Android模拟器学framework和driver之battery & backlight-----5. backlight in linux

前面介绍了battery的相关的东西，现在我们来介绍下backlight模块，背光主要是用来调节显示屏亮度的，一般背光都是用PWM控制的，调节占空比达到改变有效电压值来调节光的强弱。

背光的移植在linux中虽然不是那么难，但是背光这个组件对我们嵌入式设备的续航能力有很大的影响，一般背光上面加的电压会有20多的电压，所以这部分会很耗电的，相当于是开了个大灯泡。

现在我们先来看下android goldfish中的背光代码，哈哈，没找到吧，没有，我们打开模拟器，看sysfs中，也是没有具体的背光的文件的，所以这里我们得自己实现，自己写代码练习练习，毕竟这部分不是非常的难，参考drivers/video/backlight/下的pwm_bl.c文件，基本可以仿照，我们要做的事情很简单，创建背光相关的文件系统即可，不需要去控制硬件做什么动作，因为我们本来就没有硬件。

首先看下video中的makefile，如果backlight/没有选中就选中它，不然我们的模块不会编译进去。然后再看下backlight/下的Makefile

[cpp] obj-$(CONFIG_LCD_CLASS_DEVICE)     += lcd.o 
obj-$(CONFIG_LCD_CORGI)            += corgi_lcd.o 
obj-$(CONFIG_LCD_LTV350QV)         += ltv350qv.o 
obj-$(CONFIG_LCD_ILI9320)          += ili9320.o 
obj-$(CONFIG_LCD_PLATFORM)         += platform_lcd.o 
obj-$(CONFIG_LCD_VGG2432A4)        += vgg2432a4.o 
obj-$(CONFIG_LCD_TDO24M)           += tdo24m.o 
obj-$(CONFIG_LCD_TOSA)             += tosa_lcd.o 
 
obj-$(CONFIG_BACKLIGHT_CLASS_DEVICE) += backlight.o 
obj-$(CONFIG_BACKLIGHT_ATMEL_PWM)    += atmel-pwm-bl.o 
obj-$(CONFIG_BACKLIGHT_GENERIC) += generic_bl.o 
obj-$(CONFIG_BACKLIGHT_HP680)   += hp680_bl.o 
obj-$(CONFIG_BACKLIGHT_LOCOMO)  += locomolcd.o 
obj-$(CONFIG_BACKLIGHT_OMAP1)   += omap1_bl.o 
obj-$(CONFIG_BACKLIGHT_PROGEAR) += progear_bl.o 
obj-$(CONFIG_BACKLIGHT_CARILLO_RANCH) += cr_bllcd.o 
obj-$(CONFIG_BACKLIGHT_PWM)     += pwm_bl.o 
obj-$(CONFIG_BACKLIGHT_DA903X)  += da903x_bl.o 
obj-$(CONFIG_BACKLIGHT_MBP_NVIDIA) += mbp_nvidia_bl.o 
obj-$(CONFIG_BACKLIGHT_TOSA)    += tosa_bl.o 
obj-$(CONFIG_BACKLIGHT_SAHARA)  += kb3886_bl.o 
obj-$(CONFIG_LCD_CLASS_DEVICE)     += lcd.o
obj-$(CONFIG_LCD_CORGI)            += corgi_lcd.o
obj-$(CONFIG_LCD_LTV350QV)         += ltv350qv.o
obj-$(CONFIG_LCD_ILI9320)          += ili9320.o
obj-$(CONFIG_LCD_PLATFORM)         += platform_lcd.o
obj-$(CONFIG_LCD_VGG2432A4)        += vgg2432a4.o
obj-$(CONFIG_LCD_TDO24M)           += tdo24m.o
obj-$(CONFIG_LCD_TOSA)             += tosa_lcd.o

