内核I2C驱动GM8775

一. 屏幕参数计算

屏幕型号：星科QV185FHM-N81.FY 18.5寸LVDS接口屏幕
屏幕参数计算（详见LVDS 屏幕 M215HGE-L21 在 rk3288 上的适配过程 - 大窟窿 - 博客园）

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clock-frequency = <74000000>;
   hactive = <1920>;
   vactive = <1080>;
   hfront-porch = <100>;
   hsync-len = <5>;
   hback-porch = <35>;
   vfront-porch = <20>;
   vsync-len = <10>;
   vback-porch = <15>;
   hsync-active = <0>;
   vsync-active = <0>;
   de-active = <0>;
   pixelclk-active = <1>;

我们需要的参数就是这些，在大部分屏幕手册中，会给出上面这些相关参数，也有很多屏幕是没有参数的，需要自己计算
我们这个屏幕，手册中的参数是只有下图这些参数

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从图片中可以知道已知参数频率为clock-frequency为74.25Mhz

disp_timings 中的 hback-porch, hfront-porch, vback-porch, vfront-porch, hsync-len, vsync-len 的值。它们之间的关系如下：
Tv = vback-porch + vfront-porch + vsync-len + Tvd
Th = hback-porch + hfront-porch + hsync-len + Thd

则
vback-porch + vfront-porch + vsync-len = 1125 - 1080 = 45
hback-porch + hfront-porch + hsync-len = 1100 - 960 = 140

这里只需要保证这 3 个参数之和等于那个数就可以了，对具体的值没有特别的要求

故得到以下参数：
	hback-porch = <35>;
    hfront-porch = <100>;
    hsync-len = <5>;
    与
    vfront-porch = <20>;
    vsync-len = <10>;
    vback-porch = <15>;

二. 生成配置指令

2.1 GM8775C：MIPI转双路LVDS芯片

  • 配置方法：该芯片有一款上位机生成初始化指令的软件，分为两种方式
    I2C指令与MIPI指令

  • I2C指令：指的是使用I2C写入芯片配置参数来使得芯片将SOC的MIPI信号正确转换为LVDS信号，即上面生成的参数

  • MIPI指令：指的是上电初始化时使用MIPI写入芯片配置参数来使得芯片将SOC的MIPI信号正确转换为LVDS信号，即上面生成的参数（目前MIPI写入配置方式存在问题，待供应商支持）

  • 软件获取参数使用方法：首先填入屏幕参数，根据上面求得的参数，配置视频格式中的参考时钟、行场等参数，最后创建寄存器表，若配置方式选择I2C，直接用USB转I2C连接器写入即可显示，注意此芯片所有配置掉电丢失，也就是说每次开机都要写入配置

ID看GM8775C的手册，I2C_ADDR 管脚通过上下拉选择 I2C 器件地址，拉高为：0x5A，拉低为：0x58

Link01相关的参数看屏幕手册，VESA与8bit参数可在屏幕手册中找到

    `配置LINK EVEN / ODD奇偶场，如果屏幕显示两侧未连续，则需要交换EVEN / ODD`
    `0x27是mipi指令的密码解锁地址，0x00是I2C指令的密码解锁地址`

按照图片中的指示与上述描述，在软件上填好并生成寄存器指令表

三. 内核与设备树

3.1 编写内核代码

  • 下列代码其实就是i2c驱动示例代码，加上前面生成寄存器指令表所写的gm8775c_reg_defaults数组，通过i2c驱动形式，将数组配置写入芯片，用以配置MIPI转LVDS，在此板卡对于不同型号的屏幕，只需要改gm8775c_reg_defaults数组即可，其余不需要改动
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#include <linux/clk.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/init.h>
#include <linux/delay.h>
#include <linux/i2c.h>
#include <linux/spi/spi.h>
#include <linux/platform_device.h>
#include <linux/slab.h>
#include <linux/of_gpio.h>
#include <linux/proc_fs.h>
#include <linux/gpio.h>
#include <linux/interrupt.h>
#include <linux/irq.h>
#include <linux/regmap.h>
 
