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1. This series of CPUs is designed for system on chip embedded applications As the name suggests these processors are implemen tations of the PowerPC Architecture however they have some notable differences from classic PowerPC CPUs as used in IBM pSeries servers and Apple workstations The 4xx CPUs operate at much lower clock rates and hence are cooler and cheaper although they are in the high end by embedded standards They have a much simpler MMU just a software loaded TLB and they have no floating point unit More interestingly the 4xx CPUs include a num ber of peripheral devices built into the CPU die itself hence the term system on chip Differ ent CPUs in the family are designed for differ ent applications and so have different sets of on chip peripherals The 405LP is aimed at handheld battery powered applications and includes an LCD controller PCMCIA Compact Flash controller and real time clock along with more general purpose de vices Other 4xx chips can include devices such as Ethernet controllers HDLC interfaces PCI host bridges IDE and USB controllers The 405LP also includes a number of features to reduce power consumption some of which are quite novel The chip s base power consumption is already quite low it will run at 266MHz with no heatsink or fan The various on chip devices can be individually turned on and off saving power when they are not in use In addition the CPU
2. development sled is attached over Ethernet The kernel image will usually include an initial ramdisk with a basic userland for the device The startup scripts will then usually mount a JFFS2 filesystem which takes up the remainder of the flash memory and contains the rest the userspace programs and data For development it is also possible to use an 115 covers Linux 4xx support more generally PowerPC 405LP PDA Reference Design eLAP Debug Sled 405LP DE _ SDRAM 32MB SDRAM Buffers d PCMCIA aes a st External Bus 1O pP 5C P ee Ethernet MAC Ethernet i 1 s a USB Host is 1 tas LE u Sd c SDIO 1 I Vedic LED Ctrl Tricolour LED 1 1 FPGA W aml eee Buttons I T i i 32MB NOR Flash TCPA 1 I i LCD Panel l rog o t ee Battery ADC Frontlight Ctrl Frontlight iE i a 1 H Touchpanel I ae a a Speakers i AE Audio Codec ji 1 O Microphone 1 l i ffen Line in out l DS TS Ctrl 1 oO 1 S I 1 ie RS232 D 8 i ig 5 E PLE 1 OPB External connection l E External Bus 1 12C Bus Physical device arie DCR Bu
3. restore routine at a fixed address in memory When the device is pow ered on again the PIBS firmware checks the flag to determine that the device has been suspended and instead of booting normally passes control to the Linux restore routine This restores the register state from memory and resumes running In order for this to work properly device drivers also need to shut down devices before suspending then re initialise them and restore their state after resume The most important eLAP drivers such as the LCDC driver do this already but some other drivers still need to have suspend support added Similarly some drivers already support powering the device they control off when possible to save power Other drivers still need to be enhanced to save power in this manner The power to the on chip devices is controlled by the Clock and Power Management CPM core while power to most of the off chip devices is controlled by registers in the FPGA or by GPIO lines 3 2 1 Dynamic Power Management Of course by far the most interesting aspect of power management on the 405LP is its ability to adjust operating frequency and voltage while run ning Linux support for this is under continuing development by IBM and MontaVista This and other means of saving power while the machine is running are known as dynamic power manage ment DPM This simplest way of using this capability is to allow the user to explicitly select a particul
4. Linux on the eLAP David Gibson OzLabs IBM Linux Technology Center lt dwg aul ibm com gt lt ukuug gibson dropbear id au gt Abstract The IBM PowerPC 405LP is an embedded CPU designed especially for handheld applications Like other PowerPC 4xx CPUs it includes a number of peripheral devices built into the CPU die itself such as an LCD controller PCMCIA controller and serial ports The 405LP also includes a number of novel power management features As well as being able to power on and off the various chip compo nents as necessary it can adjust the CPU core and internal bus frequencies with very low latency al lowing the the chip to provide high performance when necessary but low power operation the rest of the time The IBM PowerPC 405LP PDA Reference De sign also known as the Embedded Linux Applica tion Platform or eLAP is a sample board made by IBM based on the 405LP see 4 or 7 It is de signed to run Linux and demonstrate the 405LP s capabilities It is expected that handheld devices derived from this design will be made and sold by third parties The hardware in this device is very different from a typical PC or server so we exam ine what is involved in getting Linux to deal with the device s peculiarities and take advantage of the hardware s features We pay particular attention to power management 1 The hardware 1 1 The CPU The IBM PowerPC 405LP is a CPU from the Pow erPC 4xx family