obj-$(CONFIG_BACKLIGHT_CLASS_DEVICE) += backlight.o
obj-$(CONFIG_BACKLIGHT_ATMEL_PWM)    += atmel-pwm-bl.o
obj-$(CONFIG_BACKLIGHT_GENERIC) += generic_bl.o
obj-$(CONFIG_BACKLIGHT_HP680)   += hp680_bl.o
obj-$(CONFIG_BACKLIGHT_LOCOMO)  += locomolcd.o
obj-$(CONFIG_BACKLIGHT_OMAP1)   += omap1_bl.o
obj-$(CONFIG_BACKLIGHT_PROGEAR) += progear_bl.o
obj-$(CONFIG_BACKLIGHT_CARILLO_RANCH) += cr_bllcd.o
obj-$(CONFIG_BACKLIGHT_PWM)     += pwm_bl.o
obj-$(CONFIG_BACKLIGHT_DA903X)  += da903x_bl.o
obj-$(CONFIG_BACKLIGHT_MBP_NVIDIA) += mbp_nvidia_bl.o
obj-$(CONFIG_BACKLIGHT_TOSA)    += tosa_bl.o
obj-$(CONFIG_BACKLIGHT_SAHARA)  += kb3886_bl.o

这里没有一个文件被编译进去的，我们要把backlight.c先编译进去，直接这样改，我比较懒 呵呵呵，

[cpp] obj-y += backlight.o 
obj-$(CONFIG_BACKLIGHT_ATMEL_PWM)    += atmel-pwm-bl.o 
obj-y += backlight.o
obj-$(CONFIG_BACKLIGHT_ATMEL_PWM)    += atmel-pwm-bl.o
然后重新编译下会生成backlight.o文件，并且在sysfs中会生成我们的backlight class


 

我们先来分析下backlight.c中的代码是如何实现的。

养成好习惯，看见代码多不用怕，首先看init函数：

[cpp] static int __init backlight_class_init(void) 
{ 
    backlight_class = class_create(THIS_MODULE, "backlight"); 
    if (IS_ERR(backlight_class)) { 
        printk(KERN_WARNING "Unable to create backlight class; errno = %ld\n", 
                PTR_ERR(backlight_class)); 
        return PTR_ERR(backlight_class); 
    } 
 
    backlight_class->dev_attrs = bl_device_attributes; 
    backlight_class->suspend = backlight_suspend; 
    backlight_class->resume = backlight_resume; 
    return 0; 
} 
 
/*
 * if this is compiled into the kernel, we need to ensure that the
 * class is registered before users of the class try to register lcd's
 */ 
postcore_initcall(backlight_class_init); 
static int __init backlight_class_init(void)
{
 backlight_class = class_create(THIS_MODULE, "backlight");
 if (IS_ERR(backlight_class)) {
  printk(KERN_WARNING "Unable to create backlight class; errno = %ld\n",
    PTR_ERR(backlight_class));
  return PTR_ERR(backlight_class);
 }

 backlight_class->dev_attrs = bl_device_attributes;
 backlight_class->suspend = backlight_suspend;
 backlight_class->resume = backlight_resume;
 return 0;
}

/*
 * if this is compiled into the kernel, we need to ensure that the
 * class is registered before users of the class try to register lcd's
 */
postcore_initcall(backlight_class_init);
很简单，这里只是用了class_create函数在sys/class下创建了backlight文件夹，然后是
backlight_class->dev_attrs = bl_device_attributes;
在backlight class中创建了一系列的文件系统，

[cpp] <pre name="code" class="cpp">static ssize_t backlight_show_power(struct device *dev, 
        struct device_attribute *attr,char *buf) 
{ 
    struct backlight_device *bd = to_backlight_device(dev); 
 
    return sprintf(buf, "%d\n", bd->props.power); 
} 
 
static ssize_t backlight_store_power(struct device *dev, 
        struct device_attribute *attr, const char *buf, size_t count) 
{ 
    int rc; 
    struct backlight_device *bd = to_backlight_device(dev); 
    unsigned long power; 
 
    rc = strict_strtoul(buf, 0, &power); 
    if (rc) 
        return rc; 
 
