设备树学习资料

【实例简介】
device tree的绝好学习材料。简明扼要，举例恰当。深入学习device tree的话还需要配合源码，入门使用的话，本材料足以。该文档出自网站：http://free-electrons.com/ 该网站也是linux内核、android、嵌入式学习的很好网站
A genda User perspective: booting with the Device Tree Basic device Tree syntax and compilation Simple example of Device Tree fragment Overall organization of a device tree Examples of device Tree usage General considerations about the device tree in linux FreeElectrons.Kernel,driversandembeddedLinuxdevelopmentconsultingtrainingandsupporthttp://free-electrons.com User perspective: before the Device Tree The kernel contains the entire description of the hardware The bootloader loads a single binary, the kernel image, and executes it e iMage or zImage The bootloader prepares some additional information, called ATAGS, which address is passed to the kernel through register r2 Contains information such as memory size and location, kernel command line, etc The bootloader tells the kernel on which board it is being booted through a machine type integer, passed in register r1 U-Boot command: bootm <kernel img addr> Barebox variable: bootm image FreeElectrons.Kernel,driversandembeddedLinuxdevelopmentconsultingtrainingandsupporthttp://free-electrons.com User perspective: before the Device Tree ATAGS RAM lmage 1 = <machine type> r2= <pointer to atags> FreeElectrons.Kernel,driversandembeddedLinuxdevelopmentconsultingtrainingandsupporthttp://free-electrons.com 5/1 User perspective: booting with a Device Tree The kernel no longer contains the description of the hardware it is located in a separate binary the device tree blob The bootloader loads two binaries: the kernel image and the DTB Kernel image remains uImage or zImage DTB located in arch/arm/boot/dts, one per board s The bootloader passes the dtb address through r 2. It is supposed to adjust the dtb with memory information, kernel command line and potentially other info No more machine type U-Boot command bootm <kernel img addr>- <dtb addr> Barebox variables: bootm image, bootm oftree FreeElectrons.Kernel,driversandembeddedLinuxdevelopmentconsultingtrainingandsupporthttp://free-electrons.com 6/1 User perspective: booting with a Device Tree DTB RAM lmage 1= don 't care r2 <pointer to DTB> FreeElectrons.Kernel,driversandembeddedLinuxdevelopmentconsultingtrainingandsupporthttp://free-electrons.com User perspective: compatibility mode for dT booting Some bootloaders have no specific support for the device Tree, or the version used on a particular device is too old to have this support To ease the transition, a compatibility mechanism was added CONFIG ARM APPENDED DTB It tells the kernel to look for a dtB right after the kernel mage there is no built-in Makefile rule to produce such kernel so one must manually do cat arch/arm/boot/zImage arch/arm/boot/dts/myboard dtb > my-zImage mkimage -d my-zImage my-uImage In addition the additional option CONFIG ARM ATAG DTB coMPAT tells the kernel to read the ATAGS information from the bootloader, and update the dt using them FreeElectrons.Kernel,driversandembeddedLinuxdevelopmentconsultingtrainingandsupporthttp://free-electrons.com 8/1 o'h What is the Device Tree? Quoted from the Power. org Standard for Embedded Power Architecture Platform Requirements(ePAPR) The ePaPr specifies a concept called a device tree to describe system hardware. a boot program loads a device tree into a client program's memory and passes a pointer to the device tree to the client ba device tree is a tree data structure with nodes that describe the physical devices in a system An ePAPR-compliant device tree describes device information in a system that cannot be dynamically detected by a client progra FreeElectrons.Kernel,driversandembeddedLinuxdevelopmentconsultingtrainingandsupporthttp://free-electrons.com Basic Device Tree syntax Node name Unit address roperty name /{ Property value node@θ a-string-property ="A string"; Properties of node@o a-string-list-property ="first string","second string a-byte-data-property =[0x01 0x23 0x34 0x56] child-node@o t first-child-property i Bytestring second-child-property a-reference-to-something =<&nodel>: child-node@l t (reference to another node Label nodel: node@l t an-empty-property a-cell-property = <1 2 3 4>; child-node@θ Four cells (32 bits values) FreeElectrons.Kernel,driversandembeddedLinuxdevelopmentconsultingtrainingandsupporthttp://free-electrons.com 【实例截图】
【核心代码】