Exynos4412——LCD之backligth

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本博文对应地址: https://hceng.cn/2017/11/22/Exynos4412——LCD之backligth/
Exynos4412的背光驱动，采用友善之臂的“一线触摸”。

1.背景

关于一线触摸友善之臂官方说明：LCD-S702/zh

• 在大部分的ARM主控板中，我们发现，直接使用CPU自带的ADC转换器并不能很好的支持大尺寸(7寸以上)的四线电阻触摸屏，市面上一般采用更加专业的USB或串口触摸屏扩展模块来解决。为了节省ARM主控芯片的有限资源以及减少外扩，我们专门开发了只使用一个普通GPIO就可以实现专业触摸效果的替代方案，并把它集成到我们的LCD模块驱动板中，我们称之为“一线触摸(1-Wire)”。它的基本原理是，使用一个低成本的MCU连接一个专业的触控芯片(在此我们使用的是ADS7843或兼容芯片)，采集并处理四线电阻模拟信号，并把滤波(未校准)后的稳定原始数据通过GPIO送给ARM主控，经我们长期反复测试，即使在19寸这样大的电阻触摸屏上，也可以实现非常精准的触摸效果，不会出现漂移抖动的现象。
• 另外，当今12寸以内的LCD显示屏，大都采用了LED背光，我们顺便也把背光调节部分也交给MCU来处理，并设置了统一的调节数值区间，最后通过“一线触摸”的GPIO传给ARM主控，这样在ARM端就可以非常方便的来设置背光了。
• 与此同时，我们还为我们设计开发的每一款带“一线触摸”的LCD模块设置了编号存储在MCU中，这样通过一线通讯读取到的编号，就可以知道这个LCD模块的具体类型了，也就可以在bootloader和内核中自动匹配相应的LCD驱动参数，以此来实现无需修改任何配置，即插即用带”一线触摸”的LCD模块。
• 在电容触摸LCD模块中，我们则去掉了电阻触控芯片，而保留了背光调节和存储LCD类型编号这2个功能，因此电容触摸通讯依然是标准的I2C接口。
• 需要注意的是，我们实现的“一线触摸”的通讯，和通常所说的单总线接口是不同的。在ARM主控端内部，我们实际采用了一路pwm timer(不是pwm管脚哦)来实现固定的通讯频率(9600Hz)，详细请查看驱动源代码。

也就是说友善之臂为了改善大尺寸四线电阻触摸屏的性能，外加了一个低成本的MCU去控制专业的触控芯片，将滤波后的稳定数据通过GPIO传回到ARM上。这是和触摸屏相关的，本次不深究，而且我的板子LCD是电容屏，以上的改进几乎不关我的事。
但，他们把背光调节部分也交给了那个低成本MCU来处理，并通过“一线触摸”来控制，这就坑了。记得以前的都是ARM的一个PWM引脚就完成LCD的背光的控制，现在由于硬件上并不能直接PWM控制，不得不采用“一线触摸”。

2.原理图

• Tiny4412SDK-1506-Schematic.pdf：
  
• Tiny4412-1412-Schematic.pdf：
  
  Exynos4412的GPX1_2与LCD相连，作为“一线触摸”引脚，也就是背光控制引脚。

3.设备树文件

--- exynos4412-tiny4412.dts     2017-11-22 01:43:28.922397104 -0800
+++ linux-4.13.9/arch/arm/boot/dts/exynos4412-tiny4412.dts      2017-11-22 01:42:47.459525364 -0800
@@ -91,6 +91,18 @@
                        clock-frequency = <24000000>;
                };
        };
+
+       backlight@139D0000{
+               compatible = "tiny4412,backlight";
+               reg = <0x139D0000  0x14>;
+               tiny4412,backlight = <&gpx1 2 GPIO_ACTIVE_HIGH>;
+               pinctrl-names = "backlight_out","backlight_in";
+               pinctrl-0 = <&backlight_out>;
+               pinctrl-1 = <&backlight_in>;
+               interrupts = <0 40 0>;
+               clocks = <&clock CLK_PWM>;
+               clock-names = "timers";
+        };
 };