    rc = -ENXIO; 
    mutex_lock(&bd->ops_lock); 
    if (bd->ops) { 
        pr_debug("backlight: set power to %lu\n", power); 
        if (bd->props.power != power) { 
            bd->props.power = power; 
            backlight_update_status(bd); 
        } 
        rc = count; 
    } 
    mutex_unlock(&bd->ops_lock); 
 
    return rc; 
} 
<pre name="code" class="cpp">static ssize_t backlight_show_power(struct device *dev,
  struct device_attribute *attr,char *buf)
{
 struct backlight_device *bd = to_backlight_device(dev);

 return sprintf(buf, "%d\n", bd->props.power);
}

static ssize_t backlight_store_power(struct device *dev,
  struct device_attribute *attr, const char *buf, size_t count)
{
 int rc;
 struct backlight_device *bd = to_backlight_device(dev);
 unsigned long power;

 rc = strict_strtoul(buf, 0, &power);
 if (rc)
  return rc;

 rc = -ENXIO;
 mutex_lock(&bd->ops_lock);
 if (bd->ops) {
  pr_debug("backlight: set power to %lu\n", power);
  if (bd->props.power != power) {
   bd->props.power = power;
   backlight_update_status(bd);
  }
  rc = count;
 }
 mutex_unlock(&bd->ops_lock);

 return rc;
}
所以我们的驱动只要填充好具体的结构体，初始化好文件系统就够了，在sysfs中生成可以让user space调用的接口，接下来的事情就交给上层开发人员去做。

ok，我们来看下我们自己写的驱动，在backlight文件夹下新建一个文件叫  android-backlight.c，我是参照pwm_bl.c来写的，具体先来看下代码，init函数

 

[cpp] static int __init android_backlight_init(void) 
{ 
    return platform_driver_register(&android_backlight_driver); 
} 
 
static void __exit android_backlight_exit(void) 
{ 
    platform_driver_unregister(&android_backlight_driver); 
} 
 
module_init(android_backlight_init); 
module_exit(android_backlight_exit); 
static int __init android_backlight_init(void)
{
 return platform_driver_register(&android_backlight_driver);
}

static void __exit android_backlight_exit(void)
{
 platform_driver_unregister(&android_backlight_driver);
}

module_init(android_backlight_init);
module_exit(android_backlight_exit);
使用platform_driver_register注册平台驱动，看下传入的参数：

[cpp] static struct platform_driver android_backlight_driver = { 
    .driver ={ 
        .name = "android-backlight", 
        .owner = THIS_MODULE, 
    }, 
    .probe  =   android_backlight_probe, 
//  .remove =   ........  
//  .suspend  
//  .resume  
 
 
}; 
static struct platform_driver android_backlight_driver = {
 .driver ={
  .name = "android-backlight",
  .owner = THIS_MODULE,
 },
 .probe = android_backlight_probe,
// .remove = ........
// .suspend
// .resume

};
这里我偷懒没写remove suspend和resume'回调函数，在移植具体驱动的时候我们都应该写上，特别是suspend和resume函数，来看下我们paltform驱动的device_register是在哪做的，在arch/arm/mach-goldfish/board-goldfish.c

[cpp] struct platform_device android_backlight_device = { 
    .name = "android-backlight", 
    .id = 0, 
}; 
 
static struct platform_pwm_backlight_data android_backlight_data = { 
    .pwm_id = 0, 
    .max_brightness = 255, 
    .dft_brightness = 128, 
//  .pwm_period_ns = ...;  
}; 
struct platform_device android_backlight_device = {
 .name = "android-backlight",
 .id = 0,
};

static struct platform_pwm_backlight_data android_backlight_data = {
 .pwm_id = 0,
 .max_brightness = 255,
 .dft_brightness = 128,
// .pwm_period_ns = ...;
};

