Section 4.6. The Device Tree
4.6. The Device TreeFrom the standpoint of system initialization and booting, Open Firmware's core data structure is the device tree, which is referred to by all interfaces supported by Open Firmware. The device tree is a representation of various hardware components in the system and their interconnections. It also contains pseudo-devices that have no corresponding physical devices. The root node of the device tree is / (as in a Unix file system): 0 > dev / ok 0 > ls ff88feb0: /cpus ff890118: /PowerPC,G5@0 ff8905c8: /l2-cache ff891550: /PowerPC,G5@1 ff891a00: /l2-cache ff891bf0: /chosen ff891d98: /memory@0,0 ff891fa8: /openprom ff892108: /client-services ff8933f8: /rom@0,ff800000 ff8935b8: /boot-rom@fff00000 ff8937a8: /macos ff893840: /options ff8938d8: /packages ff893cc8: /deblocker ff894618: /disk-label ff895098: /obp-tftp ff89fc68: /telnet ... ff9a7610: /temperatures ffa1bb70: /drive-bay@4 ffa1f370: /backside@6 ... ff9a77a8: /audible-alarm ff9a7940: /thermostats ffa1cfb8: /overtemp*-signal@5800 ok 0 > In the device tree, an individual node represents a bus, a physical device, or a pseudo-device. A node with childrenvisually indicated by indentations in the output of lsis often a bus. You can use the dev command to go to a certain node in the tree. 0 > dev /pseudo-hid ok \ Go to node /pseudo-hid 0 > ls \ List children of current node ff943ff0: /keyboard ff944788: /mouse ff944d50: /eject-key 0 > dev mouse ok \ Go to a child 0 > pwd /pseudo-hid/mouse ok \ Tell us where we are 0 > dev .. ok \ Go one level "up" 0 > pwd /pseudo-hid ok 0 > dev /cpus ok \ Go to node /cpus 0 > ls ff890118: /PowerPC,G5@0 ff8905c8: /l2-cache ff891550: /PowerPC,G5@1 ff891a00: /l2-cache ok Since the complete pathname of a device could be rather long and inconvenient to use, commonly used devices have shorthand representations, or aliases. devalias shows the current list of aliases: 0 > devalias keyboard /pseudo-hid/keyboard mouse /pseudo-hid/mouse eject-key /pseudo-hid/eject-key pci0 /pci@0,f0000000 ipc /ipc scca /ht/pci@3/mac-io/escc/ch-a nvram /nvram uni-n /u3 u3 /u3 dart /u3/dart ... first-boot /ht@0,f2000000/pci@7/k2-sata-root@c/k2-sata second-boot /ht@0,f2000000/pci@5/ata-6@d/disk last-boot /ht@0,f2000000/pci@6/ethernet screen /pci@0,f0000000/ATY,WhelkParent@10/ATY,Whelk_A@0 okdevalias followed by an alias shows the latter's expansion (note that the / separator is not used when specifying an alias): 0 > devalias hd /ht/pci@7/k2-sata-root/k2-sata@0/disk@0 ok 0 > devalias wireless /ht@0,f2000000/pci@4/pci80211@1 ok The dir command can be used to list files on an HFS Plus or HFS volume. Its argument is a device path that represents a volume and a path within that volume. The device path can be an alias or a complete path. 0 > dir hd:\ Size/ GMT File/Dir bytes date time TYPE CRTR Name 12292 6/18/ 5 15:23:14 .DS_Store 131072 5/25/ 5 10: 1:30 .hotfiles.btree 16777216 5/12/ 5 1:57:11 jrnl hfs+ .journal 4096 5/12/ 5 1:57:10 jrnl hfs+ .journal_info_block 6/18/ 5 1:32:14 .Spotlight-V100 ... 5/12/ 5 1:57:12 %00%00%00%00HFS+%20Private%20Data ok 0 > dir hd:\System\Library\CoreServices Size/ GMT File/Dir bytes date time TYPE CRTR Name 869 5/12/ 5 2:28:21 tbxj chrp .disk_label 12 5/12/ 5 2:28:21 .disk_label.contentDetails 6/ 9/ 5 4: 8:44 AppleFileServer.app 3/28/ 5 4:53:13 Automator%20Launcher.app 3/28/ 5 4:42:25 BezelUI 3/28/ 5 4:51:51 Bluetooth%20Setup%20Assistant.app 14804 3/26/ 5 22:47: 0 bluetoothlauncher 3/28/ 5 4:51:51 BluetoothUIServer.app 3/21/ 5 3:12:59 BOMArchiveHelper.app 174276 5/19/ 5 3:46:35 tbxi chrp BootX ... Each node in the device tree may have properties, methods, and data. 4.6.1. PropertiesA node's properties are externally visible data structures that describe the node and possibly its associated devices, which may further have their own specific properties. Open Firmware's client programs, as well as its own procedures, may inspect and modify properties. Access to properties is also available from the Open Firmware user interface. The .properties word shows the names and values of the current node's properties. 0 > dev enet ok 0 > .properties vendor-id 0000106b device-id 0000004c revision-id 00000000 class-code 00020000 interrupts 00000029 00000001 min-grant 00000040 ... name ethernet device_type network network-type ethernet ... local-mac-address ... gbit-phy ok 0 > dev /cpus/PowerPC,G5@0 ok 0 > .properties name PowerPC,G5 device_type cpu reg 00000000 cpu-version 003c0300 cpu# 00000000 soft-reset 00000071 state running clock-frequency 9502f900 bus-frequency 4a817c80 config-bus-frequency 4a817c80 timebase-frequency 01fca055 reservation-granule-size00000080 tlb-sets 00000100 tlb-size 00001000 d-cache-size 00008000 i-cache-size 00010000 d-cache-sets 00000080 i-cache-sets 00000200 i-cache-block-size 00000080 d-cache-block-size 00000080 graphics performance-monitor altivec data-streams dcbz 00000080 general-purpose 64-bit 32-64-bridge ... The dump-properties word can be used to show properties of all nodes in the device tree. 0 > dump-properties / PROPERTIES: model PowerMac7,3 compatible PowerMac7,3 MacRISC4 Power Macintosh ... /cpus/PowerPC,G5@0/l2-cache PROPERTIES: name l2-cache device_type cache i-cache-size 00080000 ... /sep/thermostats/overtemp*-signal@5800 PROPERTIES: name overtemp*-signal ...Properties are represented as collections of names and their corresponding values. Property names are human-readable text strings, and property values are variable-lengthpossibly zero-lengthbyte arrays representing encoded information. Standard property names found in a package include the following:
A package's registers can represent very different information depending on the nature of the package. For example, the registers of the memory package contain the physical memory addresses installed in the system. Details of installed memory can be examined through the .properties command. The machine in Figure 42 has two PC2700 DDR SDRAM memory modules installed. The two pairs of numbers shown against reg specify the starting address and size of the module. The first RAM module starts at address 0x00000000 and has a size 0x10000000 (256MB). The second module starts at 0x10000000 (256MB) and has a size 256MB. The total RAM is therefore 512MB. Figure 42. Physical memory properties in the device tree of a PowerBook G4
It is possible to delete certain properties and specify your own. In the case of memory, this would be useful if you need to reduce the installed RAM size seen by Mac OS X without physically removing a RAM module. This way, you could simulate an arbitrary memory size that is less than the total memory installed. The following command sequence disables the second of the two memory modules installed in the machine from Figure 42. The change is not permanent in that it is not written to NVRAMonce the system is rebooted, the "disabled" module will be detected and used as before. 0 > dev /memory 0 > " reg" delete-property ok 0 > 0 encode-int 10000000 encode-int encode+ " reg" property ok It must be noted that the reg properties may change from machine to machine, or more likely, with architectural changes. For example, the format of memory properties changed with the Power Mac G5. Figure 43 shows the memory properties on a quad-processor Power Mac G5 with six of its eight RAM slots filled, each with a 512MB DDR2 module. Figure 43. Physical memory properties in the device tree of a Power Mac G5
A less adventurous and more appropriate way to limit visible RAM is to use the kernel's maxmem boot argument. You can use the nvram command-line program from within Mac OS X to get and set Open Firmware NVRAM variables. For example, the following command executed at a shell prompt in Mac OS X would limit available memory size to 128MB: $ sudo nvram boot-args="maxmem=128"
A package that defines a physical address space usually contains the #address-cells and #size-cells standard properties. The value of #address-cells defines the number of cells required to encode a physical address within the address space defined by the package. The #size-cells property defines the number of cells required to represent the length of a physical address range. For example, the value of #address-cells for the root node of the device tree is 2 on the G5 and 1 on the G4. 4.6.2. MethodsA node's methods are simply software procedures supported by the device it represents. The words Forth word shows the current node's methods: 0 > dev enet ok 0 > words power-down ((open)) max-transfer block-size #blocks dma-free dma-alloc load write flush read close (open) open enet-quiesce ... show-enet-debug? enet-base my_space ok As we noted earlier, a word's definition may be viewed using see: 0 > see flush : flush " enet: Flush" 1 .enet-debug restart-rxdma ; okThe dump-device-tree word walks the entire device tree, showing each node's methods and properties. 4.6.3. DataA node may also have private data used by its methods. Such data could be either instance-specific or static. Static data, which is seen by all instances, persists across instances. There is a related abstraction called a package that we referred to earlier. Usually, a package is synonymous with a device. A distinction is made for pseudo-devices, which have no corresponding physical device. Pseudo-devices are said to have software-only packages. Here is a simple way to look at this.
There is a special category of packages: the support packages. These do not correspond to any specific devices but implement general-purpose utility methods. They live under the /packages node in the device tree. 0 > dev /packages 0 > ls ff893cc8: /deblocker ff894618: /disk-label ff895098: /obp-tftp ff89fc68: /telnet ff8a0520: /mac-parts ff8a1e48: /mac-files ff8a4fc0: /hfs-plus-files ff8aa268: /fat-files ff8ad008: /iso-9660-files ff8ade20: /bootinfo-loader ff8afa88: /xcoff-loader ff8b0560: /macho-loader ff8b33d0: /pe-loader ff8b3dd8: /elf-loader ff8b5d20: /usb-hid-class ff8b8870: /usb-ms-class ff8bb540: /usb-audio-class ff929048: /ata-disk ff92b610: /atapi-disk ff92daf0: /sbp2-disk ff931508: /bootpath-search ff9380f8: /terminal-emulator ok 0 > We came across the telnet support package earlier when we used it to connect to Open Firmware from another computer. obp-tftp implements a TFTP client for use in network booting. atapi-disk lets you communicate with an ATAPI device using the ATAPI protocol.
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