 &rtc {
@@ -151,4 +163,21 @@
 };


+&pinctrl_1 {
+        backlight_out: backlight_out{
+                samsung,pins = "gpx1-2";
+                samsung,pin-function = <1>;
+                samsung,pin-pud = <0>;
+                samsung,pin-drv = <0>;
+        };
+         backlight_in: backlight_in{
+                samsung,pins = "gpx1-2";
+                samsung,pin-function = <0>;
+                samsung,pin-pud = <0>;
+                samsung,pin-drv = <0>;
+        };
+};

本来想认真分析的，真分析起来，发现没什么看的，0x139D0000是PWMer定时器的基地址，还将gpx1_2设置成了两种模式：backlight_out（输出引脚）和backlight_in（输入引脚）。
这里为什么要用到定时器，不是很明白，文末的第二个博客说“开发板和 CPLD 通讯使用了特殊的协议，设置一次背光值，需要读写 IO 多次，因此，需要使用定时器功能”，但看原理图感觉和CPLD(EPM240T100)关系不是很大。
以上就是设备树文件的一些设置，后续驱动会用到。

4.驱动简析

驱动完整代码如下：
{% codeblock lang:c [backlight_drv.c] https://github.com/hceng/learn/blob/master/tiny4412/01_backlight_drv/backlight_drv.c %}
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/cdev.h>
#include <linux/device.h>
#include <asm/uaccess.h>
#include <linux/uaccess.h>
#include <linux/platform_device.h>
#include <linux/gpio.h>
#include <linux/of.h>
#include <linux/of_gpio.h>
#include <linux/fs.h>
#include <linux/bitops.h>
#include <linux/clk.h>
#include <linux/export.h>
#include <linux/err.h>
#include <linux/io.h>
#include <linux/pwm.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/time.h>
#include <linux/interrupt.h>
#include <linux/delay.h>
/*
PWM 时钟频率 100M
100M / 250 / 4 = 100000
1/100000 = 10us
*/

static int major;
static struct cdev backlight_cdev;
static struct class *cls;
static struct pinctrl *pctrl;
static struct pinctrl_state *pstate_in;
static struct pinctrl_state *pstate_out;
static int one_write_pin;

struct TIMER_BASE
{
unsigned int TCFG0;
unsigned int TCFG1;
unsigned int TCON;
unsigned int TCNTB0;
unsigned int TCMPB0;
unsigned int TCNTO0;
unsigned int TCNTB1;
unsigned int TCMPB1;
unsigned int TCNTO1;
unsigned int TCNTB2;
unsigned int TCMPB2;
unsigned int TCNTO2;
unsigned int TCNTB3;
unsigned int TCMPB3;
unsigned int TCNTO3;
unsigned int TCNTB4;
unsigned int TCBTO4;
unsigned int TINT_CSTAT;
};

volatile static struct TIMER_BASE *timer = NULL;

static volatile unsigned int io_bit_count;
static volatile unsigned int io_data;

enum
{
IDLE,
START,
REQUEST,
WAITING,
RESPONSE,
STOPING,
} one_wire_status = IDLE;

static inline void stop_timer_for_1wire(void)
{
unsigned long tcon;
tcon = timer->TCON;
tcon &= ~(1 << 16);
timer->TCON = tcon;
}

static irqreturn_t timer_for_1wire_interrupt(int irq, void *dev_id)
{
unsigned int tint;
tint = timer->TINT_CSTAT;
tint |= 0x100;
timer->TINT_CSTAT = tint;
//printk(“timer_for_1wire_interrupt\n”);
io_bit_count–;

switch (one_wire_status)
{
    case START:

        if (io_bit_count == 0)
        {
            io_bit_count = 16;
            one_wire_status = REQUEST;
        }

        break;
    case REQUEST:
        gpio_set_value(one_write_pin, io_data & (1U << 31));
        io_data <<= 1;

        if (io_bit_count == 0)
        {
            io_bit_count = 2;
            one_wire_status = WAITING;
        }

        break;
    case WAITING:

        if (io_bit_count == 0)
        {
            io_bit_count = 32;
            one_wire_status = RESPONSE;
        }

