XO-1/Software specification

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XO-1 Software Specification

Introduction and Related Material

Careful stewardship of battery power is critical.

This page is a work in process, collecting information related to power management on OLPC.

• Hardware Power Domains
• Power Management Software
• XO Power Management Tips and Tricks
• Misc. Papers on Energy Efficiency in Computing and Networking

In addition to effectively managing the power that is in the battery. The Battery and power page discusses many alternate power sources that could be used to supplement the existing charger.

Linux's Approach to Power Control

Linux is highly cross platform, running on just about all significant architectures, including many that are used for battery powered embedded systems. The infrastructure for power management has therefore become quite sophisticated over the last years, though it is still maturing. This means that the facilities are general, and not tied to any particular architecture. The generation one OLPC system, being from the x86 part of the world, is therefore similar and fundamentally different from other x86 based systems, for reasons that will become clear in the discussion below.

Linux is not dependent on ACPI or the older APM power management systems, which are x86 specific. As such, Linux's design has always done its power control in the operating system, and ACPI and the like are considered "platform dependent".

OLPC's Innovations

The DCON chip lets us take over refresh of our very low power flat panel and therefore completely power off the processor board. Given our flat panel is usable in gray scale mode at .1 watt, you can see that the leakage currents and power supply power consumption of the system board can dominate power consumption easily.

We are also able to leave the Marvell wireless module to operate independently, forwarding packets in the mesh while possibly everything else is powered down.

Hardware Configuration

CPU Support

More information about the CPU can be found at Wikipedia and AMD.

• BTest-1 and BTest-2 systems both use the AMD Geode GX-400
• BTest-3 and later systems will all use the AMD Geode LX-700

AMD Geode™ CS5536 Companion Device

All XO-1's use the AMD 5536. Note that the processor and southbridge chips both have extensive facilities to either automatically or under program control save power. Examples include the ability to power down the GPU when not in use, and turn off the video output.

Memory Support

The XO-1 uses soldered down memory on the mother board and cannot be expanded.

• BTest-1 systems all have 256M of RAM and 512M of flash.
• BTest-2-1 systems all have 128M of RAM and 512M of flash.
• BTest-2-2 systems all have 256M of RAM and 512M of flash.
• BTest-3 and later systems will all have 256M of RAM and 1GB of flash.

Video RAM

Video display memory on the XO-1 is taken from main memory. It is set to 16 megabytes.

Keyboard Support

The physical keyboards are all identical on any XO-1; the firmware manufacturing information indicates which variant of keyboard is installed. We chose to use a 3.3V version of a PS/2 interface to save power. There is no provision for external PS/2 devices to be plugged in.

Keyboard Power

The keyboard and touchpad are powered up continuously when the system is in any but the powered down state (BTest-3 or later) to allow the keyboard to trigger a resume of the processor. An design oversight in BTest-1 and BTest-2 means the keyboard is not powered on those versions.

Keyboard Languages Support

Language support for a keyboard involves either three or four items:

• The keyboard engravings themselves
• XKB definitions for keyboard for the window system that defines the behavior of the keyboard. These are found in /usr/share/X11/xkb.
• A console mapping of the keyboard (generally simpler than the full X Window System keyboard definition, since the console is not fully internationalized.
• Possibly input methods for some languages (e.g. Chinese).

A single keyboard design may be capable of supporting multiple languages, and be able to switch from one language to the other.

At this time, keyboard designs have been completed for the following areas:

• Arabic
• Brazilian Portuguese
• French
• Nigeria (for English, Hausa, Yoruba)
• Spanish
• Thai
• Urdu
• US International (able to be used for many western European languages)

Which keyboard is installed is encoded in the manufacturing area of the firmware, and the correct keyboard language support installed on software installation.

Additional keyboard definitions are easy to generate: input methods for complex script input may be more involved (though many already exist).

