DRM框架（vkms）分析（9）----drm驱动创建fbdevice分析（以rockchip_drm

本文主要介绍DRM框架里的fbdev兼容逻辑

一 framebuffer框架简单介绍

framebuffer框架下fbdev的注册主要三步步：

（1）创建fbdev操作函数，以rockchip为例：

static const struct fb_ops rockchip_drm_fbdev_ops = {.owner     = THIS_MODULE,DRM_FB_HELPER_DEFAULT_OPS,.fb_mmap   = rockchip_fbdev_mmap,.fb_fillrect = drm_fb_helper_cfb_fillrect,.fb_copyarea  = drm_fb_helper_cfb_copyarea,.fb_imageblit = drm_fb_helper_cfb_imageblit,
};

（2）填充struct fb_info* fbi实例的各个参数，其中就包括赋值操作函数

fbi->fbops = &rockchip_drm_fbdev_ops

（3）注册fb设备

register_framebuffer(fbi)

二 DRM注册接口

drm驱动中注册fbdev，主要是选择主要有以下几个方面:

1）创建显存drm_framebuffer 实例fb

2）填充到fb_info中并注册fbdev

3）将fb绑定到对应的crtc plane

经过上述操作，就可以通过fbdev的FBIOGET_FSCREENINFO/FBIOGET_VSCREEENINFO、mmap/FBIOPAN_DISPLAY/FBIO_WAITFORVSYNC等操作显存，而fb/crtc/plane/fb_info之间是通过drm_fb_helper来关联的，其结构体如下：

struct drm_fb_helper {//client用来关联crtc/plane/mode_set等参数struct drm_client_dev client;struct drm_client_buffer *buffer;//fb显存struct drm_framebuffer *fb;struct drm_device *dev;//fb显存的创建hook，以及部分fb_info fbdev成员的填充，该hook由各drm驱动创建const struct drm_fb_helper_funcs *funcs;//fbdevstruct fb_info *fbdev;// ....
};

（1）fb_helper->fb的创建

helper->fb是显存drm_framebuffer实例，其创建主要是通过调用drm_fb_helper_funcs实例中的fb_probe hook，如下：

static const struct drm_fb_helper_funcs rockchip_drm_fb_helper_funcs = {.fb_probe = rockchip_drm_fbdev_create,
};

static int rockchip_drm_fbdev_create(struct drm_fb_helper *helper,struct drm_fb_helper_surface_size *sizes)
{struct rockchip_drm_private *private = to_drm_private(helper);struct drm_mode_fb_cmd2 mode_cmd = { 0 };struct drm_device *dev = helper->dev;;struct drm_framebuffer *fb;unsigned int bytes_per_pixel;unsigned long offset;//根据size创建drm_gem_obj对象struct rockchip_gem_object *rk_obj rk_obj；rk_obj = rockchip_gem_create_object(dev, size, true);rok_obj = rockchip_gem_alloc_objectrockchip_gem_alloc_buf//drm_gem_obj对象                                                         private->fbdev_bo = &rk_obj->base;//创建fb_info对象并保存到fb_helper->fbdev中struct fb_info *fbi;fbi = drm_fb_helper_alloc_fbi(helper);//利用private->fbdev_bo(drm_gem_obj对象)创建drm_frame_buffer对象并保存到helper->fb中helper->fb = rockchip_drm_framebuffer_init(dev, &mode_cmd,private->fbdev_bo);//初始化fbdev的操作函数fbi->fbops = &rockchip_drm_fbdev_ops;//填充fb_info的参数fb = helper->fb;drm_fb_helper_fill_info(fbi, helper, sizes);drm_fb_helper_fill_fix //填充固定参数drm_fb_helper_fill_var //填充可变参数//将fb_helper保存到par中，fbdev操作函数（rockchip_drm_fbdev_ops）会根据//jnfo->par找到fb_helperinfo->par = fb_helper; //继续填充fb_info的参数offset = fbi->var.xoffset * bytes_per_pixel;offset += fbi->var.yoffset * fb->pitches[0];fbi->screen_base = rk_obj->kvaddr + offset;fbi->screen_size = rk_obj->base.size;fbi->fix.smem_len = rk_obj->base.size;}