在init中进行注册：

[cpp] static void __init goldfish_init(void) 
{ 
    platform_device_register(&goldfish_pdev_bus_device); 
    platform_device_register(&android_light_device); 
    platform_device_register(&android_switch_device); 
    platform_device_register(&vh_device); 
    platform_device_register(&android_temperature_device); 
    <span style="color:#ff0000;">android_register_device(&android_backlight_device, &android_backlight_data);</span> 
} 
static void __init goldfish_init(void)
{
 platform_device_register(&goldfish_pdev_bus_device);
 platform_device_register(&android_light_device);
 platform_device_register(&android_switch_device);
 platform_device_register(&vh_device);
 platform_device_register(&android_temperature_device);
 <span style="color:#ff0000;">android_register_device(&android_backlight_device, &android_backlight_data);</span>
}
这边android_backlight_data结构体主要是做一个背光的初始化。

接下来我们看一下probe函数，

[cpp] static int android_backlight_probe(struct platform_device *pdev) 
{ 
    //pass the struct from board-goldfish.c ----> init platform data  
    struct platform_pwm_backlight_data *data=pdev->dev.platform_data; 
    //local private struct  
    struct android_pwm_data *pd; 
    //backlight properties struct----> defined in include/linux/backlight.h  
 
    struct backlight_properties props; 
    struct backlight_device *bl;    //struct infomation defined in include/linux/backlight.h  
 
    int ret; 
 
    if (!data) { 
        dev_err(&pdev->dev, "failed to find platform data\n"); 
        return -EINVAL; 
    } 
    //----for here we haven't set init pointer function...  
    if(data->init) 
    { 
        ret=data->init(&pdev->dev); 
        if(ret<0) 
            return ret; 
    } 
     
    pd = kzalloc(sizeof(*pd),GFP_KERNEL); 
    if(!pd) 
    { 
        dev_err(&pdev->dev, "no memory for state\n"); 
        ret = -ENOMEM; 
        goto err_alloc; 
    } 
 
//  pd->period = data->pwm_period_ns;  
    pd->notify = data->notify; 
    pd->dev = &pdev->dev; 
 
/*  pd->pwm = pwm_request(data->pwm_id, "backlight");
    if (IS_ERR(pb->pwm)) {
        dev_err(&pdev->dev, "unable to request PWM for backlight\n");
        ret = PTR_ERR(pb->pwm);
        goto err_pwm;
    } else
        dev_dbg(&pdev->dev, "got pwm for backlight\n");
*/ 
    memset(&props,0,sizeof(struct backlight_properties)); 
    bl = backlight_device_register(dev_name(&pdev->dev), &pdev->dev, pd,&android_backlight_ops); 
    if (IS_ERR(bl)) { 
        dev_err(&pdev->dev, "failed to register backlight\n"); 
        ret = PTR_ERR(bl); 
//      goto err_bl;  
    } 
    bl->props.max_brightness = data->max_brightness; 
 
    bl->props.brightness=data->dft_brightness; 
 
    platform_set_drvdata(pdev,bl); 
 
//err_bl:  
//  pwm_free(pd->pwm);  
//err_pwm:  
//  kfree(pb);  
err_alloc: 
    if (data->exit) 
        data->exit(&pdev->dev); 
    return ret; 
} 
static int android_backlight_probe(struct platform_device *pdev)
{
 //pass the struct from board-goldfish.c ----> init platform data
 struct platform_pwm_backlight_data *data=pdev->dev.platform_data;
 //local private struct
 struct android_pwm_data *pd;
 //backlight properties struct----> defined in include/linux/backlight.h

 struct backlight_properties props;
 struct backlight_device *bl; //struct infomation defined in include/linux/backlight.h

 int ret;

 if (!data) {
  dev_err(&pdev->dev, "failed to find platform data\n");
  return -EINVAL;
 }
 //----for here we haven't set init pointer function...
 if(data->init)
 {
  ret=data->init(&pdev->dev);
  if(ret<0)
   return ret;
 }
 
 pd = kzalloc(sizeof(*pd),GFP_KERNEL);
 if(!pd)
 {
  dev_err(&pdev->dev, "no memory for state\n");
  ret = -ENOMEM;
  goto err_alloc;
 }