#define REGISTER_NUM 34
 
#define GPIO_PIN_509 509
#define GPIO_PIN_511 511
#define GPIO_PIN_493 493

struct gm8775c_priv {
	struct regmap *regmap;
	struct i2c_client *client;
    	int gpio_509;
    	int gpio_511;
    	int gpio_493;
};

static const struct reg_default gm8775c_reg_defaults[] = {   
	{0x00,0xAA},
	{0x48,0x02},
	{0xB6,0x20},
	{0x01,0x80},
	{0x02,0x38},
	{0x03,0x47},
	{0x04,0x64},
	{0x05,0x05},
	{0x06,0x23},
	{0x07,0x00},
	{0x08,0x14},
	{0x09,0x0A},
	{0x0A,0x0F},
	{0x0B,0x82},
	{0x0C,0x13},
	{0x0D,0x01},
	{0x0E,0x80},
	{0x0F,0x20},
	{0x10,0x20},
	{0x11,0x03},
	{0x12,0x1B},
	{0x13,0x63},
	{0x14,0x01},
	{0x15,0x23},
	{0x16,0x40},
	{0x17,0x00},
	{0x18,0x01},
	{0x19,0x23},
	{0x1A,0x40},
	{0x1B,0x00},
	{0x1E,0x46},
	{0x51,0x30},
	{0x1F,0x10},
	{0x2A,0x01},
};
 
static int gm8775c_read(u8 reg, u8 * rt_value, struct i2c_client *client)
{
	int ret;
	u8 read_cmd[3] = { 0 };
	u8 cmd_len = 0;
 
	read_cmd[0] = reg;
	cmd_len = 1;
 
	if (client->adapter == NULL)
		printk("gm8775c_read client->adapter==NULL\n");
 
	ret = i2c_master_send(client, read_cmd, cmd_len);
	if (ret != cmd_len) {
		printk("gm8775c_read error1\n");
		return -1;
	}
 
	ret = i2c_master_recv(client, rt_value, 1);
	if (ret != 1) {
		printk("gm8775c_read error2, ret = %d.\n", ret);
		return -1;
	}
 
	return 0;
}

static int gm8775c_write(u8 reg, unsigned char value, struct i2c_client *client)
{
	int ret = 0;
	u8 write_cmd[2] = { 0 };
 
	write_cmd[0] = reg;
	write_cmd[1] = value;
 
	ret = i2c_master_send(client, write_cmd, 2);
	if (ret != 2) {
		printk("gm8775c_write error->[REG-0x%02x,val-0x%02x]\n",
		       reg, value);
		return -1;
	}
 
	return 0;

}

 
static const struct of_device_id gm8775_of_match[] = { 
	{
		.compatible = "gm8775c,mipi_to_lvds",
	}, 
	{ }
};
MODULE_DEVICE_TABLE(of, gm8775_of_match);
 
static const struct i2c_device_id gm8775_i2c_id[] = {
	{"GM8775C_MTL", 0},
	{}
};
 
static ssize_t gm8775c_store(struct device *dev, struct device_attribute *attr, const char *buf, size_t count)
{
	int val=0, flag=0;
	u8 i=0, reg, num, value_w, value_r;
 
	
	struct gm8775c_priv *gm8775c = dev_get_drvdata(dev);
	
	val = simple_strtol(buf, NULL, 16);
	flag = (val >> 16) & 0xFF;
	
	if (flag) {
		reg = (val >> 8) & 0xFF;
		value_w = val & 0xFF;
		printk("\nWrite: start REG:0x%02x,val:0x%02x,count:0x%02x\n", reg, value_w, flag);
		while(flag--) {
			gm8775c_write(reg, value_w,  gm8775c->client);
			printk("Write 0x%02x to REG:0x%02x\n", value_w, reg);
			reg++;
		}
	} else {
		reg = (val >> 8) & 0xFF;
		num = val & 0xff;
		printk("\nRead: start REG:0x%02x,count:0x%02x\n", reg, num);
		do {
			value_r = 0;
			gm8775c_read(reg, &value_r, gm8775c->client);
			printk("REG[0x%02x]: 0x%02x;  ", reg, value_r);
			reg++;
			i++;
			if ((i==num) || (i%4==0))	printk("\n");
		} while (i<num);
	}
	
	return count;
}
 
static ssize_t gm8775c_show(struct device *dev, struct device_attribute *attr, char *buf)
{
	printk("echo flag|reg|val > gm8775c\n");
	printk("eg read star addres=0x06,count 0x10:echo 0610 >gm8775c\n");
	printk("eg write star addres=0x90,value=0x3c,count=4:echo 4903c >gm8775c\n");
	return 0;
}
 
static DEVICE_ATTR(gm8775c, 0644, gm8775c_show, gm8775c_store);
 