        if (io_bit_count == 1)
        {
            pinctrl_select_state(pctrl, pstate_in);
            gpio_set_value(one_write_pin, 1);
        }

        break;
    case RESPONSE:
        io_data = (io_data << 1) | gpio_get_value(one_write_pin);

        if (io_bit_count == 0)
        {
            io_bit_count = 2;
            one_wire_status = STOPING;
            gpio_set_value(one_write_pin, 1);
            pinctrl_select_state(pctrl, pstate_out);
            //one_wire_session_complete(one_wire_request, io_data);
        }

        break;
    case STOPING:

        if (io_bit_count == 0)
        {
            one_wire_status = IDLE;
            stop_timer_for_1wire();
        }

        break;
    default:
        stop_timer_for_1wire();
}

return IRQ_HANDLED;

}

static const unsigned char crc8_tab[] =
{
0x00, 0x07, 0x0E, 0x09, 0x1C, 0x1B, 0x12, 0x15,
0x38, 0x3F, 0x36, 0x31, 0x24, 0x23, 0x2A, 0x2D,
0x70, 0x77, 0x7E, 0x79, 0x6C, 0x6B, 0x62, 0x65,
0x48, 0x4F, 0x46, 0x41, 0x54, 0x53, 0x5A, 0x5D,
0xE0, 0xE7, 0xEE, 0xE9, 0xFC, 0xFB, 0xF2, 0xF5,
0xD8, 0xDF, 0xD6, 0xD1, 0xC4, 0xC3, 0xCA, 0xCD,
0x90, 0x97, 0x9E, 0x99, 0x8C, 0x8B, 0x82, 0x85,
0xA8, 0xAF, 0xA6, 0xA1, 0xB4, 0xB3, 0xBA, 0xBD,
0xC7, 0xC0, 0xC9, 0xCE, 0xDB, 0xDC, 0xD5, 0xD2,
0xFF, 0xF8, 0xF1, 0xF6, 0xE3, 0xE4, 0xED, 0xEA,
0xB7, 0xB0, 0xB9, 0xBE, 0xAB, 0xAC, 0xA5, 0xA2,
0x8F, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9D, 0x9A,
0x27, 0x20, 0x29, 0x2E, 0x3B, 0x3C, 0x35, 0x32,
0x1F, 0x18, 0x11, 0x16, 0x03, 0x04, 0x0D, 0x0A,
0x57, 0x50, 0x59, 0x5E, 0x4B, 0x4C, 0x45, 0x42,
0x6F, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7D, 0x7A,
0x89, 0x8E, 0x87, 0x80, 0x95, 0x92, 0x9B, 0x9C,
0xB1, 0xB6, 0xBF, 0xB8, 0xAD, 0xAA, 0xA3, 0xA4,
0xF9, 0xFE, 0xF7, 0xF0, 0xE5, 0xE2, 0xEB, 0xEC,
0xC1, 0xC6, 0xCF, 0xC8, 0xDD, 0xDA, 0xD3, 0xD4,
0x69, 0x6E, 0x67, 0x60, 0x75, 0x72, 0x7B, 0x7C,
0x51, 0x56, 0x5F, 0x58, 0x4D, 0x4A, 0x43, 0x44,
0x19, 0x1E, 0x17, 0x10, 0x05, 0x02, 0x0B, 0x0C,
0x21, 0x26, 0x2F, 0x28, 0x3D, 0x3A, 0x33, 0x34,
0x4E, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5C, 0x5B,
0x76, 0x71, 0x78, 0x7F, 0x6A, 0x6D, 0x64, 0x63,
0x3E, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2C, 0x2B,
0x06, 0x01, 0x08, 0x0F, 0x1A, 0x1D, 0x14, 0x13,
0xAE, 0xA9, 0xA0, 0xA7, 0xB2, 0xB5, 0xBC, 0xBB,
0x96, 0x91, 0x98, 0x9F, 0x8A, 0x8D, 0x84, 0x83,
0xDE, 0xD9, 0xD0, 0xD7, 0xC2, 0xC5, 0xCC, 0xCB,
0xE6, 0xE1, 0xE8, 0xEF, 0xFA, 0xFD, 0xF4, 0xF3,
};

