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

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，

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：

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，

<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

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：

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

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：

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，

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。

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

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，

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：

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。

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：

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：

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

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

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：

static const struct backlight_ops android_backlight_ops = { .update_status = android_backlight_update_status, .get_brightness = android_backlight_get_brightness, // .check_fb... };
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。

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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。