动手定制一个开发板案例

刘盼

发布于 2020-02-13 18:12:41

1K0

发布于 2020-02-13 18:12:41

前面通过学习总线、设备、驱动模型知识后，知道了设备和驱动之间都是通过总线进行绑定而匹配的；然后通过设备树的深入探究，知道了设备树的出现大大增加了驱动的通用性；接着我们一起看了 Linux 的启动流程和设备在内核里一层一层的展开。

通过前面的课程学习，相信内核初学者对内核和驱动有了一个大概的感性认识，有内核经验的小伙伴相信也有了系统化的梳理。都说“实践是检验真理的唯一标准”，为了检验前面的理论性，更为了加深理解，这里我们手把手一起从设备树入手，模拟一个电路板，上面有中断控制器、GPIO 控制器、I2C 控制器、SPI 控制器、以太网控制器等，并根据这个电路板从头到尾构建一个 DTS 文件，使内核支持我们自己定制的开发板。

添加一个新的 SOC

我们假设 csdn 是一家芯片厂商，研发了一块叫做 gitchat 的芯片，对应的开发板叫做 evb。需要做什么工作才能定制一套开发板并且使 Linux 支持我们这块电路板呢？

首先我们在 arch/arm 目录下创建一个 mach-gitchat 的目录，然后创建 arch/arm/machgitchat/Kconfig，Makefile，common.c。

其中，Kconfig 里做了一个叫 ARCH_GITCHAT 的假芯片，common.c 是对应的驱动，Makefile 里对这个驱动做了编译。我们来简单的看一下这个驱动：

就像前几节课讲的那样，我们是通过 DT_MACHINE_START 来定义一个电路板的，里面有很多回调函数，这里简单的写了 init_late 和 dt_compat，其中 dt_compat 会根据设备树里的字段进行匹配，如果字段一致就说明匹配正确，这里的字段是“csdn, gitchat”。

添加对应的 DTS

现在已经在 Linux 里添加了我们定义的 soc，接下来需要添加 soc 对应的设备树，即具体的板级文件信息。

在 arch/arm/boot/dts 下创建 csdn-gitchat.dtsi 和 csdn-gitchat-evb.dts。csdn-gitchat.dtsi 是对芯片的描述，csdn-gitchat-evb.dts 是针对这个芯片的 evb 板子描述，其内容分别如下：

/*
 * DTS file for demo platform for Dec20,2017 CSDN/Gitchat course
 *
 * Copyright (c) 2017 xxx <xxx@gmail.com>
 *
 * Licensed under GPLv2.
 */

/ {
    compatible = "csdn,gitchat";
    #address-cells = <1>;
    #size-cells = <1>;
    interrupt-parent = <&intc>;

    aliases {
        serial0 = &uart0;
    };

    cpus {
        #address-cells = <1>;
        #size-cells = <0>;

        cpu@0 {
            compatible = "arm,cortex-a9";
            device_type = "cpu";
            reg = <0x0>;
        };
        cpu@1 {
            compatible = "arm,cortex-a9";
            device_type = "cpu";
            reg = <0x1>;
        };
    };

    axi@40000000 {
        compatible = "simple-bus";
        #address-cells = <1>;
        #size-cells = <1>;
        ranges = <0x40000000 0x40000000 0x80000000>;

        l2-cache-controller@80040000 {
            compatible = "arm,pl310-cache";
            reg = <0x80040000 0x1000>;
            interrupts = <59>;
            arm,tag-latency = <1 1 1>;
            arm,data-latency = <1 1 1>;
            arm,filter-ranges = <0 0x40000000>;
        };

        intc: interrupt-controller@80020000 {
            #interrupt-cells = <1>;
            interrupt-controller;
            compatible = "csdn,gitchat";
            reg = <0x80020000 0x1000>;
        };

        peri-iobg@b0000000 {
            compatible = "simple-bus";
            #address-cells = <1>;
            #size-cells = <1>;
            ranges = <0x0 0xb0000000 0x180000>;

            ethernet@10000 {
                compatible = "davicom,dm9000";
                reg = <0x10000 0x2 0x10004 0x2>;
                interrupts = <7>;
                davicom,no-eeprom;
            };

            timer@20000 {
                compatible = "csdn,gitchat-tick";
                reg = <0x20000 0x1000>;
                interrupts = <0>;
                clocks = <&clks 11>;
            };

            clks: clock-controller@30000 {
                compatible = "csdn,gitchat-clkc";
                reg = <0x30000 0x1000>;
                #clock-cells = <1>;
            };

            gpio0: goio@40000 {
                #gpio-cells = <2>;
                #interrupt-cells = <2>;
                compatible = "csdn,gitchat-gpio";
                reg = <0x40000 0x200>;
                interrupts = <43>;
                gpio-controller;
                interrupt-controller;
            };

            uart0: uart@50000 {
                cell-index = <0>;
                compatible = "csdn,gitchat-uart";
                reg = <0x50000 0x1000>;
                interrupts = <17>;
                status = "disabled";
            };

            spi0: spi@d0000 {
                cell-index = <0>;
                compatible = "csdn,gitchat-spi";
                reg = <0xd0000 0x10000>;
                interrupts = <15>;
                #address-cells = <1>;
                #size-cells = <0>;
                status = "okay";
            };

            i2c0: i2c@e0000 {
                cell-index = <0>;
                compatible = "csdn,gitchat-i2c";
                reg = <0xe0000 0x10000>;
                interrupts = <24>;
                #address-cells = <1>;
                #size-cells = <0>;
                status = "okay";
            };
        };
    };
};

/dts-v1/;

/include/ "csdn-gitchat.dtsi"

/ {
    model = "Csdn Gitchat EVB Board";
    compatible = "csdn,gitchat-evb", "csdn,gitchat";

    memory {
        device_type = "memory";
        reg = <0x00000000 0x20000000>;
    };

    chosen {
        bootargs = "console=ttyS0,115200 root=/dev/mtdblock5 rw rootfstype=ubifs";
    };

    axi@40000000 {
        peri-iobg@b0000000 {
            spi@d0000 {
                status = "disabled";
            };
            i2c@e0000 {
                pixcir_ts@5c {
                    compatible = "pixcir,pixcir_tangoc";
                    reg = <0x5c>;
                    interrupt-parent = <&gpio0>;
                    interrupts = <17 0>;
                    attb-gpio = <&gpio0 22 0>;
                    touchscreen-size-x = <1024>;
                    touchscreen-size-y = <600>;
                };
            };
        };
    };

};

&uart0 {
    status = "okay";
};

由于篇幅限制，这里我们只写了部分代码。

DTS 的编译

在 arch/arm/boot/dts/Makefile 里添加我们的板子：