2019年8月18日 星期日

Linux Kernel(17.1)- Basic Device Tree syntax


這篇會介紹一下Device Tree的基本資料型態,並透過觀察/sys/firmware/devicetree/讓你更貼近一下DT的資料結構,下面範例是在vexpress-v2p-ca9.dts中include "brook.dtsi",然後在"brook.dtsi"撰寫DT的基本語法。DT的每個node,可包含零個以上的properties或child node。
brook@vista:~/qemu/linux-arm$ vim arch/arm/boot/dts/vexpress-v2p-ca9.dts
 #include "vexpress-v2m.dtsi"
 #include "brook.dtsi" /* add this line */

brook@vista:~/qemu/linux-arm$ vim arch/arm/boot/dts/brook.dtsi 

/ {
    node1 {
        a-string-property = "A string";
        a-string-list-property = "first string", "second string";
        // hex is implied in byte arrays. no '0x' prefix is required
        a-byte-data-property = [01 23 34 56];
        child-node1 {
            first-child-property;
            second-child-property = <1>;
            a-string-property = "Hello, world";
        };
        child-node2 {
        };
    };
    node2 {
        an-empty-property;
        a-cell-property = <1 2 3 4>; /* each number (cell) is a uint32 */
        // [@], 
        //  is a simple ascii string and can be up to 31 characters in length.
        child-node@1 {
        };
        child-node@2 {
        };
    };
};


brook@vista:~/qemu/linux-arm$ export ARCH=arm
brook@vista:~/qemu/linux-arm$ export CROSS_COMPILE=arm-linux-gnueabihf-
brook@vista:~/qemu/linux-arm$ export PATH=/opt/gcc-linaro-7.4.1-2019.02-x86_64_arm-linux-gnueabihf/bin:$PATH
brook@vista:~/qemu/linux-arm$ make dtbs


properties的value可以是empty或是以下資料型態:
  • Text strings (null terminated) are represented with double quotes: string-property = "a string";
  • 'Cells' are 32 bit unsigned integers delimited by angle brackets: cell-property = <0xbeef 123 0xabcd1234>;
  • Binary data is delimited with square brackets: binary-property = [0x01 0x23 0x45 0x67];
  • Data of differing representations can be concatenated together using a comma: mixed-property = "a string", [0x01 0x23 0x45 0x67], <0x12345678>;
  • Commas are also used to create lists of strings: string-list = "red fish", "blue fish";


brook@vista:~/qemu/linux-arm# cd ..
brook@vista:~/qemu$ qemu-system-arm -M vexpress-a9 -m 512M -kernel ./linux-arm/arch/arm/boot/zImage -dtb ./linux-arm/arch/arm/boot/dts/vexpress-v2p-ca9.dtb -initrd ./initrd-arm.img -nographic -append "console=ttyAMA0"

----- boot to VM -----

Please press Enter to activate this console.
/ # ls /sys/firmware/
devicetree  fdt
/ # ls /sys/firmware/devicetree/
base
/ # ls /sys/firmware/devicetree/base/ #根目錄(/sys/firmware/devicetree/base/) 多了node1與node2
/ # ls /sys/firmware/devicetree/base/
#address-cells                 model
#size-cells                    name
aliases                        node1
arm,hbi                        node2
arm,vexpress,site              pmu
cache-controller@1e00a000      reserved-memory
chosen                         scu@1e000000
clcd@10020000                  smb@4000000
compatible                     timer@100e4000
cpus                           timer@1e000600
dcc                            virtio_mmio@10013000
hsb@e0000000                   virtio_mmio@10013200
interrupt-controller@1e001000  virtio_mmio@10013400
interrupt-parent               virtio_mmio@10013600
memory-controller@100e0000     watchdog@100e5000
memory-controller@100e1000     watchdog@1e000620
memory@60000000

/ # find /sys/firmware/devicetree/base/node1
/sys/firmware/devicetree/base/node1
/sys/firmware/devicetree/base/node1/child-node2
/sys/firmware/devicetree/base/node1/child-node2/name
/sys/firmware/devicetree/base/node1/a-string-property
/sys/firmware/devicetree/base/node1/a-string-list-property
/sys/firmware/devicetree/base/node1/a-byte-data-property
/sys/firmware/devicetree/base/node1/name
/sys/firmware/devicetree/base/node1/child-node1
/sys/firmware/devicetree/base/node1/child-node1/first-child-property
/sys/firmware/devicetree/base/node1/child-node1/second-child-property
/sys/firmware/devicetree/base/node1/child-node1/a-string-property
/sys/firmware/devicetree/base/node1/child-node1/name


/ # find /sys/firmware/devicetree/base/node2
/sys/firmware/devicetree/base/node2
/sys/firmware/devicetree/base/node2/child-node@1
/sys/firmware/devicetree/base/node2/child-node@1/name
/sys/firmware/devicetree/base/node2/child-node@2
/sys/firmware/devicetree/base/node2/child-node@2/name
/sys/firmware/devicetree/base/node2/a-cell-property
/sys/firmware/devicetree/base/node2/name
/sys/firmware/devicetree/base/node2/an-empty-property


/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node1/a-string-property
41 20 73 74 72 69 6E 67  A string
00

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node1/a-string-list-property
66 69 72 73 74 20 73 74  first st
72 69 6E 67 00 73 65 63  ringsec
6F 6E 64 20 73 74 72 69  ond stri
6E 67 00                 ng
strings lists中的element是"0"分隔

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node1/a-byte-data-property
01 23 34 56              #4V
byte-data如其名,每個值大小就是一個byte

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node1/name
6E 6F 64 65 31 00        node1

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node1/child-node1/first-child-property
empty

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node1/child-node1/second-child-property
00 00 00 01

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node1/child-node1/a-string-property
48 65 6C 6C 6F 2C 20 77  Hello, w
6F 72 6C 64 00           orld

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node2/a-cell-property
00 00 00 01 00 00 00 02
00 00 00 03 00 00 00 04
cell每個value大小為32byte

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node2/child-node@1/name
63 68 69 6C 64 2D 6E 6F  child-no
64 65 00                 de

/ # hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"' /sys/firmware/devicetree/base/node2/child-node@2/name
63 68 69 6C 64 2D 6E 6F  child-no
64 65 00                 de


    參考資料:
  • https://elinux.org/Device_Tree_Usage, Device Tree Usage
  • https://elinux.org/images/f/f9/Petazzoni-device-tree-dummies_0.pdf, device tree dumies
  • https://blog.csdn.net/RadianceBlau/article/details/70800076, Linux DTS(Device Tree Source)设备树详解之一(背景基础知识篇)




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