Christine's Memo

最近跟研究了Device Tree這東西 , 挺有意思的 , 有時間來寫系列文章(哈) , 目前暫先簡單筆記一下心得 ^^

==========================================================================

在Linux 2.6以前 , 很多ARM base開發版上的Device Driver都用Hard Code方式直接寫到Kernel裡 , 

所以Linux Kernel漸漸地像個胖子 , 又肥又腫.

2011年後 , Device Tree採用 , 許多ARM base開發版上的Device Driver可以透過Device Tree傳遞給Linux Kernel , 

增加可移植性與幫Linux Kernel減肥 ~ 

DTS (device tree source)  

.dts採ASCII格式 , 非常人性化 , 適合直接閱讀

DTC (device tree compiler)

將.dts編譯為.dtb的工具

DTB (device Tree Blob)

給Linux Kernel看的machine code.

make dtbs可生成.dtb檔

Bootloader將帶起Kernel的過程 , 需要將.dtb放到memory , 並在bootcmd裡面宣告.dtb位址讓Kernel知道

==========================================================================

What I say in next is very important .....(抄襲某電影 XD) 

有了Device Tree這個東東後 : 

(1) 不需要在Kernel裡註冊Platform Device, 宣告IO resource, IRQ, GPIO Pins

     這些資料直接宣告在dts裡面 , 然後利用dtc編譯成dtb , Kernel會自己解析超展開~~

(2) Platform Driver裡要增加MODULE_DEVICE_TABLE

    下面是一個簡單的範例

static const struct of_device_id arm_smmu_of_match[] = {
        { .compatible = "arm,smmu-v1", .data = (void *)ARM_SMMU_V1 },
        { .compatible = "arm,smmu-v2", .data = (void *)ARM_SMMU_V2 },
        { .compatible = "arm,mmu-400", .data = (void *)ARM_SMMU_V1 },
        { .compatible = "arm,mmu-401", .data = (void *)ARM_SMMU_V1 },
        { .compatible = "arm,mmu-500", .data = (void *)ARM_SMMU_V2 },
        { },
};
MODULE_DEVICE_TABLE(of, arm_smmu_of_match);

static int arm_smmu_device_dt_probe(struct platform_device *pdev)
{
        const struct of_device_id *of_id;

        of_id = of_match_node(arm_smmu_of_match, dev->of_node);
        smmu->version = (enum arm_smmu_arch_version)of_id->data;

        ...

        if (smmu->version > ARM_SMMU_V1) {
                ...
        }
}

static struct platform_driver arm_smmu_driver = {
       .driver = {
                .name           = "arm-smmu",
                .of_match_table = of_match_ptr(arm_smmu_of_match),
        },
        .probe  = arm_smmu_device_dt_probe,
        .remove = arm_smmu_device_remove,
};