drm 驱动是如何创建 fb device 的

原文地址：https://blog.csdn.net/jingxia2008/article/details/48804859/

流程： drm_fbdev_cma_init  => drm_fb_helper_initial_config   =>drm_fb_helper_single_fb_probe =>register_framebuffer
drm 驱动是如何创建 fb device 的

    drm 驱动是如何创建 fb device 的
        什么是 drm
        如何使用 drm 接口
            libdrm
            fb device
        本文讨论的问题是 drm 驱动是如何虚拟 drm crts 为 fb device
            设备驱动创建 fbdev
            完成 fb 设备的创建
        流程图

什么是 drm？

drm 是一个 Linux 内核的显示系统驱动框架，区别于另外一个 DRM数字版权保护

    drm 是一个管理 GPU 的显示框架
    在内核级别提供内存管理，中断处理， DMA控
    为应用程序提供统一的操作接口

如何使用 drm 接口

    libdrm
    fb device

libdrm

内核提供的 IOCTRL 太多，libdrm 用于简化编程管理当前的显示器，并修改当前的模式成为 KMS （ drm-kms - Kernel Mode-Setting）

借助 libdrm 的强大 API 接口，如果内核支持 PRIME API ，也可以使用 PRIME 接口实现更为灵活的内存操作。
fb device

drm 驱动可以模拟一个 fb device， 默认是 default CRTC， 更多关于 fb device ，可以参考 内核 framebuffer 文档， fb device 是大多数 Linux 系统显示的基础。

    The X Server, Linux 桌面系统的显示服务
    Android gralloc， 安卓系统显示 HAL

本文讨论的问题是 drm 驱动是如何虚拟 drm crts 为 fb device

文章基于内核版本 linux-3.18

VERSION = 3
PATCHLEVEL = 18
SUBLEVEL = 0
EXTRAVERSION = -linux4sam_5.0-alpha7
NAME = Diseased Newt


drm 的代码位于：

drivers/gpu/drm/


1. 设备驱动创建 fbdev

    drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c: dc->fbdev = drm_fbdev_cma_init(dev, 24,
    drivers/gpu/drm/sti/sti_drm_drv.c: drm_fbdev_cma_init(dev, 32,
    drivers/gpu/drm/tilcdc/tilcdc_drv.c: priv->fbdev = drm_fbdev_cma_init(dev, bpp,
    drivers/gpu/drm/rcar-du/rcar_du_kms.c: fbdev = drm_fbdev_cma_init(dev, 32, dev->mode_config.num_crtc,

调用的是 drm_fbdev_cma_init

struct drm_fbdev_cma *drm_fbdev_cma_init(struct drm_device *dev,
unsigned int preferred_bpp, unsigned int num_crtc,
unsigned int max_conn_count)
{
struct drm_fbdev_cma *fbdev_cma;
struct drm_fb_helper *helper;
int ret;

fbdev_cma = kzalloc(sizeof(*fbdev_cma), GFP_KERNEL);

...
helper = &fbdev_cma->fb_helper;

drm_fb_helper_prepare(dev, helper, &drm_fb_cma_helper_funcs);

...
ret = drm_fb_helper_initial_config(helper, preferred_bpp);
    if (ret < 0) {
        dev_err(dev->dev, "Failed to set initial hw configuration.\n");
    goto err_drm_fb_helper_fini;
}

return fbdev_cma;
   

其中最重要的数据结构是 drm_fb_helper_funcs

static const struct drm_fb_helper_funcs drm_fb_cma_helper_funcs = {
.fb_probe = drm_fbdev_cma_create,
};
2. 完成 fb 设备的创建：

static int drm_fbdev_cma_create(struct drm_fb_helper *helper,
    struct drm_fb_helper_surface_size *sizes)
{
    struct drm_fbdev_cma *fbdev_cma = to_fbdev_cma(helper);
    struct drm_mode_fb_cmd2 mode_cmd = { 0 };
    struct drm_device *dev = helper->dev;
    struct drm_gem_cma_object *obj;
    struct drm_framebuffer *fb;
    unsigned int bytes_per_pixel;
    unsigned long offset;
    struct fb_info *fbi;
    size_t size;
    int ret;

    DRM_DEBUG_KMS("surface width(%d), height(%d) and bpp(%d)\n",
            sizes->surface_width, sizes->surface_height,
            sizes->surface_bpp);

    bytes_per_pixel = DIV_ROUND_UP(sizes->surface_bpp, 8);

    mode_cmd.width = sizes->surface_width;
    mode_cmd.height = sizes->surface_height;
    mode_cmd.pitches[0] = sizes->surface_width * bytes_per_pixel;
    mode_cmd.pixel_format = drm_mode_legacy_fb_format(sizes->surface_bpp,
        sizes->surface_depth);

    size = mode_cmd.pitches[0] * mode_cmd.height;
    obj = drm_gem_cma_create(dev, size);
    if (IS_ERR(obj))
        return -ENOMEM;

    fbi = framebuffer_alloc(0, dev->dev);
    if (!fbi) {
        dev_err(dev->dev, "Failed to allocate framebuffer info.\n");
        ret = -ENOMEM;
        goto err_drm_gem_cma_free_object;
    }

    fbdev_cma->fb = drm_fb_cma_alloc(dev, &mode_cmd, &obj, 1);
    if (IS_ERR(fbdev_cma->fb)) {
        dev_err(dev->dev, "Failed to allocate DRM framebuffer.\n");
        ret = PTR_ERR(fbdev_cma->fb);
        goto err_framebuffer_release;
    }

    fb = &fbdev_cma->fb->fb;
    helper->fb = fb;
    helper->fbdev = fbi;

    fbi->par = helper;
    fbi->flags = FBINFO_FLAG_DEFAULT;
    fbi->fbops = &drm_fbdev_cma_ops;

    ret = fb_alloc_cmap(&fbi->cmap, 256, 0);
    if (ret) {
        dev_err(dev->dev, "Failed to allocate color map.\n");
        goto err_drm_fb_cma_destroy;
    }

    drm_fb_helper_fill_fix(fbi, fb->pitches[0], fb->depth);
    drm_fb_helper_fill_var(fbi, helper, fb->width, fb->height);

    offset = fbi->var.xoffset * bytes_per_pixel;
    offset += fbi->var.yoffset * fb->pitches[0];

    dev->mode_config.fb_base = (resource_size_t)obj->paddr;
    fbi->screen_base = obj->vaddr + offset;
    fbi->fix.smem_start = (unsigned long)(obj->paddr + offset);
    fbi->screen_size = size;
    fbi->fix.smem_len = size;

    return 0;

err_drm_fb_cma_destroy:
    drm_framebuffer_unregister_private(fb);
    drm_fb_cma_destroy(fb);
err_framebuffer_release:
    framebuffer_release(fbi);
err_drm_gem_cma_free_object:
    drm_gem_cma_free_object(&obj->base);
    return ret;
}


drm_fbdev_cma_init  => drm_fb_helper_initial_config   =>drm_fb_helper_single_fb_probe =>register_framebuffer