（2）fb_helper->client的初始化

fb_helper->client是drm_client_dev对象，其成员modesets（drm_mode_set类型）关联了fb/crtc/connector/displaymode等

rockchip_drm_fbdev_inithelper = &private->fbdev_helper/*将rockchip_drm_fb_helper_funcs保存到helper->funcs*后续会调用其fb_probe，用来创建fb*/drm_fb_helper_prepare(dev, helper, &rockchip_drm_fb_helper_funcs)helper->funcs = funcs; //rockchip_drm_fb_helper_funcs//初始化drm_fb_helper对象drm_fb_helper_init(dev, helper)/*创建drm_client_dev对象fb_helper->client,这个对象很重要，*其成员modesets(drm_mode_set类型)*关联fb/crtc/connector/displaymode等*/drm_client_init（dev, &fb_helper->client）drm_client_modeset_create(client)drm_for_each_crtc(crtc, dev)根据drm设备的crtc数目创建modesetclient->modesets[i++].crtc = crtc

（3）fb_helper->fbdev的注册

注册逻辑如下：

rockchip_drm_fbdev_initdrm_fb_helper_preparedrm_fb_helper_initdrm_fb_helper_initial_config(helper, ...)__drm_fb_helper_initial_config_and_unlock(fb_helper, bpp_sel)//drm_client_modeset_probe(&fb_helper->client, width, height)//根据connector count申请crtcs/modes/offsets/enabled等变量//....//找到status为connected的connectors以及其连接的crtc和显示模式modedrm_client_firmware_config(...,connectors, crtcs, modes, offsets, enabled)//初始化client->modesets[]for(i = 0; i < connector_count;i++){/*从client->modesets[]中找到匹配的crtc* client->modesets[].crtc在drm_client_modeset_create中初始化* 其位于drm_fb_helper_init接口中调用*/drm_mode_set* modeset = drm_client_find_modeset(client, crtc)//赋值modesets的mode/connectors/x/y等成员      modeset->mode = drm_mode_dupicate(dev, mode)modeset->connectors[modeset->num_connectors++] = connector;modeset->x = offset->xmodeset->y = offset->y}//创建单显存fb，所以这里应该不支持双缓冲ret = drm_fb_helper_single_fb_probe(fb_helper, bpp_sel);/*根据connector/plane调整size，最后调用*fb_probe（即 rockchip_drm_fbdev_create）创建显存*/ret = (*fb_helper->funcs->fb_probe)(fb_helper, &sizes);//设置显存fb到modeset->fbdrm_setup_crtcs_fb(fb_helper)drm_client_for_each_modeset(modeset, client) {//这里将创建的显存fb赋给所有的modesetmodeset->fb = fb_helper->fb;}注册fb_infoinfo = fb_helper->fbdev;ret = register_framebuffer(info);

从上边的逻辑drm_fb_helper_initial_config触发所有的操作:

drm_client_modeset_probe探测所有的modesets

drm_fb_helper_single_fb_probe从所有的modesets中确定合适的fb大小并调用fb_probe创建显存fb

drm_setup_crtcs_fb将创建的显存fb保存到client所有的modeset->fb中

最后调用register_framebuffer注册fbdev

（4）显存fb绑定到crtc(plane)

我们注意到drm_setup_crtcs_fb接口只是将显存fb保存的client下所有的modeset->fb中。那么显存fb是如何绑定到crtc中的呢？

其实显存fb(modeset->fb)通过drm_client_modeset_commit_atomic接口绑定到crtc上的，如下：

总结：显存fb通过atomic modeset绑定到所有crtc的primary plane（这里是否是所有crtc？存疑）

drm_client_modeset_commit_atomic//遍历client的所有modesetdrm_client_for_each_modeset(mode_set, client){   //找到mode_set绑定的crtc的primary planestruct drm_plane* primary = mode_set->crtc->primaryret = derm_atomic_helper_set_config(mode_set, state)//将显存mode_set->fb赋值给plane_statedrm_atomic_set_fb_for_plane(primary_state, set->fb)}//通过atomic modeset 提交显存fb到crtc的primary planeret = drm_atomic_commit(state)

drm_client_modeset_commit_atomic是何时被调用的呢？梳理代码发现，在fbdev进行FBIOGET_VSCREENINFO时被调用，逻辑如下：

//用户态
fd_fb = open("/dev/fb0")
ioctl(fd_fb, FBIOPUT_VSCREENINFO, &var)
//内核态
do_fb_ioctlcase FBIOPUT_VSCREENINFO:ret = fb_set_var(info, &var)ret = info->fbos->fb_set_par(info)//drm_fb_helper_set_par__drm_fb_helper_restore_fbdev_mode_unlocked(fb_helper, force)//调用drm_client_modeset_commitret = drm_client_modeset_commit(&fb_helper->client)drm_client_modeset_commit_locked//调用drm_client_modeset_commit_atomicdrm_client_modeset_commit_atomic(client, ...)

