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SubjectRe: ACPI "hlt" mode and SMP systems?

[ acpi list cc'ed for relevance ]

> I once documented all of the states in a readable format. I will post it
> once I get back from lunch...

As promised, here is the document, slightly updated. Presented as a patch,
for easy inlcusion into your favourite kernel tree..

-pat

diff -Nur linux-2.4.14.orig/Documentation/power/acpi-states.txt linux-2.4.14/Documentation/power/acpi-states.txt
--- linux-2.4.14.orig/Documentation/power/acpi-states.txt Wed Dec 31 16:00:00 1969
+++ linux-2.4.14/Documentation/power/acpi-states.txt Tue Nov 6 15:29:24 2001
@@ -0,0 +1,284 @@
+
+Quick and Dirty Guide to ACPI States
+
+Patrick Mochel
+<mochel@osdl.org>
+
+6 November 2001
+
+1. Global States (G0 - G3)
+2. Device Power States (D0 - D3)
+3. System Power States (S0 - S5)
+4. Processor Power States (C0 - C3)
+5. Performance States (P0 - Pn)
+6. References
+
+0.
+~~
+This is the result of spending many nights and mornings deciphering the ACPI
+spec, as well as ironing out the confusion with several people concerning the
+various letter and number combinations.
+
+This is meant to be un-bias. I personaly disagree with several of the ACPI
+design points and requirements. This is meant to be accurate only. Please
+email me if you find any discrepancies between this document and the spec.
+Feel free to email me if you want an opinionated dissertation as well..
+
+1. Global States (G0 - G3)
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+Global system states (G0 - G3) are logical states that are visible to
+the user. Global states are not states that the OS can explicitly
+transition to. They instead simply describe the current state of the
+system.
+
+G0 - Working
+ - The system is normal operating mode.
+ - Devices and processors may change power and performance states.
+ - The OS is "active and responsive".
+
+G1 - Sleeping
+ - The system is in an ACPI sleep state (S1 - S4).
+ - All devices are in a sleeping state.
+ - The processor is in a low power state, or is off.
+ - The operating system is suspended and not responding to any requests.
+ - All operating context has been preserved by the OS, so it can
+ resume execution from the exact point from which it left off.
+ - There is a latency involved in transitioning to the G0 state.
+
+G2 - Soft Off
+ - The OS has transitioned the system to the ACPI sleep state S5.
+ - Operating context is not preserved, so the OS must completely
+ reinitialise itself to get to G0.
+ - The ACPI spec states that it is _not_ safe to disassemble the
+ system at this point.
+ - The system may still be powered on via some wake event.
+
+G3 - Mechanical Off
+ - The system has been physically turned off.
+ This can happen via a mechanical switch, or removing the power cable.
+ On a laptop this is achieved by removing the battery from the system.
+ - Per spec, this is the only state in which it is safe to disassemble
+ the system.
+
+
+2. Device Power States (D0 - D3)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The ACPI definition of device power states is modeled directly after
+the PCI device power states. It defines four states (D0 - D3) which
+the device can be in.
+
+What each state means to a particular device is defined by its device
+class (e.g. video device class, sound device class). These definitions
+are covered in the ACPI spec, and I will not discuss them here.
+
+In general, as the numerical power state increases (from D0 to D1, etc),
+the following things happen:
+
+- Power consumption decreases
+- Device context retained by the device decreases
+- Latency to bring the device to the D0 state increases
+- Responsibility of the driver to have knowledge of the device state
+ increases.
+
+As with the PCI definitions of low power states, the Off state (D3)
+and the working state (D0) are supported by default by every
+device. The other two states (D1 and D2) are optionaly supported by
+the device class.
+
+Note that support of these two states are optional at a device level,
+as far as PCI is concerned. However, with some more recent peripheral buses
+(e.g. PCI-X), device support for all states is mandatory.
+
+Device states are defined as follows:
+
+D0 - On
+ - Device is powered on and running.
+ - It is accepting I/O requests.
+
+D1 -
+D2 - Optional intermediate power states
+ - The device is not accepting any I/O requests.
+ - The device is still consuming power, though it may be less than it does
+ in the D0 state.
+ - The device may have lost some state, as it may powered down various
+ components to conserve power.
+ - The driver is responsible for preserving and restoring this state if it
+ receives a request to transition the device to the D0 state.
+
+D3 - Off
+ - Device is not acception I/O requests.
+ - All device context is lost.
+ - The device must be completely reinitialised upon a transition to the D0
+ state.
+ - The device is not consuming any power.
+
+ (Note that the last point is not always necessarily true; the PCI Power
+ Management specs makes a distinction between D3 "Hot" -- when the device
+ is in D3, but power is still supplied to the slot -- and D3 "Cold" -- when
+ power has been removed from the controlling bus.)
+
+
+Relation to Global System States
+
+G0
+ - Devices are typically in the D0 state, though they may be technically be
+ in any state (D0 - D3). They may have been placed in a low power state
+ based on a system-wide power policy or via direct user control.
+G1
+ - Devices are in a low power state (D1 - D2). The exact state is dictated by