Touchpad

The touch pad/tablet has provision to be "recalibrated" under program command as of BTest-3 (maybe also BTest-2-2). This readjusts the sensitivity of the capacitive sensor. Trac bug #1407 is being used to track implementation of this power related feature. As a temporary measure, recalibrating the touch pad can be forced manually.

Wireless Hardware

The XO-1 supports 88W8388+88W8015, 802.11b/g compatible; dual adjustable, rotating coaxial antennas; supports diversity reception. It also supports an implementation of what is the evolving 802.11s mesh network draft standard.

The power consumption of the Marvell wireless module has been measured at a bit over 300mw; even with power supply losses, we expect the batteries can power the wireless for > 40 hours (to be measured).

Since the "last kilometer" problem is so great, we are engineering the system to leave the wireless active for as much of the time as possible, since the wireless can run the mesh network autonomously. The module is capable of waking up the CPU via the embedded controller.

The wireless firmware dynamically adjusts transmit power; but signal processing in receive dominates power consumption. Marvell has done extensive work to minimize power consumption automatically.

Therefore, we expect to leave the wireless active in all modes except fully powered down (labeled state 1 below); this state also allows us to turn off the USB entirely as there is a signal from the wireless module that allows the XO-1 to be woken up by the wireless firmware.

Additionally, there needs to be an "airplane" mode to meet FAA and similar emissions requirements for on board airplane use, in which the wireless can be disabled. This will not be easy to access, by deliberate design. Bug #1406 is being used to track this issue.

Embedded Controller

The embedded controller is an ENE KB3700: File:KB3700-ds-01.pdf. It is used to charge the battery, emulate various legacy devices (e.g. PS/2), add more GPIO pins (since the Geode does not have enough pins, some signals have to be routed through the EC), boot the system (the SPI flash used to store the firmware is a serial ROM attached to the EC), wake up the system under various circumstances, and other miscellaneous functions. The EC Specification contains detailed information about the commands and protocol used to communicate with the EC. A number of buttons (game pad and buttons, etc.), are interfaced to the EC, and also generate scan codes as though they were keyboard keys, to simplify the programming interface. SCI events are also generated at times to inform the CPU of events, so that the XO-1 can avoid polling interfaces that would otherwise require periodic wakeups.

Status Indicators

The XO-1 has a number of status indicators; some of which are on both sides of the main unit.

The picture to the right

of a BTest-2 system has most of these, though some

will be used in a different fashion than the current use. The final production XO-1 systems will lack the keyboard lights in the picture and add indicator lights for the microphone and camera. A labeled picture of a BTest-3 system will be added as soon as possible (sometime in the last two weeks of May).

Wireless Lights

There are two wireless lights. One light looks roughly like an exclamation point, and the other like (*). These are used to indicate connectivity

• The ! LED is used to indicate association *and* connectivity via infrastructure mode.
• The (*) LED is used to indicate similar association *and* existence of a mesh portal.
• if neither is lit, then you are trying to use a mesh that is not connected.
• if both are lit, then you know you are a mesh portal for a mesh to the internet.

Note that this behavior has not been implemented yet, and will require work in the NetworkManager daemon, as it is probably the best place that knows if connectivity is available. See bug #1385 to track progress.

Power Indicator LED

This LED indicates the system is powered up. It is controlled by the embedded controller.

Battery LED

The battery LED indicates information about the battery.

• if the LED is green, it indicates the battery is fully charged.
• if the LED is orange, it indicates the battery is charging
• if the LED is red, it indicates the battery charge is critically low
• if the LED is red and flashing, it indicates an error in the battery charging system.

This LED is controlled by the embedded controller's battery charging logic.

Microphone LED

If the microphone is enabled, the microphone LED is lit. This is a hardware feature than cannot be circumvented.

Camera LED

If the camera is powered on, the camera LED will be lit. This is a hardware feature than cannot be circumvented.

Firmware (aka BIOS on conventional PC's)

Open Firmware

Rather than using a conventional BIOS and boot loader, as of the "C" series of our firmware we are solely using Open Firmware. We no longer use LinuxBIOS for the setup of our systems. This is a result of having implemented fast past resume from RAM; once you have implemented this, setup of the system initially is almost identical.