static struct attribute *egm8775c_debug_attrs[] = {
	&dev_attr_gm8775c.attr,
	NULL,
};
 
static struct attribute_group gm8775c_debug_attr_group = {
	.name   = "gm8775c_debug",
	.attrs  = egm8775c_debug_attrs,
};

 
static int gm8775c_probe(struct i2c_client *client, const struct i2c_device_id *id)
{
    struct gm8775c_priv *gm8775c;
    int i;
    int ret;

    dev_info(&client->dev, "Probing gm8775c at address 0x%02x\n", client->addr);

    if (!i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_I2C_BLOCK |
                                 I2C_FUNC_SMBUS_BYTE_DATA)) {
        dev_err(&client->dev, "doesn't support I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_I2C_BLOCK\n");
        return -ENODEV;
    }

    gm8775c = devm_kzalloc(&client->dev, sizeof(*gm8775c), GFP_KERNEL);
    if (!gm8775c)
        return -ENOMEM;

    gm8775c->client = client;
    i2c_set_clientdata(client, gm8775c);

    gm8775c->gpio_509 = GPIO_PIN_509;
    ret = gpio_request(gm8775c->gpio_509, "gm8775c_gpio_509");
    if (ret) {
        dev_err(&client->dev, "Failed to request GPIO %d\n", gm8775c->gpio_509);
        return ret;
    }

    ret = gpio_direction_output(gm8775c->gpio_509, 1);
    if (ret) {
        dev_err(&client->dev, "Failed to set GPIO %d as output\n", gm8775c->gpio_509);
        gpio_free(gm8775c->gpio_509);
        return ret;
    }
    dev_info(&client->dev, "Configured GPIO %d as output and set to 1\n", gm8775c->gpio_509);

    gm8775c->gpio_511 = GPIO_PIN_511;
    ret = gpio_request(gm8775c->gpio_511, "gm8775c_gpio_511");
    if (ret) {
        dev_err(&client->dev, "Failed to request GPIO %d\n", gm8775c->gpio_511);
        gpio_free(gm8775c->gpio_509);
        return ret;
    }

    ret = gpio_direction_output(gm8775c->gpio_511, 0);
    if (ret) {
        dev_err(&client->dev, "Failed to set GPIO %d as output\n", gm8775c->gpio_511);
        gpio_free(gm8775c->gpio_511);
        gpio_free(gm8775c->gpio_509);
        return ret;
    }
    dev_info(&client->dev, "Configured GPIO %d as output and set to 0\n", gm8775c->gpio_511);

    gm8775c->gpio_493 = GPIO_PIN_493;
    ret = gpio_request(gm8775c->gpio_493, "gm8775c_gpio_493");
    if (ret) {
        dev_err(&client->dev, "Failed to request GPIO %d\n", gm8775c->gpio_493);
        gpio_free(gm8775c->gpio_511);
        gpio_free(gm8775c->gpio_509);
        return ret;
    }

    ret = gpio_direction_output(gm8775c->gpio_493, 1);
    if (ret) {
        dev_err(&client->dev, "Failed to set GPIO %d as output\n", gm8775c->gpio_493);
        gpio_free(gm8775c->gpio_493);
        gpio_free(gm8775c->gpio_511);
        gpio_free(gm8775c->gpio_509);
        return ret;
    }
    dev_info(&client->dev, "Configured GPIO %d as output and set to 1\n", gm8775c->gpio_493);

    for (i = 0; i < REGISTER_NUM; i++) {
        ret = gm8775c_write(gm8775c_reg_defaults[i].reg, gm8775c_reg_defaults[i].def, gm8775c->client);

        dev_info(&client->dev, "Wrote and verified 0x%02x to register 0x%02x\n",
                 gm8775c_reg_defaults[i].def, gm8775c_reg_defaults[i].reg);
    }

    ret = sysfs_create_group(&client->dev.kobj, &gm8775c_debug_attr_group);
    if (ret) {
        dev_err(&client->dev, "failed to create attr group\n");