// pd->period = data->pwm_period_ns;
 pd->notify = data->notify;
 pd->dev = &pdev->dev;

/* pd->pwm = pwm_request(data->pwm_id, "backlight");
 if (IS_ERR(pb->pwm)) {
  dev_err(&pdev->dev, "unable to request PWM for backlight\n");
  ret = PTR_ERR(pb->pwm);
  goto err_pwm;
 } else
  dev_dbg(&pdev->dev, "got pwm for backlight\n");
*/
 memset(&props,0,sizeof(struct backlight_properties));
 bl = backlight_device_register(dev_name(&pdev->dev), &pdev->dev, pd,&android_backlight_ops);
 if (IS_ERR(bl)) {
  dev_err(&pdev->dev, "failed to register backlight\n");
  ret = PTR_ERR(bl);
//  goto err_bl;
 }
 bl->props.max_brightness = data->max_brightness;

 bl->props.brightness=data->dft_brightness;

 platform_set_drvdata(pdev,bl);

//err_bl:
// pwm_free(pd->pwm);
//err_pwm:
// kfree(pb);
err_alloc:
 if (data->exit)
  data->exit(&pdev->dev);
 return ret;
}
首先检查我们得到的platform_data结构体中有没有init回调函数，有的话执行，没有的话跳过。

[cpp] if(data->init) 
{ 
    ret=data->init(&pdev->dev); 
    if(ret<0) 
        return ret; 
} 
 if(data->init)
 {
  ret=data->init(&pdev->dev);
  if(ret<0)
   return ret;
 }
这边比较重要的是backlight_device_register函数www.2cto.com

[cpp] struct backlight_device *backlight_device_register(const char *name, 
        struct device *parent, void *devdata, struct backlight_ops *ops) 
{ 
    struct backlight_device *new_bd; 
    int rc; 
 
    pr_debug("backlight_device_register: name=%s\n", name); 
 
    new_bd = kzalloc(sizeof(struct backlight_device), GFP_KERNEL); 
    if (!new_bd) 
        return ERR_PTR(-ENOMEM); 
 
    mutex_init(&new_bd->update_lock); 
    mutex_init(&new_bd->ops_lock); 
 
    new_bd->dev.class = backlight_class; 
    new_bd->dev.parent = parent; 
    new_bd->dev.release = bl_device_release; 
    dev_set_name(&new_bd->dev, name); 
    dev_set_drvdata(&new_bd->dev, devdata); 
 
    rc = device_register(&new_bd->dev); 
    if (rc) { 
        kfree(new_bd); 
        return ERR_PTR(rc); 
    } 
 
    rc = backlight_register_fb(new_bd); 
    if (rc) { 
        device_unregister(&new_bd->dev); 
        return ERR_PTR(rc); 
    } 
 
    new_bd->ops = ops; 
 
#ifdef CONFIG_PMAC_BACKLIGHT  
    mutex_lock(&pmac_backlight_mutex); 
    if (!pmac_backlight) 
        pmac_backlight = new_bd; 
    mutex_unlock(&pmac_backlight_mutex); 
#endif  
 
    return new_bd; 
} 
EXPORT_SYMBOL(backlight_device_register); 
struct backlight_device *backlight_device_register(const char *name,
  struct device *parent, void *devdata, struct backlight_ops *ops)
{
 struct backlight_device *new_bd;
 int rc;

 pr_debug("backlight_device_register: name=%s\n", name);

 new_bd = kzalloc(sizeof(struct backlight_device), GFP_KERNEL);
 if (!new_bd)
  return ERR_PTR(-ENOMEM);

 mutex_init(&new_bd->update_lock);
 mutex_init(&new_bd->ops_lock);

 new_bd->dev.class = backlight_class;
 new_bd->dev.parent = parent;
 new_bd->dev.release = bl_device_release;
 dev_set_name(&new_bd->dev, name);
 dev_set_drvdata(&new_bd->dev, devdata);

 rc = device_register(&new_bd->dev);
 if (rc) {
  kfree(new_bd);
  return ERR_PTR(rc);
 }

 rc = backlight_register_fb(new_bd);
 if (rc) {
  device_unregister(&new_bd->dev);
  return ERR_PTR(rc);
 }

 new_bd->ops = ops;

