前面介紹了battery的相關的東西，現在我們來介紹下backlight模塊，背光主要是用來調節顯示屏亮度的，一般背光都是用PWM控制的，調節占空比達到改變有效電壓值來調節光的強弱。

背光的移植在linux中雖然不是那么難，但是背光這個組件對我們嵌入式設備的續航能力有很大的影響，一般背光上面加的電壓會有20多的電壓，所以這部分會很耗電的，相當于是開了個大燈泡。

現在我們先來看下android goldfish中的背光代碼，哈哈，沒找到吧，沒有，我們打開模擬器，看sysfs中，也是沒有具體的背光的文件的，所以這里我們得自己實現，自己寫代碼練習練習，畢竟這部分不是非常的難，參考drivers/video/backlight/下的pwm_bl.c文件，基本可以仿照，我們要做的事情很簡單，創建背光相關的文件系統即可，不需要去控制硬件做什么動作，因為我們本來就沒有硬件。

首先看下video中的makefile，如果backlight/沒有選中就選中它，不然我們的模塊不會編譯進去。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
這里沒有一個文件被編譯進去的，我們要把backlight.c先編譯進去，直接這樣改，我比較懶 呵呵呵，

obj-y += backlight.o obj-$(CONFIG_BACKLIGHT_ATMEL_PWM) += atmel-pwm-bl.o 然后重新編譯下會生成backlight.o文件，并且在sysfs中會生成我們的backlight class

我們先來分析下backlight.c中的代碼是如何實現的。

養成好習慣，看見代碼多不用怕，首先看init函數：

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);
很簡單，這里只是用了class_create函數在sys/class下創建了backlight文件夾，然后是
backlight_class->dev_attrs = bl_device_attributes;
在backlight class中創建了一系列的文件系統，

<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; }
所以我們的驅動只要填充好具體的結構體，初始化好文件系統就夠了，在sysfs中生成可以讓user space調用的接口，接下來的事情就交給上層開發人員去做。

ok，我們來看下我們自己寫的驅動， 在backlight文件夾下新建一個文件叫 android-backlight.c，我是參照pwm_bl.c來寫的，具體先來看下代碼，init函數

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注冊平臺驅動，看下傳入的參數：

static struct platform_driver android_backlight_driver = { .driver ={ .name = "android-backlight", .owner = THIS_MODULE, }, .probe = android_backlight_probe, // .remove = ........ // .suspend // .resume };
這里我偷懶沒寫remove suspend和resume'回調函數，在移植具體驅動的時候我們都應該寫上，特別是suspend和resume函數，來看下我們paltform驅動的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中進行注冊：

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> }
這邊android_backlight_data結構體主要是做一個背光的初始化。

接下來我們看一下probe函數，

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; }
首先檢查我們得到的platform_data結構體中有沒有init回調函數，有的話執行，沒有的話跳過。

if(data->init) { ret=data->init(&pdev->dev); if(ret<0) return ret; }
這邊比較重要的是backlight_device_register函數

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);
這里做的最主要的事情就是對一些結構體的初始化，然后調用device_register把我們具體的device掛到我們的backlight class下，具體的是如何實現的我這里不多說，我這里只做一些簡單的介紹。這里大家可以看到最重要的是backlight_device_register函數的最后一個參數，這里提供了我們可以自己定義的幾個回調函數，

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 *); };
我們這邊定義了2個回調函數掛上去：

static const struct backlight_ops android_backlight_ops = { .update_status = android_backlight_update_status, .get_brightness = android_backlight_get_brightness, // .check_fb... };
然后我們去實現這2個函數，就基本完成了我們的驅動了，看函數名字就知道這2個函數的作用，一個是用來更新我們的背光亮度，還有一個是用來得到我們的光強。

static int android_backlight_get_brightness(struct backlight_device *bl) { printk(KERN_INFO "[android]---get brightness...\n"); return bl->props.brightness; }
這個函數比較簡單，就是返回backlight_device->props->brightness，我們來看下最終我們的brightness是哪里寫進去的。這里比較繞，我們還是結合update函數一起分析：

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; }

我們姑且這么理解，我們有一個結構體，brightness_properity用來存放backlight的一些屬性信息，比如說brightness，當我們要get_brightness的時候就是去返回這個brightness，當我們要調節光強的時候就是給這個結構體中的成員變量賦值。

首先我們要了解android中用戶層是怎么做的，因為我們linux driver最終的目標就是服務用戶層，所以我們要了解。

其實android HAL層就是open backlight中的brightness這個節點，然后進行讀寫來設置背光的亮度的，好吧，先來看下讀寫這個節點會呼叫的回調函數

在backlight.c中實現：

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; }
當我們向brightness這個文件節點中寫入我們要設置的背光亮度的時候會調用store這個回調函數，我們來看下主要做了哪些事情，跟我們在driver層自己寫的update函數到底有什么關系呢？

前面都是一大堆不用看的代碼，這里最重要的看這個

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寫進我們的背光屬性結構體中，這樣就更新了我們數據結構中的背光亮度在值，但是這樣做是不夠的，因為我們最終要控制的是硬件，所以看下之后我們調用了backlight_update_status函數，ok，看下這個函數的定義：

/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); }
看下這個內聯函數，看到ops就知道了吧，這邊調用了bd->ops->update_status這里就調用到了我們自己寫的update_status回調函數：

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; }
這里咱也沒做什么，因為android模擬器沒有真正的背光的設備，我們打印了信息，還有就是一個notify回調函數，這里我們也沒有實現，這里我猜想就是這邊背光如果涉及到別的deivce的行為的話，這個notify函數可以通知到別的設備。

ok，這邊就介紹結束了，我們來啟動我們的android模擬器來看下sysfs中backlight下我們自己的節點。

大家可以看到我們自己的device的文件系統，我們cat 出來的brightness就是我們在board-goldfish.c中設置的初始值。

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OK，這部分就介紹到這，下面一篇會介紹到我們HAL層中是如何封裝我們driver中的接口的。

Android模擬器學framework和driver之battery & backlight-----5. backlight in linux