this is true of internal
suspend/resume events as well.

If a driver wants to block all suspend/resume calls during some
critical section, it can simply acquire udev->pm_mutex.
Alternatively, if the critical section might call some of the
usb_autopm_* routines, the driver can avoid deadlock by doing:

	down(&udev->dev.sem);
	rc = usb_autopm_get_interface(intf);

and at the end of the critical section:

	if (!rc)
		usb_autopm_put_interface(intf);
	up(&udev->dev.sem);

Holding the device semaphore will block all external PM calls, and the
usb_autopm_get_interface() will prevent any internal PM calls, even if
it fails.  (Exercise: Why?)

The rules for locking order are:

	Never acquire any device semaphore while holding any PM mutex.

	Never acquire udev->pm_mutex while holding the PM mutex for
	a device that isn't a descendant of udev.

In other words, PM mutexes should only be acquired going up the device
tree, and they should be acquired only after locking all the device
semaphores you need to hold.  These rules don't matter to drivers very
much; they usually affect just the USB core.

Still, drivers do need to be careful.  For example, many drivers use a
private mutex to synchronize their normal I/O activities with their
disconnect method.  Now if the driver supports autosuspend then it
must call usb_autopm_put_interface() from somewhere -- maybe from its
close method.  It should make the call while holding the private mutex,
since a driver shouldn't call any of the usb_autopm_* functions for an
interface from which it has been unbound.

But the usb_autpm_* routines always acquire the device's PM mutex, and
consequently the locking order has to be: private mutex first, PM
mutex second.  Since the suspend method is always called with the PM
mutex held, it mustn't try to acquire the private mutex.  It has to
synchronize with the driver's I/O activities in some other way.


	Interaction between dynamic PM and system PM
	--------------------------------------------

Dynamic power management and system power management can interact in
a couple of ways.

Firstly, a device may already be manually suspended or autosuspended
when a system suspend occurs.  Since system suspends are supposed to
be as transparent as possible, the device should remain suspended
following the system resume.  The 2.6.23 kernel obeys this principle
for manually suspended devices but not for autosuspended devices; they
do get resumed when the system wakes up.  (Presumably they will be
autosuspended again after their idle-delay time expires.)  In later
kernels this behavior will be fixed.

(There is an exception.  If a device would undergo a reset-resume
instead of a normal resume, and the device is enabled for remote
wakeup, then the reset-resume takes place even if the device was
already suspended when the system suspend began.  The justification is
that a reset-resume is a kind of remote-wakeup event.  Or to put it
another way, a device which needs a reset won't be able to generate
normal remote-wakeup signals, so it ought to be resumed immediately.)

Secondly, a dynamic power-management event may occur as a system
suspend is underway.  The window for this is short, since system
suspends don't take long (a few seconds usually), but it can happen.
For example, a suspended device may send a remote-wakeup signal while
the system is suspending.  The remote wakeup may succeed, which would
cause the system suspend to abort.  If the remote wakeup doesn't
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.