1. Device Tree
Rouyun Pan

2. 緣起
• 某年某⽉月的某⼀一天...
神⼈人Linus Torvalds寫了⼀一封信
有⼀一堆闌尾在⽴立法院, 可割可棄！！！

3. So, 割闌尾～
• 科科跟宅宅們參考了PowerPC等其他體系架構下
已經使⽤用的Flattened Device Tree（FDT）.起源於
OpenFirmware IEEE 1275 device-tree notion.
• ⽇日後決定採⽤用Device Tree為描述硬體平台的資料
結構.

4. Device Tree
• 由⼀一系列被命名的結點（Node）和屬性
（Property）來組成. Node可包含Sub-Node.
Property是成對的name和value. 可讀性⾼高.
• 可以在開機的時候, 在透過它將訊息傳遞給作業系
統，⽽而不需要在把這些資訊放在程式碼中.
• 未來也很⽅方便增減修改Device Tree, 容易⽀支援不同
的Soc或硬體週邊.
Z > B!!!

5. How to work with device tree
complier
Device tree
description
binary data
bootloader kernel
platform
probe
carry
HW configuration
pass
DTS
DTC
DTB

6. Device tree misc.
• DTS
- device tree source file
• DTC
- device tree compiler, It compiles DTS to DTB.
• DTB
- device tree blob, the binary data to store the
information of DTS.

7. DTS Format(1)
/{!
! properties = ”value as string” | <value as u32 list> | [ value as byte stream ] |
<&node name>!
! child_node[@address] {!
! ! child_properties!
! }!
}!
!
&node_name {!
! compatible = "<manufacturer>,<model>"!
! properties!
}!
“/“ 表⽰示根節點, 不需要寫節點名稱
屬性Properties可以⽤用混合格式表⽰示,
如“字串”,或<32位元整數列表>,或[位元組流]或
<&其他參考節點名稱>
每個 Node 都會有⼀一個’compatible’ property. 格式為
"<manufacturer>,<model>"，第⼀一個字串表⽰示確切名
稱，第⼆二個字串表⽰示可兼容的其他設備。

8. DTS Format(2)
/{!
! properties = ”value as string” | <value as u32 list> | [ value as byte stream ] |
<&node name>!
! child_node[@address] {!
! ! child_properties!
! }!
}!
!
&node_name {!
! compatible = "<manufacturer>,<model>"!
! properties!
}!
結點的命名，遵循的格式為：
<name>[@address]
name是⼀一個ASCII字串，⽤用於描述結點對應的
裝置類型. 如果⼀一個結點描述的裝置有記憶體地
址，則填⼊入@address。多個相同類型設備結點
的name可以⼀一樣，只要address不同即可

9. DTS format(3)
/ {
compatible = "acme,coyotes-revenge";
#address-cells = <1>;
#size-cells = <1>;
interrupt-parent = <&intc>;
cpus {
#address-cells = <1>;
#size-cells = <0>;
cpu@0 {
compatible = "arm,cortex-a9";
reg = <0>;
};
cpu@1 {
compatible = "arm,cortex-a9";
reg = <1>;
};
};
serial@1010adbd {
compatible = "arm,pl011";
reg = <0x101abcd 0x1000>;
reg_name = “”
};
節點1
節點2!
⼦子節點1!
⼦子節點2!
根節點

10. DTS format(4)/ {
compatible = "acme,coyotes-revenge";
#address-cells = <1>; !
#size-cells = <1>;
interrupt-parent = <&intc>;
cpus {
#address-cells = <1>; !
#size-cells = <0>;
cpu@0 {
compatible = "arm,cortex-a9";
reg = <0>;
};
cpu@1 {
compatible = "arm,cortex-a9";
reg = <1>;
};
};
serial@1010adbd {
compatible = "arm,pl011";
reg = <0x101abcd 0x1000>;!
! reg_name = “”!
};
有位址的裝置會使⽤用
#address-cells 和 #size-cells 來決定⼦子結點
的’reg’屬性的格式怎麼填
reg = <address1 length1 [address2 length2] ... >
#size-cells = <0>; 表⽰示只填⼊入Reg的starting address
#address-cells = <1>; !
#size-cells = <1>; !
填⼊入格式為 Reg = <Starting_address Size>

11. .dts & dtsi
• .dts 是採⽤用ASCII 格式的Device Tree描述檔⽂文件.
⼀一般放置位在 kernel/arch/arm/boot/dts/。
• ⼀一個SoC可能拿來開發多個產品線，為了⽅方便，
把SoC公⽤用的硬體設定或不同產品的共同部分提
煉為.dtsi，類似C語⾔言的.h。其他不同產品的.dts
可以引⽤用這些共通的.dtsi。

12. Modularization
msm1234-abc.dts
msm1234-abc.dtsi
panel-abc.dtsi msm1234.dtsi
msm1234-gpu.dtsi
msm1234-
iommu.dtsi
msm1234-
mdss.dtsi

13. for example - Driver probe
@mdss_mdp.cpp
static const struct of_device_id mdss_mdp_dt_match[] =
.compatible = "qcom,mdss_mdp"
MODULE_DEVICE_TABLE (of, mdss_mdp_dt_match);
!
static struct platform_driver mdss_mdp_driver = {
.probe = mdss_mdp_probe,
.driver = {
.name = "mdp",
.of_match_table = mdss_mdp_dt_match,
}
}
!
static int __init mdss_mdp_driver_init (void) {
platform_driver_register( &mdss_mdp_driver );
}
@msm1234-mdss.dtsi
&soc {
!
mdss_mdp: qcom,mdss_mdp@12340000 {
compatible = "qcom,mdss_mdp";
reg = <0x12340000 0xff00>
reg-names = "mdp_phys", "vbif_phys";
…
!
mdss_dsi0: qcom,mdss_dsi@12345000 {
compatible = "qcom,mdss-dsi-ctrl";
qcom,mdss-mdp = <&mdss_mdp>;
…
!
qcom,mdss_wb_panel {
compatible = "qcom,mdss_wb";
…
…

14. Parsing APIs of Device Tree
property * of_find_property();
device_node * of_find_node_by_name();
of_find_node_by_type();
of_find_compatible_node();
of_find_matching_node_and_match();
of_find_node_by_path();
of_find_node_by_phandle();
void * of_get_property();
int of_property_read_u32_index()
of_property_read_u8_array();
of_property_read_u16_array();
of_property_read_u32_array();
of_property_read_string();
of_property_read_string_index();
int of_parse_phandle_with_args(),
of_parse_phandle_with_fixed_args(),
of_count_phandle_with_args()

15. Reference
• http://www.devicetree.org/Main_Page
• http://www.devicetree.org/Device_Tree_Usage
• Device Tree Support on ARM Linux
Chih-Min Chao) http://gplus.to/cmchao COSCUP
2011 in Taiwan
• http://blog.csdn.net/21cnbao/article/details/
8457546