周六的中午还是依旧来了公司，本来也没有打算来的，既然来了，那就把上次遗留下来的一些问题给解决吧，把android下的pwm调lcd背光给总结下吧。关于android的背光，是用pwm波来控制的，通过占空比的不同，得到不同的电压值，从而点亮的lcd的亮度就不同了。具体的也就是控制了电压值而已。

要在android上层搞定背光的调节，那么必须准备底层驱动了，首先就是那个backlight了，这个linux驱动的模型已经做好了，只要选上就ok了。

Device下的graphics support

选上那个backlight就好了

还有要准备相应平台的pwm的驱动，准备好这些后，就ok了。然后跑一下就会生成一个节点：

/sys/class/backlight/pwm-backlight/brightness

只要

echo 100 >  /sys/class/backlight/pwm-backlight/brightnessecho 50 >  /sys/class/backlight/pwm-backlight/brightness

如果背光改变了，那么就说明驱动OK了，也可以用示波器看pwm引脚，是否有变换。

如果不成功，那么就继续调试底层驱动吧。接着就是要上android了。

对于背光，android代码中主要是：

Frameworks/base/services/jni/com_android_server_LightsService.cppFrameworks/base/services/java/com/android/server/LightsServer.java

这里就不用太去关注了，移植到android主要就是硬件抽象层的事情了，java层会调用jni层，jni层会调到相应的硬件抽象层，硬件抽象层再继续调用底层驱动。好了，还是看看lcd背光的硬件抽象层吧。

