We have talked about HAL data structures hw_module_t and hw_module_methods_t for the Gralloc module. The last one, hw_device_t, is initialized in the open method of the Gralloc HAL module. Now we can look at the open method of the Gralloc module as follows:

int gralloc_device_open(const hw_module_t* module, const char* name, 
        hw_device_t** device) 
{ 
    int status = -EINVAL; 
    if (!strcmp(name, GRALLOC_HARDWARE_GPU0)) { 
        gralloc_context_t *dev; 
        dev = (gralloc_context_t*)malloc(sizeof(*dev)); 

        memset(dev, 0, sizeof(*dev)); 

        dev->device.common.tag = HARDWARE_DEVICE_TAG; 
        dev->device.common.version = 0; 
        dev->device.common.module = const_cast<hw_module_t*>(module); 
        dev->device.common.close = gralloc_close; 

        dev->device.alloc   = gralloc_alloc; 
        dev->device.free    = gralloc_free; 

        *device = &dev->device.common; 
        status = 0; 
    } else { 
        status = fb_device_open(module, name, device); 
    } 
    return status; 
} 

As we can see here, the gralloc_device_open function can be used to initialize two kinds of device, GRALLOC_HARDWARE_GPU0 and GRALLOC_HARDWARE_FB0, according to the name input parameter.

Let's look at the initialization of the GPU0 device first. The output parameter of the open method is the address of the hw_device_t data structure. After the calling applications get an instance of hw_device_t, they can use the hardware device to do their work. In the open method of Gralloc HAL, it allocates the memory for the gralloc_context_t data structure first. After that, it populates its device member variable and assigns the output parameter to the address of the dev->device.common member variable. As we expect, the output is the address of an hw_device_t instance. Let's look at the relationship between gralloc_context_t, alloc_device_t, and hw_device_t:

As we can see from the preceding diagram, the first field or member variable of gralloc_context_t is device, which is data type alloc_device_t:

struct gralloc_context_t { 
    alloc_device_t  device; 
    /* our private data here */ 
}; 

The following is the definition of the alloc_device_t data structure. It is defined in the gralloc.h file:

typedef struct alloc_device_t { 
    struct hw_device_t common; 

    int (*alloc)(struct alloc_device_t* dev, 
            int w, int h, int format, int usage, 
            buffer_handle_t* handle, int* stride); 

    int (*free)(struct alloc_device_t* dev, 
            buffer_handle_t handle); 

    void (*dump)(struct alloc_device_t *dev, char *buff, int buff_len); 

    void* reserved_proc[7]; 
} alloc_device_t; 

We can see that the data type of the first field of alloc_device_t is hw_device_t. This is the technique for simulating inheritance relationships in the C language that we mentioned when we discussed the relationship between private_module_t, gralloc_module_t and hw_module_t.

The alloc and free methods of the Gralloc device are implemented in the gralloc_alloc and gralloc_free functions in the gralloc.cpp file.