SurfaceFlinger启动过程分析（三）

 

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文章出处：http://danielwood.cublog.cn

作者：Daniel Wood

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    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

Displayhardware.cpp中的init函数。

mNativeWindow= new FramebufferNativeWindow();

下面贴张图，直观一点。