#define crc8_init(crc) ((crc) = 0XACU)
#define crc8(crc, v) ( (crc) = crc8_tab[(crc) ^(v)])

static void start_one_wire_session(unsigned char req)
{
unsigned int tcon;
printk(“backlight_write\n”);
one_wire_status = START;
gpio_set_value(one_write_pin, 1);
pinctrl_select_state(pctrl, pstate_out);
// IDLE to START
{
unsigned char crc;
crc8_init(crc);
crc8(crc, req);
io_data = (req << 8) + crc;
io_data <<= 16;
}
io_bit_count = 1;
pinctrl_select_state(pctrl, pstate_out);
timer->TCNTB3 = 650;
//init tranfer and start timer
tcon = timer->TCON;
tcon &= ~(0xF << 16);
tcon |= (1 << 17);
timer->TCON = tcon;
tcon |= (1 << 16);
tcon |= (1 << 19);
tcon &= ~(1 << 17);
timer->TCON = tcon;
timer->TINT_CSTAT |= 0x08;
gpio_set_value(one_write_pin, 0);
}

static ssize_t backlight_write(struct file *file, const char __user *buf, size_t count, loff_t *off)
{
unsigned char reg, ret;
ret = copy_from_user(&reg, buf, 1);

printk("kernel: reg = %d", reg);
//printk("buf = %d", *buf);

if (ret < 0)
{
    printk("%s copy_from_user error\n", __func__);
}

/*if (reg > 127)
    { reg = 127; }

start_one_wire_session(reg + 0x80);
*/

start_one_wire_session(reg);

return 1;

}

static int backlight_open(struct inode *inode, struct file *file)
{
printk(“backlight_open\n”);
return 0;
}

static int backlight_release(struct inode *inode, struct file *file)
{
printk(“backlight_exit\n”);
return 0;
}

static struct file_operations backlight_fops =
{
.owner = THIS_MODULE,
.open = backlight_open,
.release = backlight_release,
.write = backlight_write,
};
static struct device *dev;
static struct clk *base_clk;
static struct resource *res = NULL, *irq = NULL;
static int backlight_probe(struct platform_device *pdev)
{
int ret;
dev_t devid;
dev = &pdev->dev;

printk("enter %s\n", __func__);
pctrl = devm_pinctrl_get(dev);

if (pctrl == NULL)
{
    printk("devm_pinctrl_get error\n");
    return -EINVAL;
}

pstate_in  = pinctrl_lookup_state(pctrl, "backlight_in");
pstate_out = pinctrl_lookup_state(pctrl, "backlight_out");

if (pstate_in == NULL || pstate_out == NULL)
{
    printk("pinctrl_lookup_state error\n");
    return -EINVAL;
}

one_write_pin = of_get_named_gpio(dev->of_node, "tiny4412,backlight", 0);

if (!one_write_pin)
{
    printk("of_get_named_gpio error\n");
    return -EINVAL;
}

devm_gpio_request_one(dev, one_write_pin, GPIOF_OUT_INIT_HIGH, "one_write");
//pinctrl_select_state(pctrl, pstate);
res = platform_get_resource(pdev, IORESOURCE_MEM, 0);

if (res == NULL)
{
    printk("platform_get_resource error\n");
    return -EINVAL;
}

base_clk = devm_clk_get(&pdev->dev, "timers");

if (IS_ERR(base_clk))
{
    dev_err(dev, "failed to get timer base clk\n");
    return PTR_ERR(base_clk);
}

ret = clk_prepare_enable(base_clk);

if (ret < 0)
{
    dev_err(dev, "failed to enable base clock\n");
    return ret;
}

timer = devm_ioremap_resource(&pdev->dev, res);

if (timer == NULL)
{
    printk("devm_ioremap_resource error\n");
    return -EINVAL;
}

printk("timer: %x\n", (unsigned int)timer);
timer->TCFG0  = 0xF00;
timer->TCFG1  = 0x10004;
irq = platform_get_resource(pdev, IORESOURCE_IRQ, 0);