We have also removed VSA and VESA support (the Geode Virtual Systems Support) from our firmware. Since our PCI bus is fixed, we have no need for PCI configuration registers. Similarly, using the fbdev driver on Linux, we have no need for VESA emulation; while the PCI bus emulation was free software (AMD had generously made it available), the VESA emulation was the one part of VSA that they did not own, and so we did not have source for it. We did not want an unmaintainable binary blob in our firmware that we did not need anyway, and saves space in the flash for other purposes.

Fast Resume

Resume on our system is extremely fast: even without any serious attempt to optimize resume, we can resume from RAM in 160 milliseconds (mid-April). We believe the hardware limitations for resume are about 63 milliseconds on the B2 and before systems; B3 and later are probably similar. We will work in the future to further speed resume. Note that for most uses, 100ms is considered at the edge of human perception (e.g. typing).

System Identification

The Manufacturing Data page documents the Model ID string, part number, localization information, factory, BIOS version, and many other pieces of data.

Quiet Boot

Boot is not "quiet" at this time. Linux has facilities to make a splash screen at boot overlay the boot messages, but OLPC has not implemented this as yet. Bug #1394 tracks this issue for eventual resolution.

POST Message

The system gives an progression of iconic images across the top of the screen to indicate progress of the boot process.

Prior to Q2C10, if any key is pressed during the initial countdown after the initial message, the boot sequence is stopped and OFW's command interpreter invoked. After firmware version Q2C10, if a game key is pressed after power on, the automatic boot sequence is stopped.

OFW gives a cheery "ok" message to indicate it is ready for a command.

The initial line of text on the screen provides the version, amount of memory and serial number of the machine. The second line provides the the OpenFirmware version, the version of the firmware aggregate (OpenFirmware, embedded controller, etc.)

It then reports a number of progress messages.

Here is a sample:

OLPC B2, 128MB memory installed, S/N SHF70400BD
OpenFirmware CL1   Q2C06  Q2C

Interactive boot
Keyboard probe
USB probe
USB2 devices:
/pci/usb@f,5/wlan@3,0
USB1 devices
Type any key to interrupt automatic startup
type 'help' for more information

ok

Open Firmware Command Prompt

The OFW FAQ answers some of the most common questions of how to interact with the OLPC OFW firmware.

Boot Configuration

OFW can currently boot from the following media:

• NAND Flash (using the JFFS2 file system)
• SD (using EXT3 or FAT file systems)
• USB (using EXT3 or FAT file systems)
• Marvell Wireless (via infrastructure mode or mesh) See Trac 1431 for wireless boot and installation.

Device Tree

Hardware configuration information is exposed to Linux using the device tree facilities in OFW. This facility is also used on PPC and Sparc architectures. OLPC is working with others in the community using the device tree to try to come to a common implementation.

Firmware "Blob"

The firmware stored in the SCI boot rom interfaced to the CPU via the EC includes the following:

1. 64Kbytes of EC firmware
2. The OFW firmware system
3. A copy of the Marvell wireless firmware to enable network boot and installation
4. Manufacturing Data: serial number, keyboard type, keys, etc.
5. Icons and audio for the boot screen
6. Possibly, if there is space, a cut down OFW image for recovery in the case of problems with the firmware. We do not yet know if there will be sufficient space for this backup copy of OFW.

The build process collects these pieces and generates a single image for installation into the SPI flash.

System Management BIOS Interface

Not Supported

Security

Physical Security

Cable locks can easily be arranged by use of the handles, which have two holes and a carrying handle.

Firmware and Software Security

The firmware and system security and anti-theft system is covered in the Bitfrost specification.

The SPI flash for the XO-1 is being epoxied to the motherboard, to make it very difficult to remove the boot flash intact. The boot flash contents with their keys are the basis for the Bitfrost security model.