5. a chine They are neither standard devices for the architecture nor devices found on a standard bus such as PCI which can be systematically probed At the moment the kernel locates most of these devices by ad hoc methods Many of the drivers are eLAP specific and just know that a particular device exists at a particular address This approach quickly becomes messy as the kernel supports more embedded machines especially when a particular peripheral is included in multiple embedded devices connected in slightly different ways The 4xx code in the linuxppc_2_4_devel tree includes a partial solution to this problem for the on chip devices the OCP for On Chip Periph eral subsystem uses a table of devices for a partic ular CPU to properly initialise each of the drivers which can be common across multiple 4xx CPUs However this system has a number of implemen tation problems and an improved and consolidated approach handling both on and off chip embedded devices is needed 6 covers this topic in more de tail Finally the eLAP power management code is quite 405LP specific at the moment However it has a certain amount of overlap with the function ality of the cpufreq mechanism The techniques used on the eLAP are also likely to be applicable to more embedded machines in the future At some point thus the power management code will need consolidation into a flexible and portable dynamic power management fra
6. ar fre quency and voltage mode and this mode of oper ation is already supported This approach is anal ogous to that used by the cpufreq project which allows frequency adjustment on a number of other CPUs although the 405LP implementation is for now independent of the cpufreq infrastructure However this fails to really make use of the 405LP s abilities because the 405LP can adjust frequency with particularly low latency we want to take ad vantage of this by automatically adjusting the fre quency to match runtime requirements How to best automatically manage frequency and voltage scaling is still under active investigation A promising scheme developed at IBM s Austin Re search Lab is described in 2 and partially imple mented for the eLAP The essence of this approach is for the kernel to keep track of which operating state it is in at any moment The operating states include things such as idle executing user code executing an interrupt handler and so forth If different pro cesses on the system have different compute or IO requirements they could have different operating states e g IO intensive task or compute inten sive task When the operating state changes the kernel consults a DPM policy to map the new state to an operating point that is a set of frequency and voltage parameters The DPM policy can itself be changed at runtime in order to adapt to longer te
7. clock speed and the frequencies of the several internal buses can be adjusted while the CPU is operating This allows the OS to adjust the CPU s compute and IO performance allowing a high peak perfor mance when necessary but keeping power usage to a minimum the rest of the time This adjustment can be done quickly microseconds in many cases and without disrupting the running system which allows the system to take advantage of even brief periods of inactivity to save power The 405LP can also operate at a number of differ ent voltages and the voltage can also be adjusted while the CPU is running although with much higher latency With appropriate board level sup port this allows for further savings in power con sumption Power usage is directly proportional to the frequency but to the square of the oper ating voltage for a fixed frequency The maxi mum frequency is roughly proportional to the volt age Thus voltage scaling can achieve much greater power savings than frequency adjustment alone 1 2 The eLAP The IBM PowerPC 405LP PDA Reference Design also known as the Embedded Linux Application Platform or eLAP is as the name suggests a handheld design based on the 405LP and designed to demonstrate the CPU s capabilities in a hand held device Figure 1 shows a block diagram of the eLAP including the devices built into the 405LP itself In addition to the devices within the 405LP the eLAP includes some RAM and
8. d uses one of these UARTO for the console Of course the console is only expected to be used for development and debug The kernel also has full support for the 405LP s LCD controller framebuffer device and the Semtech touchscreen controller attached to the second serial port The normal interface to the device is through the Qtopia GUI running on the handheld s built in LCD dis play 2 2 Bells and whistles In addition to the devices needed for basic op eration most of the peripherals on the 405LP and the rest of the eLAP are supported in the linuxppc_2_4 devel tree The 405LP s real time clock and GPIO lines are fully supported as is its PCCF PCMCIA and Compact Flash controller which drives the PCMCIA slot in the eLAP s de velopment sled Likewise the I C interface IIC is supported as are the eLAP s I C devices The TI audio codec which is controlled via I C is supported in conjunction with the 405LP s CSI to form a complete audio device The eLAP s buttons and its multicolour LED accessed via a custom FPGA are both supported as is the RTL8019 Ethernet on the development sled The Phillips USB chip is partially supported There is a driver for the host controller side of the chip although this has not been merged into the linuxppc_2_4 devel tree it is in MontaVista s ker nel Similarly a driver for the Atmel TCPA chip exists but only has an external module which has not been merged into the developme
