Android图形显示之硬件抽象层Gralloc【转】

1 #define GRALLOC_HARDWARE_MODULE_ID "gralloc"

gralloc_device_open(const hw_module_t* module, const char* name,  
        hw_device_t** device)  
{  
    int status = -EINVAL;  
    if (!strcmp(name, GRALLOC_HARDWARE_GPU0)) {  
       ...  
    } else {  
        status = fb_device_open(module, name, device);  
    }  
    return status;  
}  

1 static int mapFrameBuffer(struct private_module_t* module)
2 {
3     pthread_mutex_lock(&module->lock);
4     int err = mapFrameBufferLocked(module);
5     pthread_mutex_unlock(&module->lock);
6     return err;
7 }

  1 int mapFrameBufferLocked(struct private_module_t* module)
  2 {
  3     // already initialized...
  4     if (module->framebuffer) {
  5         return 0;
  6     }    
  7     char const * const device_template[] = {
  8             "/dev/graphics/fb%u",
  9             "/dev/fb%u",
 10             0 };
 11 
 12     int fd = -1;
 13     int i=0;
 14     char name[64];
 15     //检查是否存在设备文件/dev/graphics/fb0或者/dev/fb0。如果存在的话，那么就调用函数open来打开它，并且将得到的文件描述符保存在变量fd中
 16     while ((fd==-1) && device_template[i]) {
 17         snprintf(name, 64, device_template[i], 0);
 18         fd = open(name, O_RDWR, 0);
 19         i++;
 20     }
 21     if (fd < 0)
 22         return -errno;
 23     //通过IO控制命令FBIOGET_FSCREENINFO来获得系统帧缓冲区的固定信息，保存在fb_fix_screeninfo结构体finfo中
 24     struct fb_fix_screeninfo finfo;
 25     if (ioctl(fd, FBIOGET_FSCREENINFO, &finfo) == -1)
 26         return -errno;
 27     //通过IO控制命令FBIOGET_VSCREENINFO来获得系统帧缓冲区的可变信息，保存在fb_var_screeninfo结构体info中
 28     struct fb_var_screeninfo info;
 29     if (ioctl(fd, FBIOGET_VSCREENINFO, &info) == -1)
 30         return -errno;
 31     //初始化info
 32     info.reserved[0] = 0;
 33     info.reserved[1] = 0;
 34     info.reserved[2] = 0;
 35     info.xoffset = 0;
 36     info.yoffset = 0;
 37     info.activate = FB_ACTIVATE_NOW;
 38     //fb_var_screeninfo的成员变量xres和yres用来描述显示屏的可视分辨率，而成员变量xres_virtual和yres_virtual用来描述显示屏的虚拟分辨率。
 39     //将虚拟分辨率的高度值设置为可视分辨率的高度值的NUM_BUFFERS倍。
 40     info.yres_virtual = info.yres * NUM_BUFFERS; //2
 41     uint32_t flags = PAGE_FLIP;
 42     //设置设备显示屏的虚拟分辨率
 43     if (ioctl(fd, FBIOPUT_VSCREENINFO, &info) == -1) {
 44         //设置失败，重新设置显示屏的虚拟分辨率
 45         info.yres_virtual = info.yres;
 46         flags &= ~PAGE_FLIP;
 47         ALOGW("FBIOPUT_VSCREENINFO failed, page flipping not supported");
 48     }
 49     if (info.yres_virtual < info.yres * 2) {
 50         // we need at least 2 for page-flipping
 51         info.yres_virtual = info.yres;
 52         flags &= ~PAGE_FLIP;
 53         ALOGW("page flipping not supported (yres_virtual=%d, requested=%d)",info.yres_virtual, info.yres*2);
 54     }
 55     //通过IO控制命令FBIOGET_VSCREENINFO来重新获得系统帧缓冲区的可变信息
 56     if (ioctl(fd, FBIOGET_VSCREENINFO, &info) == -1)
 57         return -errno;
 58     //计算设备显示屏的刷新频率
 59     uint64_t  refreshQuotient =
 60     (
 61             uint64_t( info.upper_margin + info.lower_margin + info.yres )* ( info.left_margin  + info.right_margin + info.xres )* info.pixclock
 62     );
 63     //模拟器的info.pixclock=0，因此计算得到的refreshQuotient=0
 64     int refreshRate = refreshQuotient > 0 ? (int)(1000000000000000LLU / refreshQuotient) : 0;
 65     //如果是模拟器，设置刷新频率为60 Hz
 66     if (refreshRate == 0) {
 67         refreshRate = 60*1000;  // 60 Hz
 68     }
 69     if (int(info.width) <= 0 || int(info.height) <= 0) {
 70         info.width  = ((info.xres * 25.4f)/160.0f + 0.5f);
 71         info.height = ((info.yres * 25.4f)/160.0f + 0.5f);
 72     }
 73     //计算显示屏的密度
 74     float xdpi = (info.xres * 25.4f) / info.width;
 75     float ydpi = (info.yres * 25.4f) / info.height;
 76     float fps  = refreshRate / 1000.0f;
 77     //再次通过IO控制命令FBIOGET_FSCREENINFO来获得系统帧缓冲区的固定信息
 78     if (ioctl(fd, FBIOGET_FSCREENINFO, &finfo) == -1)
 79         return -errno;
 80     if (finfo.smem_len <= 0)
 81         return -errno;
 82     //得到的系统帧缓冲区的其它信息来初始化参数module所描述的一个private_module_t结构体。
 83     module->flags = flags;
 84     module->info = info;
 85     module->finfo = finfo;
 86     module->xdpi = xdpi;
 87     module->ydpi = ydpi;
 88     module->fps = fps;
 89     int err;
 90     //整个系统帧缓冲区的大小=虚拟分辨率的高度值info.yres_virtual * 每一行所占用的字节数finfo.line_length,并将整个系统帧缓冲区的大小对齐到页面边界
 91     size_t fbSize = roundUpToPageSize(finfo.line_length * info.yres_virtual);
 92     //创建一个private_handle_t，用来描述整个系统帧缓冲区的信息，
 93     module->framebuffer = new private_handle_t(dup(fd), fbSize, 0);
 94     //计算整个系统帧缓冲区可以划分为多少个图形缓冲区来使用
 95     module->numBuffers = info.yres_virtual / info.yres;
 96     //表示系统帧缓冲区中的所有图形缓冲区都是处于空闲状态
 97     module->bufferMask = 0;
 98     //以读写共享方式将帧缓冲区映射到当前进程地址空间中
 99     void* vaddr = mmap(0, fbSize, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
100     if (vaddr == MAP_FAILED) {
101         ALOGE("Error mapping the framebuffer (%s)", strerror(errno));
102         return -errno;
103     }
104     //系统帧缓冲区在当前进程的地址空间中的起始地址保存到private_handle_t的域base中
105     module->framebuffer->base = intptr_t(vaddr);
106     //清空大小为fbSize的帧缓冲区
107     memset(vaddr, 0, fbSize);
108     return 0;
109 }

1 #defineGRALLOC_HARDWARE_GPU0"gpu0"  

1 static inline int gralloc_open(const struct hw_module_t* module, 
2         struct alloc_device_t** device) {
3     return module->methods->open(module, 
4             GRALLOC_HARDWARE_GPU0, (struct hw_device_t**)device);
5 }