    }

    dev_info(&client->dev, "gm8775c driver loaded successfully.\n");
    return 0;


    gpio_free(gm8775c->gpio_493);
    gpio_free(gm8775c->gpio_511);
    gpio_free(gm8775c->gpio_509);
    return ret;
}
 
static void gm8775c_remove(struct i2c_client *client)
{ 
	struct gm8775c_priv *gm8775c = i2c_get_clientdata(client);

    sysfs_remove_group(&client->dev.kobj, &gm8775c_debug_attr_group);

    gpio_free(gm8775c->gpio_493);
    gpio_free(gm8775c->gpio_511);
    gpio_free(gm8775c->gpio_509);

    dev_info(&client->dev, "gm8775c driver removed.\n");
}
 
static struct i2c_driver gm8775c_driver = {
	.driver = {
		.name = "gm8775c-mipi-to-lvds",
		.of_match_table = of_match_ptr(gm8775_of_match),
	},
	.probe = gm8775c_probe,
	.remove = gm8775c_remove,
	.id_table = gm8775_i2c_id,
};
 
static int __init gm8775c_modinit(void)
{
	int ret;
	ret = i2c_add_driver(&gm8775c_driver);
	if (ret != 0)
		printk("Failed to register gm8775c i2c driver : %d \n", ret);
	return ret;
}
late_initcall(gm8775c_modinit);
 
static void __exit gm8775c_exit(void)
{
	i2c_del_driver(&gm8775c_driver);
}
 
module_exit(gm8775c_exit);
 
 
//module_i2c_driver(gm8775c_driver);
 
MODULE_AUTHOR("goa");
MODULE_DESCRIPTION("GM8775C I2C Client Driver");
MODULE_LICENSE("GPL");

3.2 写Makefile与Kconfig，代码加入内核

  • 上述代码应该创建为名为gm8775c.c的文件，需要提前创建一个名为gm8775c的文件夹用来存放gm8775c.c，同时在gm8775c文件夹下我们需要创建Makefile与Kconfig文件，用来声明编译
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我们举例说明，如果SDK的内核目录是
LinuxSDK/rk3576_linux6.1_release/kernel-6.1/
那么此文件夹可以是
LinuxSDK/rk3576_linux6.1_release/kernel-6.1/drivers/input/gm8775c
文件夹下应该有以下内容
root:~/LinuxSDK/rk3576_linux6.1_release/kernel-6.1/drivers/input/gm8775c$ ls
gm8775c.c Makefile Kconfig

  • 同时在gm8775c文件夹的上级目录，比如LinuxSDK/rk3576_linux6.1_release/kernel-6.1/drivers/input/gm8775c，上级目录就是input，对input下的Makefile与Kconfig文件增加如下内容
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/LinuxSDK/rk3576_linux6.1_release/kernel-6.1/drivers/input/Makefile
+ obj-y += gm8775c/
/LinuxSDK/rk3576_linux6.1_release/kernel-6.1/drivers/input/Kconfig
+ source "drivers/input/gm8775c/Kconfig"

完成上述操作，使用./build.sh kernel-config命令，将gm8775c配置选中后保存，进入到下一步

3.3 设备树增加对应声明

  • 将前面的屏幕参数填在对应的接口时序参数节点dsi_timing0 里，用于配置显示接口的时序参数，影响分辨率、刷新率等
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&dsi_timing0 {
    clock-frequency = <74000000>;
    hactive = <1920>;
    vactive = <1080>;
    hfront-porch = <100>;
    hsync-len = <5>;
    hback-porch = <35>;
    vfront-porch = <20>;
    vsync-len = <10>;
    vback-porch = <15>;
    hsync-active = <0>;
    vsync-active = <0>;
    de-active = <0>;
    pixelclk-active = <1>;
};
  • 在设备树MIPI转LVDS芯片所连接的i2c节点中新加gm8775c节点，启动这个驱动（文件夹附带的两个设备树文件直接覆盖原文件即可）
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&i2c6 {
    status = "okay";
    gm8775c: gm8775c@2d {
        compatible = "gm8775c,mipi_to_lvds";
        reg = <0x2d>;
    };
    gt911: gt911@5d {
            status = "okay";
            compatible = "goodix,gt911";
            reg = <0x5d>;
            interrupt-parent = <&gpio1>;
            interrupts = <18 0>;
            irq-gpios = <&gpio1 RK_PC2 0>;
            reset-gpios = <&xl8574t 0 GPIO_ACTIVE_HIGH>;
            pinctrl-names = "default";
            pinctrl-0 = <&gt911_int>;
            touchscreen-inverted-y;
            touchscreen-inverted-x;
    };
};

./build.sh重新编译，烧录系统，开机即可正常显示，每次开机自动写配置文件。

许可协议

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