9. flash memory and a number of additional peripherals Most of these are connected via a simple bus driven by the 405LP s on chip External Bus Controller EBC unit In particular it has several ways of attaching further external devices in addition to the PCM CIA controller built into the 405LP the eLAP has both a socket for Secure Digital SD memory and IO cards and a Phillips ISP1161 USB interface This chip is both a USB host controller allowing USB devices like keyboards and mice to be con nected to the eLAP and a USB client interface allowing the eLAP to be connected to a PC as a USB device An extra debug and development sled can be at tached to the eLAP also shown in Figure 1 It includes an Ethernet controller the physical PCM CIA slot driven by the 405LP s PCCF core and the physical connectors for the USB host port and serial port 2 Current support As the name Embedded Linux Application Plat form suggests the eLAP is intended to have Linux as its OS Although work is still in progress in some areas kernel support now exists for most of the hardware and features of the device The devel opment for this has mostly been done by IBM and MontaVista Software building on the code support ing other PowerPC 4xx processors which has ex isted for some time Unfortunately for various historical reasons the code for the eLAP has not yet been merged into the standard 2 4 or 2 5 Linux development trees In fac
10. ing PowerPC 4xx and other system on chip CPUs He is also the author and maintainer of the orinoco driver for Prism II based 802 11b NICs In the past he has worked on ramfs as included in the ac kernel tree and esky a userspace implementation of checkpoint resume Legal Statement This work represents the view of the author and does not necessarily represent the view of IBM IBM PowerPC PowerPC Architecture and pseries are trademarks or registered trademarks of International Business Machines Corporation in the United States and or other countries Linux is a registered trademark of Linus Tor valds Other company product and service names may be trademarks or service marks of others
11. is work as a basis for a driver for the eLAP See http opensource 1lineo com usb The 405LP s triple DES accelerator TDES is also unsupported thus far It is quite a simple de vice to operate however and a driver should ap pear in due course so far it has not had a high priority 3 2 Power management The obvious importance of reducing power con sumption in a battery powered device makes sup port for power management on the eLAP partic ularly interesting A number of power manage ment techniques are already supported on the de vice but work continues to best take advantage of the 405LP s features in this area The simplest form of power management sup ported by the eLAP is suspend and resume that is powering off most of the device while preserving the state of the OS and applications This is just like the sleep modes supported on most laptops The 405LP s APM core supports several different methods of preserving the CPU state In the eLAP suspend to RAM is implemented by saving the CPU s register state to RAM ensur ing caches are flushed then switching the SDRAM into a self refresh mode to preserve its contents The device is then powered off except for the RAM and the power management units themselves The suspend code also records a special flag in an un used bit in one of the real time clock registers which are preserved while the device is off of course and stores a pointer to a
12. mework though this might be best left until it is better understood what tech niques work well in practice References 1 Linuxppc_2_4 devel kernel tree bk ppc ppc bkbits net linuxppc_2_4 devel 2 H Blanchard B Brock M Locke M Orvek R Paulsen and K Rajamani Dynamic power management for embedded systems http www research ibm com arl projects papers DPM_V1 1 pdf 2002 3 IBM Corporation PowerPC 405LP Embedded Processor User s Manual preliminary edition 2002 4 Bruce Gain Handheld multimedia the Linux connection Tom s Hardware Guide 2003 http www6 tomshardware com business 20030129 linuxworld 02 html handh eld_ multimedia_the_linux_connection 5 David Gibson Linux on the powerpc 4xx In Conference Proceedings AUUG2002 Measure Monitor Control 2002 http www auug org au winter auug2002 AUUG2002 proceedings ps gz 6 David Gibson Device discovery and power management in embedded systems In Proceedings of the Ottawa Linus Symposium 2003 7 Linuxdevices com IBM unveils Linux based PDA reference design Linuxdevices com 2003 http www linuxdevices com news NS9222005703 htm1 About the author David Gibson is an employee of the IBM Linux Technology Center working from Canberra Aus tralia Most recently he has been working on board and device bringup for Linux on embedded Pow erPC machines along with various bits of kernel infrastructure for cleanly support
