Linux Kernel Version：3.0.35

Platform：Freescale DSA2L

通过I2C读取VGA屏的EDID信息（主要是分辨率），解析后喂给CH7036芯片（LVDS转VGA，DVI，HDMI芯片），提供文件节点给User Space。

代码流程

由于EDID协议规定I2C的读取Slave地址为0x50，所以先添加Device注册代码：

i2c.h

#define I2C_BOARD_INFO(dev_type, dev_addr) \
    .type = dev_type, .addr = (dev_addr)

board-mx6q_dsa2l.c

static struct i2c_board_info mxc_i2c2_board_info[] __initdata = {
    {
        I2C_BOARD_INFO("mxc_edid_i2c", 0x50),
        .platform_data = NULL,
        .irq = DSA2L_VGA_CABLE_IN, //not irq just a gpio
    },
};
i2c_register_board_info(2, mxc_i2c2_board_info,
            ARRAY_SIZE(mxc_i2c2_board_info)); //注册i2c设备

编写驱动文件mxcfb_ch7036_edid.c

添加I2C驱动：

static const struct i2c_device_id mxc_edid_i2c_id[] = {
    { "mxc_edid_i2c", 0 },
    {},
};
MODULE_DEVICE_TABLE(i2c, mxc_edid_i2c_id);

static struct i2c_driver mxc_edid_i2c_driver = {
    .driver = {
           .name = "mxc_edid_i2c",
           },
    .probe = mxc_edid_i2c_probe,
    .remove = mxc_edid_i2c_remove,
    .id_table = mxc_edid_i2c_id,
};

static int __init mxc_edid_i2c_init(void)
{
    return i2c_add_driver(&mxc_edid_i2c_driver);
}

static void __exit mxc_edid_i2c_exit(void)
{
    i2c_del_driver(&mxc_edid_i2c_driver);
}

module_init(mxc_edid_i2c_init);
module_exit(mxc_edid_i2c_exit);

probe函数，创建了文件节点给用户空间读取：

static int __devinit mxc_edid_i2c_probe(struct i2c_client *client, const struct i2c_device_id *id){
    int ret;
    edid_vga_client = client;

    if (!i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_BYTE | I2C_FUNC_I2C))
        return -ENODEV;

    edid_all_buf = kmalloc(edid_buf_size, GFP_KERNEL);

    edid_vga_class = class_create(THIS_MODULE, "mxc_edid_class");
    edid_vga_dev = device_create(edid_vga_class, NULL, MKDEV(I2C_MAJOR, 50), NULL, "mxc_edid_dev");

    ret = device_create_file(edid_vga_dev, &dev_attr_all_register_value);
    if (ret < 0){
        dev_warn(&client->dev, "cound not create sys node for dev_attr_all_register_value\n");
    }
    ret = device_create_file(edid_vga_dev, &dev_attr_plug_state);
    if (ret < 0){
        dev_warn(&client->dev, "cound not create sys node for dev_attr_plug_state\n");
    }
    ret = device_create_file(edid_vga_dev, &dev_attr_timing);
    if (ret < 0){
        dev_warn(&client->dev, "cound not create sys node for dev_attr_timing\n");
    }

    return 0;
}

DEVICE_ATTR，列举一个：

static DEVICE_ATTR(all_register_value, S_IRUGO, mxc_edid_show_state, NULL);

读取文件节点对应的处理函数：

static ssize_t mxc_edid_show_state(struct device *dev, struct device_attribute *attr, char *buf)
{
    int ret, i;
    ret = mxc_edid_edidread(edid_vga_client);
    if (ret <= 0)
        return ret;

    for(i = 0; i < edid_buf_size; i++){
        if(i!=0 && i%10 == 0){
            printk("\n");
            printk("%02X ", edid_all_buf[i]);
        }else{
            printk("%02X ", edid_all_buf[i]);
        }
    }
    printk("\n");
    return strlen(buf);
}

通过I2C读取EDID：

static inline int edid_i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num){
    int ret = 0;

    ret = gpio_get_value(edid_vga_client->irq); // 读取vga_detect pin，低电平表示接入vga
    if(!ret){
        ret = i2c_transfer(adap, msgs, num);
        if (ret != num) {
            return -EIO;
        }
    }
    return ret;
}

static int mxc_edid_edidread(struct i2c_client *client){
    int ret = 0;
    unsigned char offset = 0;
    struct i2c_adapter *adp = client->adapter;
    struct i2c_msg msg[2] = {
        {
        .addr    = edid_addr,
        .flags    = 0,
        .len    = 1,
        .buf    = &offset,
        }, {
        .addr    = edid_addr,
        .flags    = I2C_M_RD,
        .len    = edid_buf_size,
        .buf    = edid_all_buf,
        },
    };

    ret = edid_i2c_transfer(adp, msg, ARRAY_SIZE(msg));

    return ret;
}

知识细节

I2C时序图

所以前面 i2c_msg 数组定义有两个msg，先是写一个寄存器地址，然后是读取指定长度。

i2c_transfer 和 i2c_smbus_write_byte_data

smbus 是 i2c 的子集，是轻量级的i2c，最大传输速度比i2c慢，常见于电源管理。

追踪代码可以发现 i2c_smbus_write_byte_data 其实本质就是对 i2c_transfer 的封装，先不做深究。

关于/dev/i2c-x

用户空间通过操作/dev/i2c-x这些i2c适配器（比如ioctl），也可以对i2c设备进行操作。TODO：研究此部分

关于文件节点的操作

代码流程：

struct device *edid_vga_dev;
static struct class *edid_vga_class;

static DEVICE_ATTR(all_register_value, S_IRUGO, mxc_edid_show_state, NULL);

edid_vga_class = class_create(THIS_MODULE, "mxc_edid_class");
edid_vga_dev = device_create(edid_vga_class, NULL, MKDEV(I2C_MAJOR, 50), NULL, "mxc_edid_dev");

ret = device_create_file(edid_vga_dev, &dev_attr_all_register_value);

class_create：创建一个class，如“mxc_edid_class”，这样，会在对应的设备目录下创建class目录，比如：/sys/devices/virtual/mxc_edid_class

device_create：创建一个device，第一个参数：class，参数二：parent device，参数三，设备号，参数四：相关连的device，参数五：device name

device_create_file：创建sysfs attribute file

关于DEVICE_ATTR，device.h：

#define DEVICE_ATTR(_name, _mode, _show, _store) struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)

sysfs.h：

#define __ATTR(_name,_mode,_show,_store) { \
    .attr = {.name = __stringify(_name), .mode = _mode },        .show    = _show,                        .store    = _store,                    }

所以，第一个参数为名字，第二个参数是访问权限，第三个参数是用户空间 cat 时调用的函数，第四个参数是用户空间 echo 时调用的函数。