Firmware Recovery

Space permitting, OLPC hopes to have a (subset) copy of OFW in left-over flash space to be used if somehow the original is corrupted. We do not yet know if there will be sufficient space for a recovery copy; if it exists, it is clear it will have to be a subset of the full OFW.

Power Management Support

As discussed above, Linux does not depend on ACPI. To meet our fast resume goals and transparency into the firmware, we do not use ACPI, which would significantly slow our resume from suspend while adding no benefit. In this regard, we differ significantly from other x86 systems. This is the normal case for Linux on other architectures, so should not be regarded as unusual for Linux overall.

Power Management States

User Visible States

The following are the major operating states of the system. For simplicity's sake in using commonly understood terminology, see Wikipedia's ACPI article.

1. Powered Down: This state is called G3 in ACPI terminology Everything is off and the battery can be swapped. The operating system will have to be booted to start operation; the RAM contents are not preserved. If power is available, the EC will be powered up and potentially charging the battery.
2. Suspended, with Mesh Active, No screen: A common mode of use will be the system not being used, but still active forwarding in the mesh network on behalf of others in the mesh, but otherwise unused. This differs from our powered down state by the fact the Marvell wireless will be powered up, and active. In ACPI terminology, the closest match is G1/S3. The processor is suspended to RAM (in self-refresh).
3. Suspended, with Mesh Active, Screen Enabled: Another common mode of use is sometimes called "ebook mode". Both the screen and the Marvell wireless are left operational: the screen by use of the DCON chip. This differs from our powered down state by the fact the Marvell wireless will be powered up, and active along with the display. In ACPI terminology, the closest match is G1/S3. The processor is suspended to RAM (in self-refresh). Note that the DCON has facilities to implement a "screen saver" where it can disable itself and the backlight after a preset time without requiring the system to be resumed from RAM.
4. System fully operational: In this state, the system is available for normal use. The ACPI processor states that this corresponds to are C0 and C1 (note that C1 is not useful on a GX, but does save power on the LX).

Processor Power Management States

The Geode LX processor itself supports the following states (excerpt from the LX processor handbook):

• On (S0/C0): All internal and external clocks with respect to the AMD Geode LX processor are running and all functional blocks (CPU Core, Memory Controller, Display Controller, etc.) are actively generating cycles. This is equivalent to the ACPI specification’s “S0/C0” state.
• Active Idle (S0/C1): The CPU Core has been halted and all other functional blocks (including the Display Controller for refreshing the display) are actively generating cycles. This state is entered when a HLT instruction is executed by the CPU Core. From a user’s perspective, this state is indistinguishable from the On state and is equivalent to the ACPI specification’s “S0/C1” state.
• Sleep (S1): This is the lowest power state the AMD Geode LX processor can be in with voltage still applied to the device’s core and I/O supply pins. This is equivalent to the ACPI specification’s “S1” state. Given that the Geode LX does not drive the video output, and therefore a full DCON synchronization is required to get the display running, it is not clear that S1 will be useful for us (though it would allow computation to start faster than full suspend to RAM).

Power Button

The power button on OLPC serves as a power button.

Momentary Button Push

The system will suspend to RAM after the button is pressed momentarily. Wireless will be left operational when suspended this way. (Prior to deployment of suspend/resume, this button currently performs a clean Linux shutdown). See bug #1396 for more information.

Four Second Button Push

Pressing the power button for four seconds does a hard reset of the system and all state is lost.

Battery Subsystem

Battery charging is controlled by the embedded controller. Both NiMH and LiFePo battery chemistries are supported by the firmware. The current state of battery charge is reflected in the Home Mode display. It is also available in the developer's console.

