Messages in this thread Patch in this message | | | From | David Brownell <> | Subject | [PATCH 2.6.9-rc1-bk+] update Documentation/power/pci.txt | Date | Wed, 8 Sep 2004 10:46:24 -0700 |
| |
That document was wrong on some things, misleading on others; this fixes some of the issues I noticed.
However it probably needs to say that drivers for devices that implement the PCI PM spec "should" always use pci_set_power_state() to reduce the power usage. If I get ambitions I might submit a patch to the PCI core to print a nag message for drivers that don't do that.
- Dave
Updates the PCI PM docs, better matching the specs and code.
- List both D3 states (D3hot, D3cold) up front.
- Clarify that suspend() methods should disable I/0 (including DMA) and IRQs; it's not optional.
- More accurately describe resume(); there are common cases where device re-initialization isn't appropriate. The previous text said re-init was always required; that's false.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
--- 1.4/Documentation/power/pci.txt Mon Jun 14 09:54:41 2004 +++ edited/Documentation/power/pci.txt Wed Sep 8 09:41:05 2004 @@ -5,6 +5,7 @@ An overview of the concepts and the related functions in the Linux kernel Patrick Mochel <mochel@transmeta.com> +(and others) --------------------------------------------------------------------------- @@ -31,10 +32,15 @@ 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: @@ -204,15 +210,16 @@ 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); @@ -223,7 +230,7 @@ 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 ------ @@ -237,16 +244,28 @@ 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(). -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. + * 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. + +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: @@ -263,7 +282,7 @@ 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 | |