#ifdef CONFIG_PMAC_BACKLIGHT
 mutex_lock(&pmac_backlight_mutex);
 if (!pmac_backlight)
  pmac_backlight = new_bd;
 mutex_unlock(&pmac_backlight_mutex);
#endif

 return new_bd;
}
EXPORT_SYMBOL(backlight_device_register);

这里做的最主要的事情就是对一些结构体的初始化，然后调用device_register把我们具体的device挂到我们的backlight class下，具体的是如何实现的我这里不多说，我这里只做一些简单的介绍。这里大家可以看到最重要的是backlight_device_register函数的最后一个参数，这里提供了我们可以自己定义的几个回调函数，

[cpp] struct backlight_ops { 
    unsigned int options; 
 
#define BL_CORE_SUSPENDRESUME   (1 << 0)  
 
    /* Notify the backlight driver some property has changed */ 
    int (*update_status)(struct backlight_device *); 
    /* Return the current backlight brightness (accounting for power,
       fb_blank etc.) */ 
    int (*get_brightness)(struct backlight_device *); 
    /* Check if given framebuffer device is the one bound to this backlight;
       return 0 if not, !=0 if it is. If NULL, backlight always matches the fb. */ 
    int (*check_fb)(struct fb_info *); 
}; 
struct backlight_ops {
 unsigned int options;

#define BL_CORE_SUSPENDRESUME (1 << 0)

 /* Notify the backlight driver some property has changed */
 int (*update_status)(struct backlight_device *);
 /* Return the current backlight brightness (accounting for power,
    fb_blank etc.) */
 int (*get_brightness)(struct backlight_device *);
 /* Check if given framebuffer device is the one bound to this backlight;
    return 0 if not, !=0 if it is. If NULL, backlight always matches the fb. */
 int (*check_fb)(struct fb_info *);
};
我们这边定义了2个回调函数挂上去：

[cpp] static const struct backlight_ops android_backlight_ops = { 
    .update_status  = android_backlight_update_status, 
    .get_brightness = android_backlight_get_brightness, 
//        .check_fb...    
}; 
static const struct backlight_ops android_backlight_ops = {
 .update_status  = android_backlight_update_status,
 .get_brightness = android_backlight_get_brightness,
//        .check_fb... 
};
然后我们去实现这2个函数，就基本完成了我们的驱动了，看函数名字就知道这2个函数的作用，一个是用来更新我们的背光亮度，还有一个是用来得到我们的光强。

[cpp] static int android_backlight_get_brightness(struct backlight_device *bl) 
{ 
    printk(KERN_INFO "[android]---get brightness...\n"); 
    return bl->props.brightness; 
} 
static int android_backlight_get_brightness(struct backlight_device *bl)
{
 printk(KERN_INFO "[android]---get brightness...\n");
 return bl->props.brightness;
}

这个函数比较简单，就是返回backlight_device->props->brightness，我们来看下最终我们的brightness是哪里写进去的。这里比较绕，我们还是结合update函数一起分析：

[cpp] static int android_backlight_update_status(struct backlight_device *bl) 
{ 
    struct android_pwm_data *pd = dev_get_drvdata(&bl->dev); 
    int brightness = bl->props.brightness; 
    int max=bl->props.max_brightness; 
 
/*  if (bl->props.power != FB_BLANK_UNBLANK)
        brightness = 0;
 
    if (bl->props.fb_blank != FB_BLANK_UNBLANK)
        brightness = 0;
*/ 
    printk(KERN_INFO "update brightness...\n"); 
    if (pd->notify) 
        brightness = pd->notify(pd->dev, brightness); 
    //+++add  
    global_brightness = brightness; 
//  complete(&priv_event);  
    printk(KERN_INFO "complete event....\n"); 
    return 0; 
} 
static int android_backlight_update_status(struct backlight_device *bl)
{
 struct android_pwm_data *pd = dev_get_drvdata(&bl->dev);
 int brightness = bl->props.brightness;
 int max=bl->props.max_brightness;

