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Power Management Operation
Power management of the PCH SATA controller and ports will cover operations of the host controller and the SATA link.
Power State Mappings
The D0 PCI Power Management (PM) state for device is supported by the PCH SATA controller.
SATA devices may also have multiple power states. SATA adopted 3 main power states from parallel ATA. The three device states are supported through ACPI. They are:
- D0 – Device is working and instantly available.
- D1 – Device enters when it receives a STANDBY IMMEDIATE command. Exit latency from this state is in seconds.
- D3 – From the SATA device’s perspective, no different than a D1 state, in that it is entered using the STANDBY IMMEDIATE command. However, an ACPI method is also called which will reset the device and then cut its power.
Each of these device states are subsets of the host controller’s D0 state.
Finally, the SATA specification defines three PHY layer power states, which have no equivalent mappings to parallel ATA. They are:
- PHY READY – PHY logic and PLL are both on and in active state.
- Partial – PHY logic is powered up, and in a reduced power state. The link PM exit latency to active state maximum is 10 ns.
- Slumber – PHY logic is powered up, and in a reduced power state. The link PM exit latency to active state maximum is 10 ms.
- Devslp – PHY logic is powered down. The link PM exit latency from this state to active state maximum is 20 ms, unless otherwise specified by DETO in Identify Device Data Log page 08h (Refer SATA Rev3.2 Gold specification).
Since these states have much lower exit latency than the ACPI D1 and D3 states, the SATA controller specification defines these states as sub-states of the device D0 state.
Power State Transitions
Partial and Slumber State Entry/Exit
The partial and slumber states save interface power when the interface is idle. It would be most analogous to CLKRUN# (in power savings, not in mechanism), where the interface can have power saved while no commands are pending. The SATA controller defines PHY layer power management (as performed using primitives) as a driver operation from the host side, and a device proprietary mechanism on the device side. The SATA controller accepts device transition types, but does not issue any transitions as a host. All received requests from a SATA device will be ACKed.
When an operation is performed to the SATA controller such that it needs to use the SATA cable, the controller must check whether the link is in the Partial or Slumber states, and if so, must issue a COMWAKE to bring the link back online. Similarly, the SATA device must perform the same COMWAKE action.
DEVSLP State Entry/Exit
Device Sleep (DEVSLP) is a host-controlled SATA interface power state. To support a hardware autonomous approach that is software agnostic Intel is recommending that BIOS configure the AHCI controller and the device to enable Device Sleep. This allows the AHCI controller and associated device to automatically enter and exit Device Sleep without the involvement of OS software.
To enter Device Sleep the link must first be in Slumber. By enabling HIPM (with Slumber) or DIPM on a Slumber capable device, the device/host link may enter the DevSleep Interface Power state.
The device must be DevSleep capable. Device Sleep is only entered when the link is in slumber, therefore when exiting the Device Sleep state, the device must resume with the COMWAKE out-of-band signal (and not the COMINIT out-of-band signal). Assuming Device Sleep was asserted when the link was in slumber, the device is expected to exit DEVSLP to the DR_Slumber state. Devices that do not support this feature will not be able to take advantage of the hardware automated entry to Device Sleep that is part of the AHCI 1.3.1 specification and supported by Intel platforms.
Device D1 and D3 States
These states are entered after some period of time when software has determined that no commands will be sent to this device for some time. The mechanism for putting a device in these states does not involve any work on the host controller, other then sending commands over the interface to the device. The command most likely to be used in ATA/ATAPI is the “STANDBY IMMEDIATE” command.
Host Controller D3HOT State
After the interface and device have been put into a low power state, the SATA host controller may be put into a low power state. This is performed using the PCI power management registers in configuration space. There are two very important aspects to note when using PCI power management.
- When the power state is D3, only accesses to configuration space are allowed. Any attempt to access the memory or I/O spaces will result in master abort.
- When the power state is D3, no interrupts may be generated, even if they are enabled. If an interrupt status bit is pending when the controller transitions to D0, an interrupt may be generated.
When the controller is put into D3, it is assumed that software has properly shut down the device and disabled the ports. Therefore, there is no need to sustain any values on the port wires. The interface will be treated as if no device is present on the cable, and power will be minimized.
When returning from a D3 state, an internal reset will not be performed.
Low Power Platform Consideration
When low power feature is enabled, the Intel SATA controller may power off PLLs or OOB detection circuitry while in the Slumber link power state. As a result, a device initiated wake may not be recognized by the host. For example, when the low power feature is enabled it can prevent a Zero Power ODD (ZPODD) device from successfully communicating with the host on media insertion.
The SATA MPHY Dynamic Power Gating (PHYDPGEPx) can be enabled/disabled for each SATA ports.