基于tiny4412的Linux内核移植 -- 设备树的展开

作者信息

作者: 彭东林

邮箱:pengdonglin137@163.com

QQ:405728433

平台简介

开发板:tiny4412ADK + S700 + 4GB Flash

要移植的内核版本:Linux-4.4.0 (支持device tree)

u-boot版本:友善之臂自带的 U-Boot 2010.12 (为支持uImage启动,做了少许改动)

busybox版本:busybox 1.25

交叉编译工具链: arm-none-linux-gnueabi-gcc

(gcc version 4.8.3 20140320 (prerelease) (Sourcery CodeBench Lite 2014.05-29))

摘要

在Linux引入设备树之后,将原来写在代码中的大量的硬件信息全部移到了设备树中,然后在Linux启动的时候会解析设备树,利用解析到的硬件信息构造device,然后注册到相应的bus上,如果有对应的driver,则会调用driver的probe函数。那么这个过程是怎么进行的?Linux系统有各种device,如对于platform子系统来说有platform_device、对于I2C子系统有i2c_client、对于SPI子系统来说有spi_device,那么这些device是怎么跟设备树关联起来的呢?

在分析的过程中参考了下面的几篇博文:

http://www.wowotech.net/device_model/dt-code-analysis.html

http://www.wowotech.net/comm/i2c_overview.html

http://www.wowotech.net/comm/i2c_provider.html

这几篇博文讲的非常好,下面一篇是之前总结的:

http://www.cnblogs.com/pengdonglin137/p/4495056.html

内核文档:

Documentation/devicetree/booting-without-of.txt

Documentation/devicetree/usage-model.txt

官方文档:

Power_ePAPR_APPROVED_v1.1.pdf

正文

设备树的populate过程大致有如下几个阶段(下文中“节点”与“device node”可以理解为一个意思):

一、根据设备树创建device node链表

start_kernel

---> setup_arch

---> unflatten_device_tree

在u-boot引导内核的时候,会将设备树在物理内存中的物理起始地址(存放在寄存器r2中)传递给Linux内核,然后Linux内核在函数unflatten_device_tree中会解析设备树镜像,并利用扫描到的信息创建由device node构成的链表,全局变量of_root指向链表的根节点,设备树的每个节点都会有一个struct device_node与之对应。

二、遍历device node链表,创建并注册platform_device

start_kernel

---> rest_init

---> kernel_init

---> kernel_init_freeable

---> do_basic_setup

---> do_initcalls

在do_initcalls函数中,kernel会依次执行各个initcall函数,在这个过程中,会调用 customize_machine,具体如下:

static int __init customize_machine(void)
{
    /*
     * customizes platform devices, or adds new ones
     * On DT based machines, we fall back to populating the
     * machine from the device tree, if no callback is provided,
     * otherwise we would always need an init_machine callback.
     */
    of_iommu_init();
    if (machine_desc->init_machine)
        machine_desc->init_machine();
#ifdef CONFIG_OF
    else
        of_platform_populate(NULL, of_default_bus_match_table,
                    NULL, NULL);
#endif
    return 0;
}
arch_initcall(customize_machine);

这样就可调用到exynos_dt_machine_init:

static void __init exynos_dt_machine_init(void)
{
    ......
 
    of_platform_populate(NULL, of_default_bus_match_table, NULL, NULL);
}

在of_platform_populate中会调用of_platform_bus_create ---> of_platform_device_create_pdata,完成platform_device的创建和注册。那么Linux系统是怎么知道哪些device node要注册为platform_device,哪些是用于i2c_client,哪些是用于spi_device?不知道你有没有注意到调用of_platform_populate的时候给它传递了一个参数of_default_bus_match_table,它的定义如下:

const struct of_device_id of_default_bus_match_table[] = {
    { .compatible = "simple-bus", },
    { .compatible = "simple-mfd", },
#ifdef CONFIG_ARM_AMBA
    { .compatible = "arm,amba-bus", },
#endif /* CONFIG_ARM_AMBA */
    {} /* Empty terminated list */
};