/* if (bl->props.power != FB_BLANK_UNBLANK)
  brightness = 0;

 if (bl->props.fb_blank != FB_BLANK_UNBLANK)
  brightness = 0;
*/
 printk(KERN_INFO "update brightness...\n");
 if (pd->notify)
  brightness = pd->notify(pd->dev, brightness);
 //+++add
 global_brightness = brightness;
// complete(&priv_event);
 printk(KERN_INFO "complete event....\n");
 return 0;
}

我们姑且这么理解，我们有一个结构体，brightness_properity用来存放backlight的一些属性信息，比如说brightness，当我们要get_brightness的时候就是去返回这个brightness，当我们要调节光强的时候就是给这个结构体中的成员变量赋值。

首先我们要了解android中用户层是怎么做的，因为我们linux driver最终的目标就是服务用户层，所以我们要了解。

其实android HAL层就是open backlight中的brightness这个节点，然后进行读写来设置背光的亮度的，好吧，先来看下读写这个节点会呼叫的回调函数

在backlight.c中实现：

[cpp] static ssize_t backlight_show_brightness(struct device *dev, 
        struct device_attribute *attr, char *buf) 
{ 
    struct backlight_device *bd = to_backlight_device(dev); 
 
    return sprintf(buf, "%d\n", bd->props.brightness); 
} 
 
static ssize_t backlight_store_brightness(struct device *dev, 
        struct device_attribute *attr, const char *buf, size_t count) 
{ 
    int rc; 
    struct backlight_device *bd = to_backlight_device(dev); 
    unsigned long brightness; 
 
    rc = strict_strtoul(buf, 0, &brightness); 
    if (rc) 
        return rc; 
 
    rc = -ENXIO; 
 
    mutex_lock(&bd->ops_lock); 
    if (bd->ops) { 
        if (brightness > bd->props.max_brightness) 
            rc = -EINVAL; 
        else { 
            pr_debug("backlight: set brightness to %lu\n", 
                 brightness); 
            bd->props.brightness = brightness; 
            backlight_update_status(bd); 
            rc = count; 
        } 
    } 
    mutex_unlock(&bd->ops_lock); 
 
    return rc; 
} 
static ssize_t backlight_show_brightness(struct device *dev,
  struct device_attribute *attr, char *buf)
{
 struct backlight_device *bd = to_backlight_device(dev);

 return sprintf(buf, "%d\n", bd->props.brightness);
}

static ssize_t backlight_store_brightness(struct device *dev,
  struct device_attribute *attr, const char *buf, size_t count)
{
 int rc;
 struct backlight_device *bd = to_backlight_device(dev);
 unsigned long brightness;

 rc = strict_strtoul(buf, 0, &brightness);
 if (rc)
  return rc;

 rc = -ENXIO;

 mutex_lock(&bd->ops_lock);
 if (bd->ops) {
  if (brightness > bd->props.max_brightness)
   rc = -EINVAL;
  else {
   pr_debug("backlight: set brightness to %lu\n",
     brightness);
   bd->props.brightness = brightness;
   backlight_update_status(bd);
   rc = count;
  }
 }
 mutex_unlock(&bd->ops_lock);

 return rc;
}

当我们向brightness这个文件节点中写入我们要设置的背光亮度的时候会调用store这个回调函数，我们来看下主要做了哪些事情，跟我们在driver层自己写的update函数到底有什么关系呢？