Linux reflects the battery information into the /sys/class/battery/psu-0 directory. The files found in this directory include:

• capacity_percentage - percentage of battery remaining
• current - the current being subtracted/added to the battery
• device - link to ../../../../devices/platform/olpc-battery.0
• name - OLPC battery
• power
  • modaliasas - olpc-battery
  • power
  • psu_0
  • psu_1
  • subsystem - link to ../../../bus/platform
  • uevent
• status - e.g. present discharging or present charging discharging
• status-cap - capabilities e.g. present low full charging discharging overtemp critical
• subsystem - link to ../../../../class/battery
• technology - unknown?? (see bug #1408)
• temp1 - in millivolts
• temp1-name - battery
• temp2 - in millivolts
• temp2_name - ambient
• type - battery
• uevent
• voltage - voltage in

Linux reflects the line in information into the /sys/class/battery/psu-1 directory. The files found in this directory include:

• name - OLPC AC
• power
  • wakeup
• status - on-line or off-line
• status_cap capabilities on-line ?? (see bug #1409)
• subsystem link to ../../../../class/battery
• type - ac
• uevent

Note that Linux is in the process of revising its battery interface and that this interface will change to match that revision. We also have to convert the battery driver from indexed reads to recently defined EC commands. Trac #1430 will track this work.

Switches

There are a number of switches that are intended to help Linux know what the user is doing and take appropriate action. These are not hardwired in the firmware at all, but the following describes the expected behavior under Linux. This policy is typically controlled by a user level daemon process (trac #31).

Lid Close Switch

This switch indicates the laptop has been closed. Depending upon the policy set in the power manager, the system may:

• suspend to RAM, powering down everything except the wireless (default behavior), and corresponds to state 1, as described above.
• forcing the screen off, power down the DCON, the video unit, and the video drivers. In principle, the GPU could also be disabled and applications be asked to regenerate their contents on reenable, but we'll probably get most of the power benefit by disabling the GPU when not in use.

Ebook sense switch

The Ebook sense switch indicates that the hardware is configured as an ebook, though this is just a physical configuration rather than a requirement of what sort of application is running. Depending upon the chosen application, the behavior may differ.

An ebook reader itself will ask the power manager for aggressive power management. (State 3 as described above; wireless enabled, and screen in DCON driven mode). The screen will be on, being driven at 25 hz, with the DCON configured to turn the screen and backlight off after a short timeout.

Thermal Management

The CPU is protected by a fixed over-temperature cut-out at 85 degrees C. By using the ambient temperature reported by the battery system and the known temperature rise over ambient in the system enclosure, we hope to be able to warn users of possible overheating before running into this over temperature cut-out. The CPU can also be "throttled" when over temperature. There may be some ways of reducing internal heat generation, though experimentation of this will wait until a later date. Bug #1410 has been established to track this.

Note that different battery chemistries have limitations on the maximum temperature that they can be charged, and the system can report over temperature to programs via the /sys/class/battery/psu_0/status interface.

Video

Compression Buffer

The Geode has the capability of compressing the video data stream from RAM to the video processor. This could greatly reduce the memory bandwidth used, thereby saving power, and reducing memory contention, increasing system performance. The power versus RAM tradeoffs are not obvious, particularly with the DCON able to take over driving the flat panel, and this will need serious careful investigation with real power numbers and possibly real use data. Bug #1411 has been established to track work on determining if this feature is worthwhile for the XO-1.

Video Drivers

If the GPU is idle for some length of time, (say, several frame times), the data sheet makes clear it might be worth turning off the video drivers to the DCON and switch to DCON mode, to reduce power consumption. As always, we should measure the benefit: the cost here is the latency for the Geode to resume driving the display. We cannot presume the 37ma typical figure is relevant to the XO-1, as we are driving the DCON chip rather than a TFT panel directly.

Trac bug #1412 has been established to track this issue.

Video Input Port

Is not used, and should be disabled in the driver. See the GLD_MSR_PM Bit Descriptions in the Geode LX processor handbook.

Security Block

The security hardware block in the processor should be turned off whenever not in use.

Operating System Power Management

Linux is working very hard to remove "ticks"; the Linux kernel is now "tickless" and this is operational on OLPC, meaning that it no longer uses a periodic 250 hz timer clock interrupt to drive the scheduling of processes. The X0-1 has been observed at 40 wakeups per second. Work is underway in user space to abolish polling of hardware that might force wakeups, and private communications are that a full Gnome environment has been seen as low as only a few wakeups/second. Note: Independent of the CPU lower state, Linux may have many parts of the system powered down: e.g. audio, video output, etc. as described in detail below.

