dtc - Device Tree Compiler

從dtc manpage可以簡略瞭解一下DTC(Device Tree Compiler)功能, 就是將device-tree的format轉成另一種format, 其input格式有三種,

1. dts - device tree source text
2. dtb - device tree blob
3. fs - /proc/device-tree style directory

而output格式也有三種,

1. dts - device tree source text
2. dtb - device tree blob
3. asm - assembler source

而dtc支援的phandle可支援三種

1. legacy - "linux,phandle" properties only
2. epapr - "phandle" properties only
3. both - Both "linux,phandle" and "phandle" properties

根據Device Tree Mysteries,

what is the reason for having a phandle? 
It is really just a hack to get around the fact that device tree does 
not have a pointer data type. 
It is a way to reference 
"that node over there that is related to this node for some reason". 

簡單來說, phandler就是pointer data type, 用個例子說明

pic@10000000 {
    phandle = < 1 >;
};

A phandle value of 1 is defined. 
Another device node could reference the pic node with a
phandle value of 1:

uart@20000000 {
    interrupt-parent = > 1 >;
};

"phandle = < 1 >"的"1"是一個隨意的unit32數值, 只要不衝突即可, 但是這並不是很好記, 所以DTC貼心的可以用lable來建立phandle,如下範例

PIC_3: pic@10000000 {
    interrupt-controller;
};

uart@20000000 {
    interrupt-parent = < &PIC_3 >;
};

這裡的"&"是告訴DTC後面接一個字串, 是個phandle參考到某個lable, 然後DTC就會幫user建立unit32的數值, 建立出phandle. 除此之外也可以使用full path來取代lable, 如下範例

/{
    soc {
        PIC_3: pic@10000000 {
            interrupt-controller;
        };
    };

    uart@20000000 {
        interrupt-parent = < &PIC_3 >;
    };

    uart@30000000 {
        interrupt-parent = < &{/soc/pic@10000000} >;
    };
};

下面再用一個例子同時說明DTC與phandle

brook@vista:~/dts$ cat test_phandle.dts
/dts-v1/;
/{
    soc {
        PIC_3: pic@10000000 {
           interrupt-controller;
        };
    };

    uart@20000000 {
        interrupt-parent = < &PIC_3 >;
    };

    uart@30000000 {
        interrupt-parent = < &{/soc/pic@10000000} >;
    };
};
brook@vista:~/dts$ dtc -O dtb -I dts test_phandle.dts > test_phandle.dtb
brook@vista:~/dts$ dtc -O dts -I dtb test_phandle.dtb
/dts-v1/;

/ {

        soc {

                pic@10000000 {
                        interrupt-controller;
                        linux,phandle = <0x1>;
                        phandle = <0x1>;
                };
        };

        uart@20000000 {
                interrupt-parent = <0x1>;
        };

        uart@30000000 {
                interrupt-parent = <0x1>;
        };
};

這篇文章主要簡單講解利用dtc來轉換dtb與dts, 順帶說明一下phandle是一種pointer data type, 與其基本原理

參考資料:
• Device Tree Mysteries
• Device Tree Compiler Manual
• Device Tree Source Format (version 1)
• Device Tree Usage