对于不同平台那个lights.c是放在不同地方的，比如你的是samsung的tuna平台，那么就

在/device/Samsung/tuna/liblights/lights.c下。

#define LOG_TAG "lights"
#include <cutils/log.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <hardware/lights.h>
#include <linux/leds-an30259a.h>static pthread_once_t g_init = PTHREAD_ONCE_INIT;
static pthread_mutex_t g_lock = PTHREAD_MUTEX_INITIALIZER;char const *const LCD_FILE = "/sys/class/backlight/s6e8aa0/brightness";char const *const LED_FILE = "/dev/an30259a_leds";#define IMAX 0 // 12.75mA power consumption// Slope values, based on total blink of 1000ms#define SLOPE_UP_1          450#define SLOPE_UP_2          (500-SLOPE_UP_1)#define SLOPE_DOWN_1    SLOPE_UP_2#define SLOPE_DOWN_2    SLOPE_UP_1// brightness at mid-slope, on 0 - 127 scale#define MID_BRIGHTNESS  31void init_g_lock(void)
{pthread_mutex_init(&g_lock, NULL);
}static int write_int(char const *path, int value)
{int fd;static int already_warned;already_warned = 0;LOGV("write_int: path %s, value %d", path, value);fd = open(path, O_RDWR);if (fd >= 0) {char buffer[20];int bytes = sprintf(buffer, "%d\n", value);int amt = write(fd, buffer, bytes);close(fd);return amt == -1 ? -errno : 0;} else {if (already_warned == 0) {LOGE("write_int failed to open %s\n", path);already_warned = 1;}return -errno;}
}static int rgb_to_brightness(struct light_state_t const *state)
{int color = state->color & 0x00ffffff;return ((77*((color>>16) & 0x00ff))+ (150*((color>>8) & 0x00ff)) + (29*(color & 0x00ff))) >> 8;
}static int set_light_backlight(struct light_device_t *dev,struct light_state_t const *state)
{int err = 0;int brightness = rgb_to_brightness(state);pthread_mutex_lock(&g_lock);err = write_int(LCD_FILE, brightness);pthread_mutex_unlock(&g_lock);return err;
}static int close_lights(struct light_device_t *dev)
{LOGV("close_light is called");if (dev)free(dev);return 0;
}/* LEDs */
static int write_leds(struct an30259a_pr_control *led)
{int err = 0;int imax = IMAX;int fd;pthread_mutex_lock(&g_lock);fd = open(LED_FILE, O_RDWR);if (fd >= 0) {err = ioctl(fd, AN30259A_PR_SET_IMAX, &imax);if (err)LOGE("failed to set imax");err = ioctl(fd, AN30259A_PR_SET_LED, led);if (err < 0)LOGE("failed to set leds!");close(fd);} else {LOGE("failed to open %s!", LED_FILE);err =  -errno;}pthread_mutex_unlock(&g_lock);return err;
}static int set_light_leds(struct light_state_t const *state, int type)
{struct an30259a_pr_control led;memset(&led, 0, sizeof(led));switch (state->flashMode) {case LIGHT_FLASH_NONE:led.state = LED_LIGHT_OFF;break;case LIGHT_FLASH_TIMED:case LIGHT_FLASH_HARDWARE:led.state = LED_LIGHT_SLOPE;led.color = state->color & 0x00ffffff;// tweak to eliminate purplish tint from white colorif (led.color == 0x00ffffff)led.color = 0x80ff80;// scale slope times based on flashOnMSled.time_slope_up_1 = (SLOPE_UP_1 * state->flashOnMS) / 1000;led.time_slope_up_2 = (SLOPE_UP_2 * state->flashOnMS) / 1000;led.time_slope_down_1 = (SLOPE_DOWN_1 * state->flashOnMS) / 1000;led.time_slope_down_2 = (SLOPE_DOWN_2 * state->flashOnMS) / 1000;led.mid_brightness = MID_BRIGHTNESS;led.time_off = state->flashOffMS;break;default:return -EINVAL;} return write_leds(&led);
}static int set_light_leds_notifications(struct light_device_t *dev,struct light_state_t const *state)
{return set_light_leds(state, 0);
}static int set_light_leds_attention(struct light_device_t *dev,struct light_state_t const *state)
{return set_light_leds(state, 1);
}static int open_lights(const struct hw_module_t *module, char const *name,struct hw_device_t **device)
{int (*set_light)(struct light_device_t *dev,struct light_state_t const *state);if (0 == strcmp(LIGHT_ID_BACKLIGHT, name))set_light = set_light_backlight;else if (0 == strcmp(LIGHT_ID_NOTIFICATIONS, name))set_light = set_light_leds_notifications;else if (0 == strcmp(LIGHT_ID_ATTENTION, name))set_light = set_light_leds_attention;elsereturn -EINVAL;pthread_once(&g_init, init_g_lock);struct light_device_t *dev = malloc(sizeof(struct light_device_t));memset(dev, 0, sizeof(*dev));dev->common.tag = HARDWARE_DEVICE_TAG;dev->common.version = 0;dev->common.module = (struct hw_module_t *)module;dev->common.close = (int (*)(struct hw_device_t *))close_lights;dev->set_light = set_light;*device = (struct hw_device_t *)dev;return 0;
}static struct hw_module_methods_t lights_module_methods = {.open =  open_lights,
};const struct hw_module_t HAL_MODULE_INFO_SYM = {.tag = HARDWARE_MODULE_TAG,.version_major = 1,.version_minor = 0,.id = LIGHTS_HARDWARE_MODULE_ID,.name = "lights Module",.author = "Google, Inc.",.methods = &lights_module_methods,
};

对于这个代码，主要是修改下

char const *const LCD_FILE = "/sys/class/backlight/s6e8aa0/brightness";

背光其实就是打开这个节点，往里面写数据而已，所以，要把这个节点改为自己的平台的节点就可以了。

这里我们改为

char const *const LCD_FILE = "/sys/class/backlight/pwm-backlight/brightness";

至于

char const *const LED_FILE = "/dev/an30259a_leds";

这个，应该就是提示灯的节点吧，不管他。然后再mm下，得到了一个lights.default.so，然后替换掉这个.so就好了。

还有就是权限问题了，打开这个节点是要权限的，那么我们可以在init.xxx.rc中加上权限就好了。

chown system system /sys/class/backlight/pwm-backlight/brightness

OK，这样，那么android那个调节屏幕亮度的UI界面就可以控制lcd的背光了。