是这个意思:如果某个device node的compatible属性的值与数组of_default_bus_match_table中的任意一个元素的compatible的值match(但是对于compatible属性的值是arm,primecell的节点有些特殊,它是单独处理的),那么这个device node的child device node(device_node的child成员变量指向的是这个device node的子节点,也是一个链表)仍旧会被注册为platform_device。

of_platform_populate:

1: int of_platform_populate(struct device_node *root,
   2:             const struct of_device_id *matches,
   3:             const struct of_dev_auxdata *lookup,
   4:             struct device *parent)
   5: {
   6:     struct device_node *child;
   7:     int rc = 0;
   8:  
   9:     root = root ? of_node_get(root) : of_find_node_by_path("/");  // 找到root device node
  10:     if (!root)
  11:         return -EINVAL;
  12:  
  13:     for_each_child_of_node(root, child) { // 遍历root device node的child device node
  14:         rc = of_platform_bus_create(child, matches, lookup, parent, true);
  15:         if (rc) {
  16:             of_node_put(child);
  17:             break;
  18:         }
  19:     }
  20:     of_node_set_flag(root, OF_POPULATED_BUS);
  21:  
  22:     of_node_put(root);
  23:     return rc;
  24: }

of_platform_bus_create :

1: static int of_platform_bus_create(struct device_node *bus,
   2:                   const struct of_device_id *matches,
   3:                   const struct of_dev_auxdata *lookup,
   4:                   struct device *parent, bool strict)
   5: {
   6:     const struct of_dev_auxdata *auxdata;
   7:     struct device_node *child;
   8:     struct platform_device *dev;
   9:     const char *bus_id = NULL;
  10:     void *platform_data = NULL;
  11:     int rc = 0;
  12:  
  13:     /* Make sure it has a compatible property */
  14:     if (strict && (!of_get_property(bus, "compatible", NULL))) { // 这样可以把chosen、aliases、memory等没有compatible属性的节点排除在外
  15:         pr_debug("%s() - skipping %s, no compatible prop\n",
  16:              __func__, bus->full_name);
  17:         return 0;
  18:     }
  19:  
  20:     auxdata = of_dev_lookup(lookup, bus);  // tiny4412给lookup传递的是NULL
  21:     if (auxdata) {
  22:         bus_id = auxdata->name;
  23:         platform_data = auxdata->platform_data;
  24:     }
  25:  
  26:     if (of_device_is_compatible(bus, "arm,primecell")) {
  27:         /*
  28:          * Don't return an error here to keep compatibility with older
  29:          * device tree files.
  30:          */
  31:         of_amba_device_create(bus, bus_id, platform_data, parent);
  32:         return 0;
  33:     }
  34:  
  35:     dev = of_platform_device_create_pdata(bus, bus_id, platform_data, parent); // 根据device node创建 platform_device并注册
  36:     if (!dev || !of_match_node(matches, bus)) // 判断是不是需要继续遍历这个device node下的child device node
  37:         return 0;
  38:  
  39:     for_each_child_of_node(bus, child) { // 遍历这个device node下的child device node,将child device node也注册为platform_device
  40:         pr_debug("   create child: %s\n", child->full_name);
  41:         rc = of_platform_bus_create(child, matches, lookup, &dev->dev, strict);
  42:         if (rc) {
  43:             of_node_put(child);
  44:             break;
  45:         }
  46:     }
  47:     of_node_set_flag(bus, OF_POPULATED_BUS);
  48:     return rc;
  49: }

三、注册其他设备

I2C设备的注册

下面说一下i2c_client是如何注册的。先看下面一张图(来自蜗窝科技):

下面是从http://www.wowotech.net/comm/i2c_overview.html摘抄的一段话:

1)platform bus(/sys/bus/platform)是驱动工程师常见的bus,用于挂载和CPU通过系统总线连接的各类外设。在I2C framework中,I2C控制器直接从属于platform bus,我们在linux kernel中常说的I2C driver,都是指I2C controller driver,都是以platform driver的形式存在,当然,对应的控制器是platform device。 2)与此同时,kernel抽象出I2C bus(/sys/bus/i2c),用于挂载和I2C controller通过I2C总线连接的各个I2C slave device。 3)比较特殊的地方是,I2C core使用一个虚拟实体----I2C adapter,抽象I2C controller有关的功能(主要是数据的收发),I2C adapter也挂载在I2C bus上。 4)I2C adapter和I2C slave device都挂载在I2C bus上,就可以方便的进行Master(I2C adapter)和Slave之间的匹配操作,并通过I2C core提供的统一接口,访问I2C salve device,进行数据的收发。

