SurfaceFlinger启动过程分析（三）

文章出处：http://danielwood.cublog.cn

作者：Daniel Wood

内存映射对于framebuffer来说非常重要，因为通常用户是不能直接操作物理地址空间的（也就是物理内存？），然而通过mmap映射之后，将framebuffer的物理地址空间映射到用户空间的一段虚拟地址中，用户就可以通过操作这段虚拟内存而间接操作framebuffer了，你在那段虚拟内存中画了图，相应的图就会显示到屏幕上。

——这段是自己的理解，有错必究！

下面是framebuffer.cpp中的mapFrameBufferLocked函数。

int mapFrameBufferLocked(struct private_module_t* module) {     // already initialized...     if (module->framebuffer) {         return 0;     }             char const * const device_template[] = {             "/dev/graphics/fb%u",             "/dev/fb%u",             0 };     int fd = -1;     int i=0;     char name[64];     while ((fd==-1) && device_template[i]) {         snprintf(name, 64, device_template[i], 0);         fd = open(name, O_RDWR, 0);         i++;     }     if (fd < 0)         return -errno;     struct fb_fix_screeninfo finfo;     if (ioctl(fd, FBIOGET_FSCREENINFO, &finfo) == -1)         return -errno;     struct fb_var_screeninfo info;     if (ioctl(fd, FBIOGET_VSCREENINFO, &info) == -1)         return -errno;     info.reserved[0] = 0;     info.reserved[1] = 0;     info.reserved[2] = 0;     info.xoffset = 0;     info.yoffset = 0;     info.activate = FB_ACTIVATE_NOW;     /*      * Explicitly request 5/6/5      */     info.bits_per_pixel = 16;     info.red.offset = 11;     info.red.length = 5;     info.green.offset = 5;     info.green.length = 6;     info.blue.offset = 0;     info.blue.length = 5;     info.transp.offset = 0;     info.transp.length = 0;     /*      * Request NUM_BUFFERS screens (at lest 2 for page flipping)      */     info.yres_virtual = info.yres * NUM_BUFFERS;     uint32_t flags = PAGE_FLIP;     if (ioctl(fd, FBIOPUT_VSCREENINFO, &info) == -1) {         info.yres_virtual = info.yres;         flags &= ~PAGE_FLIP;         LOGW("FBIOPUT_VSCREENINFO failed, page flipping not supported");     }     if (info.yres_virtual < info.yres * 2) {         // we need at least 2 for page-flipping         info.yres_virtual = info.yres;         flags &= ~PAGE_FLIP;         LOGW("page flipping not supported (yres_virtual=%d, requested=%d)",                 info.yres_virtual, info.yres*2);     }     if (ioctl(fd, FBIOGET_VSCREENINFO, &info) == -1)         return -errno;     int refreshRate = 1000000000000000LLU /     (             uint64_t( info.upper_margin + info.lower_margin + info.yres )             * ( info.left_margin + info.right_margin + info.xres )             * info.pixclock     );     if (refreshRate == 0) {         // bleagh, bad info from the driver         refreshRate = 60*1000; // 60 Hz     }     if (int(info.width) <= 0 || int(info.height) <= 0) {         // the driver doesn't return that information         // default to 160 dpi         info.width = ((info.xres * 25.4f)/160.0f + 0.5f);         info.height = ((info.yres * 25.4f)/160.0f + 0.5f);     }     float xdpi = (info.xres * 25.4f) / info.width;     float ydpi = (info.yres * 25.4f) / info.height;     float fps = refreshRate / 1000.0f;     LOGI( "using (fd=%d)/n"             "id = %s/n"             "xres = %d px/n"             "yres = %d px/n"             "xres_virtual = %d px/n"             "yres_virtual = %d px/n"             "bpp = %d/n"             "r = %2u:%u/n"             "g = %2u:%u/n"             "b = %2u:%u/n",             fd,             finfo.id,             info.xres,             info.yres,             info.xres_virtual,             info.yres_virtual,             info.bits_per_pixel,             info.red.offset, info.red.length,             info.green.offset, info.green.length,             info.blue.offset, info.blue.length     );     LOGI( "width = %d mm (%f dpi)/n"             "height = %d mm (%f dpi)/n"             "refresh rate = %.2f Hz/n",             info.width, xdpi,             info.height, ydpi,             fps     );     if (ioctl(fd, FBIOGET_FSCREENINFO, &finfo) == -1)         return -errno;     if (finfo.smem_len <= 0)         return -errno;     module->flags = flags;     module->info = info;     module->finfo = finfo;     module->xdpi = xdpi;     module->ydpi = ydpi;     module->fps = fps;     /*      * map the framebuffer      */     int err;     size_t fbSize = roundUpToPageSize(finfo.line_length * info.yres_virtual);//对齐页     module->framebuffer = new private_handle_t(dup(fd), fbSize, 0);     module->numBuffers = info.yres_virtual / info.yres;     module->bufferMask = 0;     void* vaddr = mmap(0, fbSize, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);     if (vaddr == MAP_FAILED) {         LOGE("Error mapping the framebuffer (%s)", strerror(errno));         return -errno;     }     module->framebuffer->base = intptr_t(vaddr);     memset(vaddr, 0, fbSize);     return 0; }

这个函数就是和驱动相关的调用，其实结合驱动去看代码是很有意思的，把一路都打通了。

该函数首先通过open函数打开设备结点。

"/dev/graphics/fb%u"和"/dev/fb%u"，如果前一个顺利打开的话，那么就不打开第二个。我的Log显示打开的是第一个设备结点/dev/graphics/fb%u。

    然后通过ioctl读取设备的固定参数（FBIOGET_FSCREENINFO）和可变参数（FBIOGET_VSCREENINFO）。

【kernel部分的代码在drivers/video/fbmem.c中。】

    然后对可变参数进行修改，通过ioctl设置（FBIOPUT_VSCREENINFO）显示屏的可变参数。

    设置好以后再ioctl-FBIOGET_VSCREENINFO获得可变参数，然后在log上打出显示屏的各个参数设置，也就是我们开机看到的一长串log。

I/gralloc ( 1620): using (fd=8) I/gralloc ( 1620): id = truly-ILI9327 I/gralloc ( 1620): xres = 240 px I/gralloc ( 1620): yres = 400 px I/gralloc ( 1620): xres_virtual = 240 px I/gralloc ( 1620): yres_virtual = 800 px I/gralloc ( 1620): bpp = 16 I/gralloc ( 1620): r = 11:5 I/gralloc ( 1620): g = 5:6 I/gralloc ( 1620): b = 0:5 I/gralloc ( 1620): width = 38 mm (160.421051 dpi) I/gralloc ( 1620): height = 64 mm (158.750000 dpi) I/gralloc ( 1620): refresh rate = 60.00 Hz

    然后通过mmap完成对显示缓存区的映射。这样mapFrameBufferLocked函数的任务算是完成了。

好了，以上所讲的只是（1）中的第一句话而已