+ both what the device supports and by what the driver is capable of
+ restoring. (Some drivers may not have enough knowledge of the device to
+ restore all of its state.)
+G2 -
+ All devices are off.
+ Some devices may have auxillary power lines that allow the device to wake
+ the system up under certain circumstances (e.g. Wake-On-Lan).
+G3 -
+ All devices are off.
+ No device is consuming any power.
+
+
+3. System Sleep States (S0 - S5)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ACPI defines 6 states which the system can be in, S0 - S5.
+
+S0 is the working state. S5 is the soft-off state. S1 - S4 are intermediate
+sleeping states. Similar to device power states, there are tradeoffs in time
+and power consumption for each system sleep state.
+
+In general, as the numerical sleep state increases, the following are true:
+
+- Power consumption decreases
+- Context retained by system components decreases; the responsibility of the OS
+ to account for this context increases.
+- Latency to bring the system to the S0 state increases.
+
+System sleep states are defined as follows:
+
+S0 - On
+ - System is running.
+
+S1 - "Power On Suspend"
+ - Processor power, and hence execution context, is preserved.
+ - Devices may have been put into a low power state.
+
+S2 - "Pseudo-Suspend To Ram"
+ - Processor power is removed.
+ - Devices may be placed into low power state.
+ - Memory is placed into self-refresh and retains context.
+ - Execution starts again from processor's reset vector
+ - Cache and MTRR configuration is lost during this state; the
+ firmware is responsible for restoring it to some known state.
+
+S3 - "Suspend to Ram"
+ - Processor power is removed.
+ - Devices are placed into a low power state.
+ - Power may be removed from all system buses.
+ - Memory context is preserved by placing memory in Self-Refresh
+ - Execution begins from processor's reset vector
+ - Cache and MTRR configuration is lost during this state; the
+ firmware is responsible for restoring it to some known state.
+
+S4 - "Suspend to Disk", aka "Hibernate"
+ - All power may be removed from the system.
+ - All device context may be lost.
+ - Context is usually written to persistant storage (e.g. disk).
+ - Execution begins at processor's reset vector; the firmware
+ will usually load the OS boot loader.
+
+S5 - Soft Off
+ - Technically not a sleep state
+ - No context is retained
+ - OS is not responsible for saving context
+ - OS must completely reinitialise itself on 'resume' (power-on)
+ - Wake events may still trigger the system to resume from sleep.
+
+When the system is in a sleep state, it can be woken up via a 'wake event'.
+The type of wake events available to a user is dependent on the system
+components and the configuration.
+
+Relation to Global System States
+
+G0 -
+ The system is in the S0 state.
+G1 -
+ The system is in one of the S1 - S4 sleep states.
+G2 -
+ The system is in the S4 sleep state.
+G3 -
+ The system is completely off.
+
+Relation to Device Power States
+
+S0 -
+ Devices are typically in the D0 state, though they may be individually
+ powered down.
+S1 -
+ Devices should be placed in a low-latency low power state (i.e. D1) if
+ they support it.
+S2 -
+ Devices are placed in a low power state if they support it. The resume
+ latency D2 is commonly acceptable because of the savings in power.
+S3 -
+ Devices are placed in a low power state, and power is typically removed
+ from all buses.
+ Some devices may retain power and context, based on their function in the
+ system (e.g. A memory controller may retain power so it can safely
+ resume).
+S4 -
+S5 -
+ All devices are powered off.
+ Some residual power may be consumed by devices that support wake events
+ and need some amount of power to generate an interrupt.
+
+4. Processor States (C0 - C3)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ACPI defines 4 power states for processors. Each state is entered when the
+system is running (in the G0 state), but when the processor is idle. The
+longer the processor is idle, the deeper the sleep state it enters.
+
+The four processor states are:
+
+C0 - On
+ - The processor is executing instructions at this point
+
+C1 - "AutoHalt"
+ - The processor executes the equivalent to the "hlt" instruction
+ on x86.
+ - All processors must support this state.
+ - Latency should be transparent to Operating System.
+
+C2 - "Quick Start"
+ - Entered by reading from a system port.
+
+C3 - Deep Sleep
+ - Entered by reading from a system port.
+ - Caches may not be snooped in this state.
+
+The ACPI firmware tables (specifically the FADT table) tell what port to read
+from to enter C2 and C3. The table also contains the latency involved in
+returning from each state. (The processor states are supposed to nearly
+transparent to the OS; so, if a latency was deemed to be too high, the OS
+could simply choose not to enter it.)
+
+5. Performance States (P0 - Pn)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Performance states are a mechanism for processors and devices to
+compromise performance for lower power consumption while in the
+fully on state (C0 or D0).
+
+An example of this would be a video controller with a 3d graphics
+engine. When placed into a lower performance state, Px, the 3d
+graphics engine is disabled.
+
+By default, all devices support and operate in the P0 state. A device or
+processor may define a maximum of 16 performance states which the OS or an
+application may use.
+
+
+6. References
+~~~~~~~~~~~~~
+
+ACPI 2.0 Specification; esp. Ch 1 - 3.
+<http://acpi.info>
+
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