开发Android硬件抽象层(HAL)模块

文件列表

hardware/libhardware/include/hardware/hello.h
hardware/libhardware/modules/hello.c
hardware/libhardware/modules/Android.mk

HAL层包括两个部分

1. 增加硬件抽象层（HAL）模块访问Linux内核驱动程序

2. 硬件抽象层（HAL）模块编写JNI方法提供Java访问硬件服务接口

编写硬件抽象层（HAL)模块接口

本章节是将通过设备文件/dev/hello来连接硬件抽象层模块和Linux内核驱动程序模块

hello.h###

在android\qiku\hardware\libhardware\include\hardware目录，新增hello.h

#ifndef ANDROIDDRIVER_HELLO_H
#define ANDROIDDRIVER_HELLO_H
#include <hardware/hardware.h>


struct hello_module_t {
    struct hw_module_t common;
};

struct hello_device_t {
    struct hw_device_t common;
    int fd;
    int (*get_val)(struct hello_device_t *dev,int *val);
    int (*set_val)(struct hello_device_t *dev,int val);
};

int hello_dri_open(const struct hw_module_t* module, const char* id,
              struct hw_device_t** device);

#endif //ANDROIDDRIVER_HELLO_H

hello.c###

在android\qiku\hardware\libhardware\modules\目录创建hello目录，创建Android.mk和hello.c文件
hello.c

#include <hardware/hardware.h>
#include <stdlib.h>
#include <fcntl.h>
#include <hardware/hello.h>
#include <cutils/log.h>
#include <errno.h>
#include <string.h>


struct hw_module_methods_t   hello_method = {
        open : hello_dri_open
};

struct hello_module_t HAL_MODULE_INFO_SYM = {
        common : {
                tag : HARDWARE_MODULE_TAG,
                version_major : 1,
                version_minor : 0,
                id : "hello",
                name : "hello",
                author : "jdf",
                methods : &hello_method
        }
};


int hello_get_val(struct hello_device_t *dev,int *val){
    if(!dev) {
        ALOGI("HelloHal Null dev pointer.");
        return -EFAULT;
    }
    if(!val) {
        ALOGI("HelloHal Null val pointer.");
        return -EFAULT;
    }
    read(dev->fd, val, sizeof(*val));
    ALOGI("HelloHal Get value %d from device file /dev/hello.", *val);
    return *val;
}

int hello_set_val(struct hello_device_t *dev,int val){
    if(!dev) {
        ALOGI("HelloHal Null dev pointer.");
        return -EFAULT;
    }
    ALOGI("HelloHal Set value %d to device file /dev/hello.", val);
    write(dev->fd, &val, sizeof(val));

    return 0;
}

int hello_close(struct hw_device_t* device){
  //关闭文件，释放内存
    struct hello_device_t *dev = (struct hello_device_t *)device;
    if(dev) {
        close(dev->fd);
        free(dev);
    }
    return 0;
}

int hello_dri_open(const struct hw_module_t* module, const char* id,struct hw_device_t** device){
    ALOGI("HelloHal hello_device_open...%s",id);
    ALOGI("HelloHal YES,IT IS HELLO");
    struct hello_device_t* dev;
    dev = (struct hello_device_t*)malloc(sizeof(struct hello_device_t));
    if(!dev) {
        ALOGI("HelloHal: failed to alloc space");
        return -EFAULT;
    }
    memset(dev,0, sizeof(struct hello_device_t));
    ALOGI("HelloHal memset sucess /dev/hello");
    dev->common.tag = HARDWARE_DEVICE_TAG;
    dev->common.version = 0;
    dev->common.module = (hw_module_t*)module;
    dev->common.close = hello_close;
    dev->set_val = hello_set_val;
    dev->get_val = hello_get_val;
    if((dev->fd = open("/dev/hello",O_RDWR))== -1){
        ALOGI("HelloHal  fail open /dev/hello -- %s.", strerror(errno));
        return -EFAULT;
    }
   *device = &(dev->common);
   ALOGI("HelloHal Open device file /dev/hello successfully."); 
   return 0;
}

OPEN方法中的 /dev/hello也就是我们上一节创建的字符虚拟设备

Android.mk###

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE_TAGS := optional
LOCAL_PRELINK_MODULE := false
LOCAL_MODULE_PATH := $(TARGET_OUT_SHARED_LIBRARIES)/hw
LOCAL_SHARED_LIBRARIES := liblog
LOCAL_SRC_FILES := hello.c
LOCAL_MODULE := hello.default
include $(BUILD_SHARED_LIBRARY)

处理硬件访问权限问题##

DEVICE_NAME定义为"/dev/hello"。由于设备文件是在内核驱动里面通过device_create创建的，而device_create创建的设备文件默认只有root用户可读写，而hello_device_open一般是由上层APP来调用的，这些APP一般不具有root权限，这时候为/dev/hello添加root权限

打开Android源代码工程目录下，进入到system/core/rootdir目录，里面有一个名为ueventd.rc文件，往里面添加一行：

diff --git a/core/rootdir/ueventd.rc b/core/rootdir/ueventd.rc
old mode 100644
new mode 100755
index b735dc3..d6607f4
--- a/core/rootdir/ueventd.rc
+++ b/core/rootdir/ueventd.rc
@@ -1,6 +1,7 @@
 subsystem adf
        devname uevent_devname
 
+/dev/hello                0666   root       root
 /dev/null                 0666   root       root
 /dev/zero                 0666   root       root
 /dev/full                 0666   root       root

编译验证##

通过mmm单独编译下改模块

jdf@ubuntu:~/mtk_6737_6.x_wtwd_8_qiku_LE_PLUS/android/qiku$ mmm hardware/libhardware/modules/hello/ -j8

查看生成结果：

xuzhike@ubuntu:~/mtk_6737_6.x_wtwd_8_qiku_LE_PLUS/android/qiku$ ls out/target/product/yk658_37m_lwtg_35g/system/lib/hw | grep hello
hello.default.so

重新打包编译system.image或者全编工程

xuzhike@ubuntu:~/mtk_6737_6.x_wtwd_8_qiku_LE_PLUS/android/qiku$ make snod