Link Power Management in USB 3.0 Hardware

Link Power Management in USB3.0 HardwareCommon Problems in Implementing U1 and U2StatesOctober 19, 2011AbstractThis paper provides information about certain limitations of the Universal Serial Bus(USB) 2.0 Selective Suspend mechanism. It provides guidelines for independenthardware vendors (IHVs) and original equipment manufacturers (OEMs) toimplement power management for USB devices by using Link Power Management(LPM) in conjunction with Selective Suspend. It also provides information aboutcommon pitfalls in LPM implementation in USB controllers, hubs, and devices.This paper assumes that the reader is familiar with:• The official USB 3.0 specification.• The USB Selective Suspend mechanism.The mechanism is described in the blog post, Demystifying USB Selective Suspend.This information applies to the following operating systems:The next version of Windows® (code-named Windows 8)The current version of this paper is maintained on the web at:Link Power Management in USB 3.0 HardwareDisclaimer: This document is provided “as-is”. Information and views expressed in this document, includingURL and other Internet website references, may change without notice. Some information relates to prereleasedproduct which may be substantially modified before it’s commercially released. Microsoft makes nowarranties, express or implied, with respect to the information provided here. You bear the risk of using it.Some examples depicted herein are provided for illustration only and are fictitious. No real association orconnection is intended or should be inferred.This document does not provide you with any legal rights to any intellectual property in any Microsoftproduct. You may copy and use this document for your internal, reference purposes.© 2011 Microsoft. All rights reserved.

Link Power Management in USB 3.0 Hardware - 2ContentsDocument HistoryDateChangeOctober 19, 2011 First publicationIntroduction.................................................................................................................... 3Limitations of USB 2.0 Mechanism.............................................................................3USB 3.0 LPM Mechanism............................................................................................ 3What Is a Link......................................................................................................... 3U States...................................................................................................................4Advantages of U1 and U2.......................................................................................4U1 and U2 Transitions.................................................................................................4Initial Setup by Software........................................................................................ 4Hardware Transitions............................................................................................. 5Common Hardware Problems with U1 or U2 Implementation..................................8Device-related Issues..............................................................................................8Hub or Controller-Related Issues........................................................................... 9Conclusion.................................................................................................................10Resources...................................................................................................................... 10October 19, 2011© 2011 Microsoft. All rights reserved.

Link Power Management in USB 3.0 Hardware - 4U StatesAs per the USB 2.0 specification, a link enters a low power state (consuming lesspower than the working state) only when the downstream device enters thesuspended state through the Selective Suspend mechanism. The USB 3.0 specificationdecouples link power states from device power states. The specification defines theLPM feature (see Section C.1 in the specification) that refers to power managementof physical and logical layers of a pair of ports that constitute a link. The specificationdefines four link power states known as U states, from U0 to U3. An active link is instate U0. After remaining idle for a certain period of time, link partners progressivelyenter U1 (standby with fast exit) and then U2 (standby with slower exit). After theyare idle for sufficient time, software initiates the transition to U3 by sending acommand to the DS port link partner. The steps that are required by software, tosend the link to U3, are identical to the steps required for USB 2.0 Selective suspend.The device must enter the suspended state when the link enters U3. As a result, thedevice is subject to similar restrictions as with USB 2.0 Selective Suspend. Toovercome those limitations, the USB 3.0 specification defines U1 and U2 states.Advantages of U1 and U2U1 and U2 states are designed to complement Selective Suspend, which can lead tosignificant power savings. After the software configures the link partners for U1 or U2transition, the hardware enters the states autonomously without any softwareintervention. The exit times from U1 and U2 are really fast (from microseconds to afew milliseconds) and have less of an impact on the performance of the devices. Thisallows for much better power management where links can enter and exit thesestates even when the device is in use. For example, a device with isochronousendpoints can put the link to U1 or U2 between service intervals. To save somepower, when the device is idle, it can send its upstream link to those states evenbefore Selective Suspend gets invoked. There are no restrictions on how much powerthe device can draw when the link is in U1 or U2. A device could remain fully poweredwhen the link is in U1 or U2. Therefore, unlike Selective Suspend, a device can sendits link to U1 or U2 without losing any capabilities.U1 and U2 TransitionsThis section first describes the initial setup that is done by the software to enable U1and U2 transitions and then describes how these transitions occur in the hardware.Initial Setup by SoftwareIn order for U1 or U2 transitions to occur, software performs the following stepsduring the enumeration of a device.1. Software exchanges U1 or U2 exit latency information with the device during theenumeration process. As the first part of this exchange, the device-specificlatencies are filled in by the device in bU1DevExitLat and wU2DevExitLat fields ofthe SuperSpeed USB device capability (defined in Section 9.6.2.2 of the USB 3.0specification). As the second part of the exchange, the host informs the deviceabout overall exit latencies for the device by sending a SET_SEL control transfer,as per section 9.4.12 of the USB 3.0 specification. The latency informationincludes the latencies that are associated with upstream links and controller.October 19, 2011© 2011 Microsoft. All rights reserved.