我们知道,i2c控制器在i2c驱动模型中被抽象为i2c_adapter,但是i2c控制器驱动实际上是在platform_bus上,所以i2c控制器对应的是platform_device,因此会在上面调用of_platform_populate时注册,然后i2c控制器驱动的probe函数会被调用。以tiny4412开发板为例,在drivers/i2c/busses/i2c-s3c2410.c的probe函数中调用注册adapter的函数接口:i2c_add_numbered_adapter ---> i2c_add_adapter ---> i2c_register_adapter ---> of_i2c_register_devices,在函数of_i2c_register_devices中会遍历这个adapter对应的device node的child device node,这些child device node对应的就是挂载i2c bus上的板级外设的硬件信息(这些板级外设使用I2C接口跟SOC通信),如 MMA7660。然后调用of_i2c_register_device,这个函数根据每个child device node的信息构造i2c_board_info,并调用i2c_new_device,在i2c_new_device中会创建并注册i2c_client,注册i2c_client的时候如果找到了对应的设备驱动程序(如 MMA7660的驱动程序),设备驱动程序的probe函数就会被调动。

SPI设备的注册

由于SPI驱动模型跟I2C类似,spi_device的注册过程也跟i2c_client的很类似。spi控制器在spi子系统中被抽象为spi_master,spi控制器驱动实际上也在platform_bus上,所以spi控制器对应的是platform_device。当调用of_platform_populate注册spi控制器对应的platform_device的时候,spi控制器驱动的probe函数会被执行,在probe函数中会向spi子系统注册spi_master。以tiny4412为例,在drivers/spi/spi-s3c64xx.c的s3c64xx_spi_probe函数中调用devm_spi_register_master ---> spi_register_master ---> of_register_spi_devices,在of_register_spi_devices中会遍历与这个spi_master对应的device node的child device node,这些child device node就是挂在spi bus上的板级外设,如spi接口的存储器等等。然后调用of_register_spi_device,根据每个child device node的信息创建spi_device,并调用spi_add_device完成注册,注册spi_device的时候如果找到了对应的设备驱动程序(如 SPI接口的存储器的驱动程序),设备驱动程序的probe函数就会被调动。

其他platform device的注册

在上面说如果在of_platform_populate的时候如果给matches传递了of_default_bus_match_table,那么跟matches匹配的device_node的直接child device node会也会自动被注册为platform_device。假如跟matches不匹配的话,这个device_node的直接child device node不会被再被处理了。比如像下面的设备树结构:

 1 / {
 2     #address-cells = <0x2>;
 3     #size-cells = <0x2>;
 4     model = "Qualcomm Technologies";
 5     compatible = "qcom,msm8996";
 6     interrupt-parent = <0x1>;
 7 
 8     soc {
 9         compatible = "simple-bus";
10 
11         qcom,msm-dai-mi2s {
12             compatible = "qcom,msm-dai-mi2s";
13 
14             qcom,msm-dai-q6-mi2s-quat {
15                 compatible = "qcom,msm-dai-q6-mi2s";
16             };
17         };
18     };
19 };

如上,节点"qcom,msm-dai-mi2s"会被注册为platform_device,而其child device node是"qcom,msm-dai-q6-mi2s-quat",并不会被注册为platform_device。如果此时需要把"qcom,msm-dai-q6-mi2s-quat"也注册为 platform_device的话,也可以在"qcom,msm-dai-mi2s"对应的platform device_driver在被probe的时候重新调用of_platform_populate。如下:

 1 static int msm_dai_mi2s_q6_probe(struct platform_device *pdev)
 2 {
 3     int rc;
 4     rc = of_platform_populate(pdev->dev.of_node, NULL, NULL, &pdev->dev);
 5     if (rc) {
 6         dev_err(&pdev->dev, "%s: failed to add child nodes, rc=%d\n",
 7             __func__, rc);
 8     } else
 9         dev_dbg(&pdev->dev, "%s: added child node\n", __func__);
10     return rc;
11 }
12 
13 static int msm_dai_mi2s_q6_remove(struct platform_device *pdev)
14 {
15     return 0;
16 }
17 
18 static const struct of_device_id msm_dai_mi2s_dt_match[] = {
19     { .compatible = "qcom,msm-dai-mi2s", },
20     { }
21 };
22 
23 MODULE_DEVICE_TABLE(of, msm_dai_mi2s_dt_match);
24 
25 static struct platform_driver msm_dai_mi2s_q6 = {
26     .probe  = msm_dai_mi2s_q6_probe,
27     .remove = msm_dai_mi2s_q6_remove,
28     .driver = {
29         .name = "msm-dai-mi2s",
30         .owner = THIS_MODULE,
31         .of_match_table = msm_dai_mi2s_dt_match,
32     },
33 };

其中,在第4行又重新调用了of_platform_populate,它的第一个参数是"qcom,msm-dai-mi2s"的device node,通过这个就可以遍历其child device node,并将其注册为platform device。

其他

在Linux系统起来后,会将解析完成的设备树导出到用户空间。

一、/proc/device-tree

这个目录下的目录和文件是根据device node的结构组织的,顶层目录是root device node,其他的子目录是root device node 的 child device node,同时子目录又可以再嵌套子目录,以此表示这些device node的父子关系。

[root@tiny4412 root]# cd /proc/device-tree/
[root@tiny4412 base]# ls
#address-cells                   pinctrl@106E0000
#size-cells                      pinctrl@11000000
adc@126C0000                     pinctrl@11400000
aliases                          pmu
amba                             ppmu_acp@10ae0000
backlight                        ppmu_camif@11ac0000
cam-power-domain@10023C00        ppmu_cpu@106c0000
camera                           ppmu_dmc0@106a0000
chipid@10000000                  ppmu_dmc1@106b0000
chosen                           ppmu_g3d@12630000
clock-controller@03810000        ppmu_g3d@13220000
clock-controller@10030000        ppmu_image@12aa0000
codec@13400000                   ppmu_lcd0@11e40000
compatible                       ppmu_leftbus0@116a0000
cpus                             ppmu_mfc_left@13660000
dsi@11C80000                     ppmu_mfc_right@13670000
ehci@12580000                    ppmu_rightbus@112a0000
exynos-usbphy@125B0000           ppmu_tv@12e40000
fimd@11c00000                    pwm@139D0000
fixed-rate-clocks                regulators
g2d@10800000                     rtc@10070000
g3d-power-domain@10023C60        sdhci@12510000
gps-alive-power-domain@10023D00  sdhci@12520000
gps-power-domain@10023CE0        sdhci@12530000
hdmi@12D00000                    sdhci@12540000
hsotg@12480000                   serial@13800000
i2c-gpio-0                       serial@13810000
i2c@13860000                     serial@13820000
i2c@13870000                     serial@13830000
i2c@13880000                     spi@13920000
 
......

可以看看上一篇博文中的用软件I2C控制MMA7660的设备树的结构:

[root@tiny4412 base]# cd i2c-gpio-0
[root@tiny4412 i2c-gpio-0]# ls
#address-cells     compatible         i2c-gpio,delay-us  name
#size-cells        gpios              mma7660@4c         status
[root@tiny4412 i2c-gpio-0]# cd mma7660@4c/
[root@tiny4412 mma7660@4c]# ls
compatible        interrupt-parent  poll_interval
input_flat        interrupts        reg
input_fuzz        name              status
[root@tiny4412 mma7660@4c]#

可以看到,mma7660@4c确实是i2c-gpio-0的子目录,而且我们也知道mma7660对应的device node确实是i2c-gpio-0对应的device node的child device node。

可以看看platform device的注册情况:

[root@tiny4412 root]# cd /sys/bus/platform/
[root@tiny4412 platform]# ls
devices            drivers_autoprobe  uevent
drivers            drivers_probe
[root@tiny4412 platform]# cd devices/
[root@tiny4412 devices]# ls
10000000.chipid                  12530000.sdhci
10010000.syscon                  12550000.mmc
10020000.system-controller       12580000.ehci
10023c00.cam-power-domain        12590000.ohci
10023c20.tv-power-domain         125b0000.exynos-usbphy
10023c40.mfc-power-domain        12a30000.sysmmu
10023c60.g3d-power-domain        12e20000.sysmmu
10023c80.lcd0-power-domain       13620000.sysmmu
10023ca0.isp-power-domain        13630000.sysmmu
10023ce0.gps-power-domain        13800000.serial
10023d00.gps-alive-power-domain  13810000.serial
10030000.clock-controller        13820000.serial
10050000.mct                     13830000.serial
10070000.rtc                     139d0000.pwm
10440000.interrupt-controller    2020000.sysram
10490000.interrupt-controller    3810000.clock-controller
10502000.l2-cache-controller     3860000.pinctrl
106e0000.pinctrl                 alarmtimer
10a40000.sysmmu                  amba
11000000.pinctrl                 backlight
11400000.pinctrl                 cpufreq-dt
11840000.jpeg-codec              exynos-drm
11a20000.sysmmu                  i2c-gpio-0
11a30000.sysmmu                  leds
11a40000.sysmmu                  opp_table0
11a50000.sysmmu                  pmu
11a60000.sysmmu                  reg-dummy
11e20000.sysmmu                  regulators
12260000.sysmmu                  regulators:regulator@0
12270000.sysmmu                  regulatory.0
122a0000.sysmmu                  serial8250
122b0000.sysmmu                  snd-soc-dummy
123b0000.sysmmu                  usb-hub
123c0000.sysmmu                  video-phy@10020710
12480000.hsotg

可以看到,在设备树中:

regulators {
    compatible = "simple-bus";
    #address-cells = <0x1>;
    #size-cells = <0x0>;
 
    regulator@0 {
        compatible = "regulator-fixed";
        reg = <0x0>;
        regulator-name = "VMEM_VDD_2.8V";
        regulator-min-microvolt = <0x2ab980>;
        regulator-max-microvolt = <0x2ab980>;
        linux,phandle = <0x19>;
        phandle = <0x19>;
    };
};

regulator@0虽然是regulator的child device node,而在/proc/device-tree(用于呈现device node的父子关系)中却看不到regulator@0对应的目录(其实是放在了regulator目录的下面),但是在/sys/bus/platform/devices/下却可以看得到(说明regulator@0这个device node也被注册为了platform_device)。

二、/sys/firmware

在/sys/firmware下也可以看到devicetree的导出信息:

[root@tiny4412 root]# cd /sys/firmware/
[root@tiny4412 firmware]# ls -F
devicetree/ fdt

其中fdt是一个二进制文件,其中是完整的设备树镜像,也就是bootloader最终传给kernel的设备树镜像文件,如果是在Andriod系统上,可以用adb pull将该文件导出到开发机上,然后使用dtc对导出的文件进行反编译:

adb pull /sys/firmware/fdt ./fdt
dtc -I dtb -O dts -o fdt.dts ./fdt

这样就可以用编辑器查看fdt.dts文件了。

此外,这个文件可以用hexdump查看:

[root@tiny4412 root]# hexdump -C  /sys/firmware/fdt | head -n 100
00000000  d0 0d fe ed 00 00 dc 2d  00 00 00 48 00 00 a3 ec  |.......-...H....|
00000010  00 00 00 28 00 00 00 11  00 00 00 10 00 00 00 00  |...(............|
00000020  00 00 08 ad 00 00 a3 a4  00 00 00 00 43 a7 f0 00  |............C...|
00000030  00 00 00 00 00 27 bb 09  00 00 00 00 00 00 00 00  |.....'..........|
00000040  00 00 00 00 00 00 00 00  00 00 00 01 00 00 00 00  |................|
00000050  00 00 00 03 00 00 00 04  00 00 00 00 00 00 00 01  |................|
00000060  00 00 00 03 00 00 00 04  00 00 00 0f 00 00 00 01  |................|
00000070  00 00 00 03 00 00 00 04  00 00 00 1b 00 00 00 01  |................|
00000080  00 00 00 03 00 00 00 38  00 00 00 2c 66 72 69 65  |.......8...,frie|
00000090  6e 64 6c 79 61 72 6d 2c  74 69 6e 79 34 34 31 32  |ndlyarm,tiny4412|
000000a0  00 73 61 6d 73 75 6e 67  2c 65 78 79 6e 6f 73 34  |.samsung,exynos4|
000000b0  34 31 32 00 73 61 6d 73  75 6e 67 2c 65 78 79 6e  |412.samsung,exyn|
000000c0  6f 73 34 00 00 00 00 03  00 00 00 2f 00 00 00 37  |os4......../...7|
000000d0  46 72 69 65 6e 64 6c 79  41 52 4d 20 54 49 4e 59  |FriendlyARM TINY|
000000e0  34 34 31 32 20 62 6f 61  72 64 20 62 61 73 65 64  |4412 board based|
000000f0  20 6f 6e 20 45 78 79 6e  6f 73 34 34 31 32 00 00  | on Exynos4412..|
00000100  00 00 00 01 63 68 6f 73  65 6e 00 00 00 00 00 03  |....chosen......|
00000110  00 00 00 04 00 00 08 9c  43 cf ab 08 00 00 00 03  |........C.......|
00000120  00 00 00 04 00 00 08 89  43 a7 f0 00 00 00 00 03  |........C.......|
00000130  00 00 00 11 00 00 00 3d  2f 73 65 72 69 61 6c 40  |.......=/serial@|

可以看到开头的四个字节正好是d00dfeed,这个文件跟原始的设备树文件还是有些不同的,如chosen节点和memory节点。因为在用u-boot引导的时候,u-boot根据当前的环境对设备树镜像内容进行修改,下面是不同的地方:

我dump的方法是将fdt的内容用上面的命令重定向到一个文件中(hexdump –C /sys/firmware/fdt > /mnt/fdt.txt),然后通过U盘拷贝到电脑上,复制其中的部分信息,利用winhex文件创建一个二进制文件。再用fdtdump工具(fdtdump dtb文件)将dtb的文件信息导出到一个文本文件中,最后再做比较。

在/sys/firmware/devicetree/base/下也是以device node的父子关系创建的文件和目录,其实会发现,/proc/device-tree是一个软连接,指向的就是/sys/firmware/devicetree/base/:

[root@tiny4412 root]# ls /proc/device-tree -l
lrwxrwxrwx    1 0        0               29 Jan  1 06:11 /proc/device-tree -> /sys/firmware/devicetree/base
[root@tiny4412 root]#

那么/sys/firmware/fdt以及/sys/firmware/devicetree是在什么地方创建的呢?

/sys/firmware/devicetree的创建:

start_kernel ---> rest_init ---> kernel_init ---> kernel_init_freeable ---> do_basic_setup ---> driver_init ---> of_core_init

在of_core_init函数中(drivers/of/base.c):

void __init of_core_init(void)
{
    struct device_node *np;

    /* Create the kset, and register existing nodes */
    mutex_lock(&of_mutex);
    of_kset = kset_create_and_add("devicetree", NULL, firmware_kobj);
    if (!of_kset) {
        mutex_unlock(&of_mutex);
        pr_err("devicetree: failed to register existing nodes\n");
        return;
    }
    for_each_of_allnodes(np)
        __of_attach_node_sysfs(np);
    mutex_unlock(&of_mutex);

    /* Symlink in /proc as required by userspace ABI */
    if (of_root)
        proc_symlink("device-tree", NULL, "/sys/firmware/devicetree/base");
}

/sys/firmware/fdt的创建(drivers/of/fdt.c):

#ifdef CONFIG_SYSFS
static ssize_t of_fdt_raw_read(struct file *filp, struct kobject *kobj,
                   struct bin_attribute *bin_attr,
                   char *buf, loff_t off, size_t count)
{
    memcpy(buf, initial_boot_params + off, count);
    return count;
}

static int __init of_fdt_raw_init(void)
{
    static struct bin_attribute of_fdt_raw_attr =
        __BIN_ATTR(fdt, S_IRUSR, of_fdt_raw_read, NULL, 0);

    if (!initial_boot_params)
        return 0;

    if (of_fdt_crc32 != crc32_be(~0, initial_boot_params,
                     fdt_totalsize(initial_boot_params))) {
        pr_warn("fdt: not creating '/sys/firmware/fdt': CRC check failed\n");
        return 0;
    }
    of_fdt_raw_attr.size = fdt_totalsize(initial_boot_params);
    return sysfs_create_bin_file(firmware_kobj, &of_fdt_raw_attr);
}
late_initcall(of_fdt_raw_init);