前面都是一大堆不用看的代码，这里最重要的看这个

[cpp]     if (bd->ops) { 
        if (brightness > bd->props.max_brightness) 
            rc = -EINVAL; 
        else { 
            pr_debug("backlight: set brightness to %lu\n", 
                 brightness); 
<span style="color:#ff0000;">         bd->props.brightness = brightness; 
            backlight_update_status(bd);</span> 
            rc = count; 
        } 
    } 
 if (bd->ops) {
  if (brightness > bd->props.max_brightness)
   rc = -EINVAL;
  else {
   pr_debug("backlight: set brightness to %lu\n",
     brightness);
<span style="color:#ff0000;">   bd->props.brightness = brightness;
   backlight_update_status(bd);</span>
   rc = count;
  }
 }
首先是把brightness写进我们的背光属性结构体中，这样就更新了我们数据结构中的背光亮度在值，但是这样做是不够的，因为我们最终要控制的是硬件，所以看下之后我们调用了backlight_update_status函数，ok，看下这个函数的定义：

/include/linux/backlight.h

[cpp] static inline void backlight_update_status(struct backlight_device *bd) 
{ 
    mutex_lock(&bd->update_lock); 
    if (bd->ops && bd->ops->update_status) 
        bd->ops->update_status(bd); 
    mutex_unlock(&bd->update_lock); 
} 
static inline void backlight_update_status(struct backlight_device *bd)
{
 mutex_lock(&bd->update_lock);
 if (bd->ops && bd->ops->update_status)
  bd->ops->update_status(bd);
 mutex_unlock(&bd->update_lock);
}
看下这个内联函数，看到ops就知道了吧，这边调用了bd->ops->update_status这里就调用到了我们自己写的update_status回调函数：

[cpp] static const struct backlight_ops android_backlight_ops = { 
    .update_status  = android_backlight_update_status, 
    .get_brightness = android_backlight_get_brightness, 
//        .check_fb...    
}; 
static const struct backlight_ops android_backlight_ops = {
 .update_status  = android_backlight_update_status,
 .get_brightness = android_backlight_get_brightness,
//        .check_fb... 
};
[cpp] static int android_backlight_update_status(struct backlight_device *bl) 
{ 
    struct android_pwm_data *pd = dev_get_drvdata(&bl->dev); 
    int brightness = bl->props.brightness; 
    int max=bl->props.max_brightness; 
 
/*  if (bl->props.power != FB_BLANK_UNBLANK)
        brightness = 0;
 
    if (bl->props.fb_blank != FB_BLANK_UNBLANK)
        brightness = 0;
*/ 
    printk(KERN_INFO "update brightness...\n"); 
    if (pd->notify) 
        brightness = pd->notify(pd->dev, brightness); 
    //+++add  
    global_brightness = brightness; 
//  complete(&priv_event);  
    printk(KERN_INFO "complete event....\n"); 
    return 0; 
} 
static int android_backlight_update_status(struct backlight_device *bl)
{
 struct android_pwm_data *pd = dev_get_drvdata(&bl->dev);
 int brightness = bl->props.brightness;
 int max=bl->props.max_brightness;

/* if (bl->props.power != FB_BLANK_UNBLANK)
  brightness = 0;

 if (bl->props.fb_blank != FB_BLANK_UNBLANK)
  brightness = 0;
*/
 printk(KERN_INFO "update brightness...\n");
 if (pd->notify)
  brightness = pd->notify(pd->dev, brightness);
 //+++add
 global_brightness = brightness;
// complete(&priv_event);
 printk(KERN_INFO "complete event....\n");
 return 0;
}
这里咱也没做什么，因为android模拟器没有真正的背光的设备，我们打印了信息，还有就是一个notify回调函数，这里我们也没有实现，这里我猜想就是这边背光如果涉及到别的deivce的行为的话，这个notify函数可以通知到别的设备。

ok，这边就介绍结束了，我们来启动我们的android模拟器来看下sysfs中backlight下我们自己的节点。

 

大家可以看到我们自己的device的文件系统，我们cat 出来的brightness就是我们在board-goldfish.c中设置的初始值。

=====================================================

OK，这部分就介绍到这，下面一篇会介绍到我们HAL层中是如何封装我们driver中的接口的。

摘自  zhangjie201412的专栏