Link Power Management in USB 3.0 Hardware - 8to initiate transfers with other endpoints. In parallel, the hub initiates a U0 transitionon the link and then informs the device about the deferred packet. The device thensends ERDY to the host to indicate that the device is now ready for the transfer. Thehost can then reschedule the transfer to the device.An important responsibility of the device is that after sending ERDY, the device isresponsible for keeping the link in U0 until the host sends a response to ERDY or untilthe tERDYTimeout (500 milliseconds) time elapses. During that time, the device mustnot initiate a U1 or U2 transition and should also reject any transition initiated by itslink partner.Common Hardware Problems with U1 or U2 ImplementationLPM can significantly save power and lead to longer battery life. Therefore, it isimperative that both software and hardware should support LPM to its fullest extent.However, some of the early prototypes of USB 3.0 hardware have common problemsin the LPM implementation that can lead to poor end-user experience. The purposeof this section is to identify those problems.Device-related IssuesNo support for U1 or U2Some devices never initiate U1 and U2 transitions and always reject transitions thatare initiated by the host, even though there are no transfers for a long time and theperformance effect of LPM is not likely to be significant. Those devices not onlyprevent power savings for their link but also prevent any upstream links fromentering U1 or U2.Incorrect deferred packet implementationAs described in the Packet Deferring section, after a device has sent ERDY, the devicemust keep the link in U0 until the host sends a response to ERDY or tERDYTimeoutoccurs. Some devices fail to send ERDY after getting a deferred packet notification.This can lead to a problematic situation where a transfer never completes.Failure to send Ping.LPFS in U1The US port of the device should keep sending Ping.LPFS when the link is in U1. Somedevices fail to do that, which causes the link partner to assume that the device hasbeen removed. That can cause the link to enter an error state and can cause reenumerationof the device.Failure of SET_SEL transferThe software sends a SET_SEL control transfer to inform the device about the variousexit latencies from U1 and U2. Some devices stall that transfer. That can lead toenumeration failure or can lead to software not enabling U1 or U2 for the device.Failure of SET_FEATURE (U1_ENABLE or U2_ENABLE) transferThe software enables or disables the ability of the device to initiate a U1 or U2transition by sending a SET_FEATURE control transfer. Some devices stall thattransfer. This can lead to enumeration failure or the software not enabling U1 or U2for the device.October 19, 2011© 2011 Microsoft. All rights reserved.

Link Power Management in USB 3.0 Hardware - 9Hub or Controller-Related IssuesNo support of U1 or U2 timersOne of the most common problems with LPM implementation is the failure to initiateU1 or U2 transition when the timer expires. Even after the software has programmedU1 or U2 timeout values for DS ports, some hubs or controllers do not initiate atransition to U1 or U2 on the expiration of the timer. This behavior prevents powersavings through LPM.Hard-coded U1 or U2 time-out valuesSome host controllers support U1 and U2 transitions but have a hard-coded time-outvalue. Before this time-out, they do not initiate these transitions and rejecttransitions initiated by the link partner. This behavior results in missed opportunitiesfor U1 and U2 transitions and thus can prevent some power savings.Incorrect implementation of deferred packetAs described in the Packet Deferring section, hubs are responsible for sending thedeferred bit packet header back to the host that must process the packet, similar to aNRDY notification from the device. Some hubs fail to send the deferred packet to thehost or the device. Some hosts do not correctly process the deferred bit packet or resendthe transfer when the device ultimately sends ERDY. This leads to transferfailures and unreliable behavior.Not sending upstream port to U2 when no device connectedSome hubs fail to initiate a U1 or U2 transition for the US port when there are nodownstream devices connected. Some hubs send the link to U1 but do not send to U2in this scenario. This issue has been observed in many of the current shippingimplementations of USB 3.0 hardware, as of the release date of this paper. Thisbehavior prevents optimum power savings.Not transitioning the US port from U1 to U2Some hubs fail to transition the US port from U1 to U0 to U2 when the hub’s DS portstransition from U1 to U2 or a lower state. That occurs if the inactivity timer of the DSport to which the hub is connected is set to 0xFF. This behavior prevents optimumpower savings.Transition from U1/U2 to U3If a DS port of a hub or controller is in U1 or U2 and the software initiates a U3transition on the port, the parent hub or controller is responsible for firsttransitioning the link to U0 and then to U3. Some hubs and controllers do not handlethat requirement properly. This can cause the link to enter an error state and cancause re-enumeration of the device.October 19, 2011© 2011 Microsoft. All rights reserved.

Link Power Management in USB 3.0 Hardware - 10ConclusionResourcesIn this paper, we have discussed the LPM mechanism for saving power and describedvarious common problems seen in current USB 3.0 hardware. USB-IF certificationrequires that devices, hubs, and controllers implement U1 and U2 correctly. Thecertification aims at enforcing that requirement through compliance tests. TheMicrosoft USB driver stack (included with the next version of Windows®, code-namedWindows 8) takes full advantage of the U1 and U2 mechanism to achieve maximumpower savings. Therefore problems such as those described in this document will beseen more frequently. Those problems can lead to poor user experience and mightprevent Windows from achieving the power savings offered by the USB 3.0specification.Hardware vendors must take steps to avoid the issues that are described in thisdocument. For currently released hardware with problems, vendors should releaseupdated firmware with fixes as soon as possible and must work with their partners toensure that the updates are provided to end customers.The following list contains links to documents that contain related information.USB 3.0 Specificationhttp://go.microsoft.com/fwlink/?linkid=230961USB Selective Suspendhttp://go.microsoft.com/fwlink/?linkid=230964Demystifying USB Selective Suspendhttp://go.microsoft.com/fwlink/?linkid=230962October 19, 2011© 2011 Microsoft. All rights reserved.