CPU and Support Chip

The CPU and 5536 automatically gate clocks when the system is idle on most hardware in both chips. Trac #1417 has been created to track verification of the configuration registers that control clock gating.

Given the progressively improving reduction in wakeups in Linux, use of the CPU's S1 is expected to be possible much of the time. We do not yet know how much power this may save. Trac #1416 has been created to track this measurement.

The additional possible savings under program control internal to the LX processor and 5536 are the GPU, video TFT panel drivers, and refresh rate.

The X Window System is in a good position to control the GPU: when the X server is idle, it can shut down the GPU, and drop the refresh rate (see below). Trac #1418 has been entered to track the GPU power management.

Device Power Down

Many external devices in the XO-1 can be powered down when not in use.

Screen Refresh Rate

Up to 100mw of power can be saved by controlling the refresh speed of the screen. This is very significant in Ebook mode.

Whenever the X server transitions from not drawing to drawing, the refresh rate should be increased to its normal 50hz at the next vertical retrace. Whenever the X server goes idle, the refresh rate should be dropped to a frame rate of (tbd) at the next vertical retrace. Trac #1419 has been established to track this behavior.

DCON mode

See above section about the video drivers.

Backlight

Given the back light can consume a significant portion of total power, particularly in Ebook mode, its management is important.

In Ebook mode (CPU suspended), there are registers in the DCON chip that allow for the backlight to be turned off automatically without processor intervention.

Recent Gnome versions have dbus messages defined to allow applications (such as DVD players) to temporarily disable the screen saver/DPMS functions of the system (e.g. the Totem video player). The XO-1 power manager and activities should follow these conventions.

The backlight is controlled by the X DPMS extension Trac #1429 has been established to track its implementation..

Audio

The Analog Devices AD1888 can disable all sections it is not using. As it can drive 5.1 audio output, it is important that we verify that all but the stereo channels are disabled.

It also has a "deep sleep" feature, in which it can be entirely disabled. If its device driver is closed, the AD1888 should be put into deep sleep, and restored when the device opened. Trac #1420 has been established to track this verification.

We also need to audit our audio applications that they close the audio device when idle. Trac #1421 has been established to track this verification.

USB

USB and power management are "iffy" at best. Our power manager will probably need to have an option to indicate whether STR (suspend to RAM) should be performed if USB devices are plugged in or not.

We also need to verify that we are properly disabling USB as we suspend and resume. Trac #1423 has been established to track this issue.

Keyboard/Touchpad

At this time, it does not appear that the keyboard and touchpad consume enough power (a few milliwatts) to be a significant battery drain in Ebook mode. This may need to be revisited when we transition to a new lower power wireless module. In BTest-2 and BTest-1, it was inadvertently on the wrong power rail that is not powered during suspend: this is fixed in B3.

SD

The SD slot and CaFE chip section should be powered down whenever the device is closed. Trac #1426 has been established to track verification of this behavior.

Camera

An LED is in series electrically with the camera (B3 and later) to guarantee that users will know if the camera is enabled. The camera and that segment of the CaFE' chip should be disabled when the device driver is closed. Trac 1424 has been established to track this issue.

Camera based activities should be audited that they close the device when idle or when not the current activity; eventual automation of this for standard activities in the tinderbox would be desirable if we can arrange it. Trac #1425 has been established to track this task.

Special Function Keys

One Touch Buttons

The Sugar Instructions documents the function of buttons in the Sugar user interface. Note that there are no built in function keys that are built into the BIOS firmware as there may be on a conventional PC: instead, all keys and buttons are left to the user interface software for interpretation. The sole exception to this rule is the power button: by holding the button down for four seconds you can force a hard reset of the XO-1 system.

The two sets of keys which directly affect power consumption are the "volume" control keys for the audio, and the back light control keys.