13. nt tree In any case the driver is of very limited use without con siderable userspace library support which is still in progress A driver for the Toshiba SDIO chip has been written by Toshiba with IBM s assistance but is only available in binary form SD Association rules prevent Toshiba from releasing specifications to write an open source driver with luck this will change eventually 3 Continuing and future work 3 1 Missing drivers Although as we have seen most of the the eLAP s hardware is already supported a few devices still lack drivers The most obvious of these is the M Systems Disk on Chip MDOC which com prises the bulk of the handheld s storage A driver for this device developed by M Systems and adapted for the eLAP with IBM and MontaVista s assistance does now exist Unfortunately a num ber of technical hitches hardware and software have delayed getting problems ironed out of this driver The most difficult problems now seem to have been resolved so MDOC support should be working in the near future Using the USB target side of the Phillips USB chip allowing the eLAP to act as a USB device is as yet wholly unsupported Adding support for this is made more difficult by the fact that unlike USB host controller support USB gadget support is not yet in the mainstream kernel trees However some groups notably Lineo have worked on USB gadget support so it should be possible to use th
14. rm changes in performance requirements This is handled by a userspace program known as a DPM policy manager Suitably modified application pro 3See http www brodo de cpufreq_old grams could give hints to the policy manager to let it better anticipate their performance requirements For example a video playback program might in form the policy manager that it needs a certain amount of compute time in order to maintain full speed playback The problem is complicated by the fact that some devices in the system may only operate properly at certain bus frequencies These devices place con straints on what operating points can be selected when they are active The scheme in 2 allows drivers to register these constraints with the DPM subsystem so that they can be properly taken into account 3 3 Consolidation and cleanups In addition to ongoing work to add support for cur rently missing features on the eLAP some work on consolidation and cleanup of the existing code will be needed as the eLAP support is merged back into the main kernel trees Many of the problems faced supporting the eLAP also affect other PowerPC 4xx based machines or other embedded devices gener ally This work is somewhat delayed by the need for consolidation and cleanup of the general PowerPC 4xx kernel code Most of the devices on the eLAP particularly those built into the 405LP are quite different to what would be found on a normal PC or server m
15. s PowerPC 405 Other connection Socket Coe poco co JEEE TEET EEE weet e tere reteee Interrupt Line APM Power management unit implements the CPU sleep suspend states MDOC 64M of M Systems Millennium Pluie Diskan Chip NAND NB this is not related to the APM BIOS in PC laptops PCCF kederi a cee ller 16 bit onl Card Audio Codec Texas Instruments TLV320AIC23 stereo audio codec and Compact Flash controller 16 bit only not Card Battery ADC ADS7823 ADC monitoring battery voltage 5 pus mae CPM Clock and Power Management unit c Re ae nice csi Codec Serial Interface interface to audio codec devices s 5 eal eee loc s a d SDIO i f DMAC DMA controller can DMA to from devices on both the PLB and OPB DI Toshiba TCO380AF Secure Digital and SDIQ interface SDRAM SDRAM controller EBC External Bus Controller c SC Ethernet MAC RTL8109 Ethernet controller i ne Atme A 320M TEM Frontlight Ctrl DS1050 LCD frontlight controller TDE Triple DES accelerator GPIO General Purpose IO interface TS Ctrl Semtech UR7HCT52 S840L touchscreen controller Ic 2c N f UARTx NS16550 compatible UARTs TA 14C bus mer S both master and slave capable UIC Universal Interrupt Controller ED Col BU8770KN tricolour LED driver USB Phillips ISP1161 USB interface includes both host controller LEDE LCD display controller and target interface Figure 1 Block diagram of the eLAP NFS root filesystem The kernel includes drivers for the 405LP s built in serial ports an
16. t what little 4xx support is in the 2 4 tree is broken and the 4xx code in 2 5 is quite in complete Eventually the 4xx and eLAP code should be merged back into the mainstream trees once someone can find the time to do the nec essary cleanups consolidation and merging For now most of the development for 4xx based ma chines including the eLAP takes place in the linuxppc_2_4 devel BitKeeper tree 1 MontaVista Software maintains a distribution for embedded devices which has been used as the basis for the userspace software on the eLAP This distribution also includes a kernel which in cludes the eLAP and other 4xx support from linuxppc_2 4 devel or at least will in a future release It also includes a few extra pieces for the eLAP which have not yet been merged into the linuxppc_2_4_devel tree 2 1 Booting and the basics As a handheld device the eLAP is designed to boot from flash memory The eLAP s flash is wired so that the CPU will execute the eLAP firmware PIBS PowerPC Initialization and Boot Software from flash at power on PIBS performs some ba sic hardware configuration such as initialising the 405LP s SDRAM controller then boots the Linux kernel It does this by loading the kernel image into RAM then jumping to it much as LILO does on a normal PC Usually the kernel image is stored in another sec tion of flash but for development purposes PIBS can also load a kernel over the serial port or if the

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