if (irq == NULL)
{
    printk("platform_get_resource irq error\n");
    return -EINVAL;
}

ret = devm_request_irq(dev, irq->start, timer_for_1wire_interrupt , IRQF_TIMER, "backlight", NULL);

if (ret)
{
    dev_err(dev, "unable to request irq\n");
    return -EINVAL;
}

start_one_wire_session(0x60);

if (alloc_chrdev_region(&devid, 0, 1, "backlight") < 0)
{
    printk("%s ERROR\n", __func__);
    return -EINVAL;
}

major = MAJOR(devid);
cdev_init(&backlight_cdev, &backlight_fops);
cdev_add(&backlight_cdev, devid, 1);
cls = class_create(THIS_MODULE, "onewire_backlight");
device_create(cls, NULL, MKDEV(major, 0), NULL, "tiny4412_backlight");
return 0;

}

static int backlight_remove(struct platform_device *pdev)
{
printk(“enter %s\n”, func);
device_destroy(cls, MKDEV(major, 0));
class_destroy(cls);
cdev_del(&backlight_cdev);
unregister_chrdev_region(MKDEV(major, 0), 1);
devm_pinctrl_put(pctrl);
devm_free_irq(dev, irq->start, NULL);
clk_disable_unprepare(base_clk);
devm_gpio_free(dev, one_write_pin);
return 0;
}

static const struct of_device_id backlight_dt_ids[] =
{
{ .compatible = “tiny4412,backlight”, },
{},
};

MODULE_DEVICE_TABLE(of, backlight_dt_ids);

static struct platform_driver backlight_driver =
{
.driver = {
.name = “tiny4412_backlight”,
.of_match_table = of_match_ptr(backlight_dt_ids),
},
.probe = backlight_probe,
.remove = backlight_remove,
};

static int backlight_init(void)
{
int ret;
printk(“enter %s\n”, func);
ret = platform_driver_register(&backlight_driver);

if (ret)
{
    printk(KERN_ERR "Tiny4412 backlight: probe faid backlight: %d\n", ret);
}

return ret;

}

static void backlight_exit(void)
{
printk(“enter %s\n”, func);
platform_driver_unregister(&backlight_driver);
}

module_init(backlight_init);
module_exit(backlight_exit);

MODULE_LICENSE(“GPL”);
MODULE_AUTHOR(“hceng [email protected]”);
MODULE_DESCRIPTION(“Tiny4412 LCD backlight driver.”);
MODULE_ALIAS(“Exynos4412_backlight”);
MODULE_VERSION(“V1.0”);
{% endcodeblock %}

首先insmod该驱动，会调用backlight_init(),注册平台设备platform_driver_register(&backlight_driver)。
backlight_driver结构体里面的.compatible与设备树文件里的compatible完全匹配上后，即调用最核心的backlight_probe()函数。
在backlight_probe()函数里获取设备树的各种信息、地址映射、设置时钟等，最后再申请设备号、注册设备、创建类、创建设备。
在backlight_fops操作函数里，主要的是写函数，它获取用户层的数据，然后调用start_one_wire_session()实现背光的修改。
这里的start_one_wire_session()就不太好分析了，协议是友善自定的。

5.测试程序

{% codeblock lang:c [app.c] https://github.com/hceng/learn/blob/master/tiny4412/01_backlight_drv/app.c %}
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#define msleep(x) usleep(x*1000)

int main(int argc, char **argv)
{
int fd;
int val;

if (argc != 2)
{
    printf("%s+val\n",argv[0]);
    return -1;
}

fd = open("/dev/tiny4412_backlight", O_RDWR);
 
if (fd < 0)
{
    printf("can't open /dev/tiny4412_backlight\n");
    return 0;
}
val = strtoul(argv[1], NULL, 0);   

printf("app: val = %d",val);    

write(fd, &val, 1);

return 0;	

}
{% endcodeblock %}

6.实测结果

参考博客：
学习设备树之（十一）Backlight
[(20）设备树LCD背光驱动]((20）设备树LCD背光驱动 “http://muasy.com/40690/”)