Android from hardware to application: Step by Step 3-Hardware Abstraction Layer access hardware driver

Android from hardware to application: Step by Step 3-Hardware Abstraction Layer access hardware driver

The Android standard hardware driver is divided into two parts: one is the hardware driver running in the Linux kernel, and the other is the hardware abstraction layer running in the user space. By using this method, the system can be unrelated to hardware and protect the interests of some vendors. From hardware to application in Android: Step by Step, step by step. 1. Write the underlying hardware driver from scratch to the Linux kernel, next, let's take a look at how to add a hardware module to the hardware abstraction layer to interact with our kernel driver to complete hardware control.

Go to the hardware/libhardware/include/hardware directory and create android_gpio.h:

#ifndef ANDROID_ANDROID_GPIO_INTERFACE_H  #define ANDROID_ANDROID_GPIO_INTERFACE_H  #include 
 
          __BEGIN_DECLS        /*module ID*/  #define ANDROID_GPIO_HARDWARE_MODULE_ID "android_gpio"  /*module struct*/  struct android_gpio_module_t {struct hw_module_t common;  };       /*interface struct*/  struct android_gpio_device_t {struct hw_device_t common;  int fd;  int (*set_val)(struct android_gpio_device_t* dev, int val);  int (*get_val)(struct android_gpio_device_t* dev, int* val);  }; __END_DECLS#endif  
 

Set_val and get_val are the APIS provided by the upper-layer applications of the HAL layer.

Cd to the hardware/libhardware/modules directory, create the android_gpio directory, and create the android. c file in it:

#include 
 
    #include 
  
     #include 
   
      #include 
    
       #include 
     
        #include 
      
        #define DEVICE_NAME "/dev/AdrIO" #define MODULE_NAME "Android_gpio" //open and closestatic int android_gpio_device_open(const struct hw_module_t* module, const char* name, struct hw_device_t** device); static int android_gpio_device_close(struct hw_device_t* device); //device accessstatic int android_gpio_set_val(struct android_gpio_device_t* dev, int val); static int android_gpio_get_val(struct android_gpio_device_t* dev, int* val); static struct hw_module_methods_t android_gpio_module_methods = { open: android_gpio_device_open }; struct android_gpio_module_t HAL_MODULE_INFO_SYM = { common: { tag: HARDWARE_MODULE_TAG, version_major: 1, version_minor: 0, id: ANDROID_GPIO_HARDWARE_MODULE_ID, name: MODULE_NAME, author: "HAL", methods: &android_gpio_module_methods, } }; static int android_gpio_device_open(const struct hw_module_t* module, const char* name, struct hw_device_t** device) { struct android_gpio_device_t* dev;dev = (struct android_gpio_device_t*)malloc(sizeof(struct android_gpio_device_t)); memset(dev, 0, sizeof(struct android_gpio_device_t)); dev->common.tag = HARDWARE_DEVICE_TAG; dev->common.version = 0; dev->common.module = (hw_module_t*)module; dev->common.close = android_gpio_device_close; dev->set_val = android_gpio_set_val;dev->get_val = android_gpio_get_val; if((dev->fd = open(DEVICE_NAME, O_RDWR)) == -1) { LOGE("android_gpio: failed to open /dev/AdrIO -- %s.", strerror(errno));free(dev); return -EFAULT; } *device = &(dev->common); return 0; } static int android_gpio_device_close(struct hw_device_t* device) { struct android_gpio_device_t* android_gpio_device = (struct android_gpio_device_t*)device; if(android_gpio_device) { close(android_gpio_device->fd); free(android_gpio_device); } return 0; } static int android_gpio_set_val(struct android_gpio_device_t* dev, int val) { LOGI("android_gpio: set value %d to device.", val); write(dev->fd, &val, sizeof(val)); return 0; } static int android_gpio_get_val(struct android_gpio_device_t* dev, int* val) { return 0;}
      
     
    
   
  
 

To prevent calling with Permission denied:

Open the system/core/rootdir directory and open ueventd. rc to add:

/dev/android_gpio 0666 root root

Add the Android. mk file in the android_gpio directory:

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE_TAGS := optionalLOCAL_PRELINK_MODULE := falseLOCAL_MODULE_PATH := $(TARGET_OUT_SHARED_LIBRARIES)/hwLOCAL_SHARED_LIBRARIES := liblogLOCAL_SRC_FILES := android_gpio.cLOCAL_MODULE := android_gpio.defaultinclude $(BUILD_SHARED_LIBRARY)

Compile the HAL layer:

mmm hardware/libhardware/modules/android_gpio

If an error occurs, refer:

No command 'mmm' found

You can create/lib/liblog. so without rules.

If successful, you can generate android_gpio.default.so

Install: out/target/product/generic/system/lib/hw/android_gpio.default.so

This is the hardware abstraction layer module we need. After this step is completed, we need to go up and finally complete the hardware call.