三、测试

下面测试一下根据parent device node的compatible的不同,在populate的时候会把不同的device node注册为不同的device。

修改arch/arm/boot/dts/exynos4412-tiny4412.dts:

1: diff --git a/arch/arm/boot/dts/exynos4412-tiny4412.dts b/arch/arm/boot/dts/exynos4412-tiny4412.dts
   2: index 579a507..ae29aa8 100644
   3: --- a/arch/arm/boot/dts/exynos4412-tiny4412.dts
   4: +++ b/arch/arm/boot/dts/exynos4412-tiny4412.dts
   5: @@ -129,6 +129,30 @@
   6:          };
   7:      };
   8:  #endif
   9: +
  10: +    demo_parent0 {
  11: +        compatible = "simple-bus";
  12: +
  13: +        child0{
  14: +            compatible = "child0";
  15: +        };
  16: +
  17: +        child1{
  18: +            compatible = "child1";
  19: +        };
  20: +    };
  21: +
  22: +    demo_parent1 {
  23: +        compatible = "demo_parent1";
  24: +
  25: +        child3{
  26: +            compatible = "child3";
  27: +        };
  28: +
  29: +        child4{
  30: +            compatible = "child4";
  31: +        };
  32: +    };
  33:  };
  34:  
  35:  &rtc {

其中第11行表示demo_parent0对应的device node将来会在of_platform_populate时递归注册它的的child device node为platform_device。其实只要demo_parent1的compatible字段含有字符串"simple-bug"即可(如:compatible = "demo_parent1","simple-bus"),字符串"simple-bus"来自数组of_default_bus_match_table:

1:  
   2: const struct of_device_id of_default_bus_match_table[] = {
   3:     { .compatible = "simple-bus", },
   4:     { .compatible = "simple-mfd", },
   5: #ifdef CONFIG_ARM_AMBA
   6:     { .compatible = "arm,amba-bus", },
   7: #endif /* CONFIG_ARM_AMBA */
   8:     {} /* Empty terminated list */
   9: };

重新编译设备树(make dtbs),启动内核可以发现:

在/proc/device-tree/下(仅表示device node之间的父子逻辑关系):

1: [root@tiny4412 root]# cd /proc/device-tree/
   2: [root@tiny4412 base]# ls
   3: ......
   4: demo_parent0                     ppmu_mfc_left@13660000
   5: demo_parent1                     ppmu_mfc_right@13670000
   6: ......
   7: [root@tiny4412 base]# ls -R demo_parent* 
   8: demo_parent0:
   9: child0      child1      compatible  name
  10:  
  11: demo_parent0/child0:
  12: compatible  name
  13:  
  14: demo_parent0/child1:
  15: compatible  name
  16:  
  17: demo_parent1:
  18: child3      child4      compatible  name
  19:  
  20: demo_parent1/child3:
  21: compatible  name
  22:  
  23: demo_parent1/child4:
  24: compatible  name

在/sys/bus/platform/devices/下:

1: [root@tiny4412 root]# cd /sys/bus/platform/devices/
   2: [root@tiny4412 devices]# ls
   3: ......
   4: 11000000.pinctrl                 demo_parent0
   5: 11400000.pinctrl                 demo_parent0:child0
   6: 11840000.jpeg-codec              demo_parent0:child1
   7: 11a20000.sysmmu                  demo_parent1
   8: 11a30000.sysmmu                  exynos-drm
   9: ......

可以看到demo_parent0、child0和child1都被注册为了platform_device,demo_parent1也被注册为了platform_device,而child3和child4却没有。child3和child4的具体被注册为什么设备需要由demo_parent1对应的platform_device的驱动程序决定。

未完待续……

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