An overview of the concepts and the related functions in the Linux kernel
Patrick Mochel <mochel@transmeta.com>
+(and others)
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the higher the number, the longer the latency is for the device to return to
an operational state (D0).
+There are actually two D3 states. When someone talks about D3, they usually
+mean D3hot, which corresponds to an ACPI D2 state (power is reduced, the
+device may lose some context). But they may also mean D3cold, which is an
+ACPI D3 state (power is fully off, all state was discarded); or both.
+
Bus power management is not covered in this version of this document.
-Note that all PCI devices support D0 and D3 by default, regardless of whether or
-not they implement any of the PCI PM spec.
+Note that all PCI devices support D0 and D3cold by default, regardless of
+whether or not they implement any of the PCI PM spec.
The possible state transitions that a device can undergo are:
dev->driver->suspend(dev,state);
A driver uses this function to actually transition the device into a low power
-state. This may include disabling I/O, memory and bus-mastering, as well as
-physically transitioning the device to a lower power state.
+state. This should include disabling I/O, IRQs, and bus-mastering, as well as
+physically transitioning the device to a lower power state; it may also include
+calls to pci_enable_wake().
Bus mastering may be disabled by doing:
pci_disable_device(dev);
For devices that support the PCI PM Spec, this may be used to set the device's
-power state:
+power state to match the suspend() parameter:
pci_set_power_state(dev,state);
obviate the need for some operations.
The driver should update the current_state field in its pci_dev structure in
-this function.
+this function, except for PM-capable devices when pci_set_power_state is used.
resume
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transition the device to the D0 state.
The driver is responsible for reenabling any features of the device that had
-been disabled during previous suspend calls and restoring all state that was
-saved in previous save_state calls.
+been disabled during previous suspend calls, such as IRQs and bus mastering,
+as well as calling pci_restore_state().
+
+If the device is currently in D3, it may need to be reinitialized in resume().
+
+ * Some types of devices, like bus controllers, will preserve context in D3hot
+ (using Vcc power). Their drivers will often want to avoid re-initializing
+ them after re-entering D0 (perhaps to avoid resetting downstream devices).
+
+ * Other kinds of devices in D3hot will discard device context as part of a
+ soft reset when re-entering the D0 state.
+
+ * Devices resuming from D3cold always go through a power-on reset. Some
+ device context can also be preserved using Vaux power.
+
+ * Some systems hide D3cold resume paths from drivers. For example, on PCs
+ the resume path for suspend-to-disk often runs BIOS powerup code, which
+ will sometimes re-initialize the device.
-If the device is currently in D3, it must be completely reinitialized, as it
-must be assumed that the device has lost all of its context (even that of its
-PCI config space). For almost all current drivers, this means that the
-initialization code that the driver does at boot must be separated out and
-called again from the resume callback. Note that some values for the device may
-not have to be probed for this time around if they are saved before entering the
-low power state.
+To handle resets during D3 to D0 transitions, it may be convenient to share
+device initialization code between probe() and resume(). Device parameters
+can also be saved before the driver suspends into D3, avoiding re-probe.
If the device supports the PCI PM Spec, it can use this to physically transition
the device to D0:
ensure correct (and speedy) operation.
The driver should update the current_state field in its pci_dev structure in
-this function.
+this function, except for PM-capable devices when pci_set_power_state is used.
enable_wake
git://git.onelab.eu / linux-2.6.git / blobdiff