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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。
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; }
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。
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。
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
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。
标签：style blog http color io ar strong for 文件
原文地址：http://www.cnblogs.com/all-for-fiona/p/3949654.html