需要注意的是，drm_client_modeset_commit_atomic接口其他地方也会调用，待梳理。

三 fbdev操作

fbdev操作按顺序来主要有FBIOGET_FSCREENINFO/FBIOGET_VSCREENINFO/mmap/FBIO_PUT_VSCREENINFO/

FBIOPAN_DISPLAY/FBIO_WAITFORVSYNC，对应的操作函数如下：

#define DRM_FB_HELPER_DEFAULT_OPS \.fb_check_var = drm_fb_helper_check_var, \.fb_set_par    = drm_fb_helper_set_par, \.fb_setcmap  = drm_fb_helper_setcmap, \.fb_blank    = drm_fb_helper_blank, \.fb_pan_display    = drm_fb_helper_pan_display, \.fb_debug_enter = drm_fb_helper_debug_enter, \.fb_debug_leave = drm_fb_helper_debug_leave, \.fb_ioctl  = drm_fb_helper_ioctlstatic const struct fb_ops rockchip_drm_fbdev_ops = {.owner      = THIS_MODULE,DRM_FB_HELPER_DEFAULT_OPS,.fb_mmap   = rockchip_fbdev_mmap,.fb_fillrect = drm_fb_helper_cfb_fillrect,.fb_copyarea  = drm_fb_helper_cfb_copyarea,.fb_imageblit = drm_fb_helper_cfb_imageblit,
};

（1） FBIOGET_FSCREENINFO

获取固定参数，比较简单不做介绍

（2）FBIOGET_VSCREENINFO

获取可变参数，比较简单不做介绍

（3）mmap

映射显存fb虚拟地址，供用户空间使用，内核态对应接口fb_mmap

static int rockchip_fbdev_mmap(struct fb_info *info,struct vm_area_struct *vma)
{//根据info->par找到helperstruct drm_fb_helper *helper = info->par;struct rockchip_drm_private *private = to_drm_private(helper);//fbdev_bo就是显存fb对应的drm_gem_obj实例rockchip_gem_mmap_buf(private->fbdev_bo, vma);//映射操作drm_gem_mmap_obj
}

（4）FBIO_PUT_VSCREENINFO

设置可变参数，其用法之一就是设置多显存buffer, 用户态设置方法如下：

ioctl(fd, FBIOGET_FSCREENINFO, &fix);
ioctl(fd, FBIOGET_VSCREENINFO, &var);//获取当前显存buffer个数
curr_num = fix.smem_len/screen_size;//设置双缓冲
var.yres_virtual = 2 * var.yes
ioctl(fd, FBIOPUT_VSCREENINFO, &var);

内核态对应接口fb_set_par ，这个接口我们在前文已经分析过，他会触发drm_client_modeset_commit_atomic接口绑定fb到plane，当我们没有分析双缓冲的逻辑，这个后续补充

（5）FBIOPAN_DISPLAY

双缓冲时，切换缓冲使用，对用内核态接口fb_pan_display

//用户态
var.yoffset = buf_idx*var.yes
ioctl(fd, FBIOPAN_DISPLAY, &var)
//内核态
drm_fb_helper_pan_display(fb_var_screeninfo*var, fb_info* info)pan_display_atomic(var, info)//设置modeset->x/ypan_set(fb_helper, var->xoffset, var->yoffset);modeset->x = x; //var->xoffsetmodeset->y = y; // var->yoffset//更新plane的参数x,ydrm_client_modeset_commit_locked(&fb_helper->client);

从（4）（5）可以看出，所谓的双缓冲，在内核态实际上申请一块大的显存（2倍的单缓冲大小），然后通过var.yoffset（内核态对应modeset->y）来切换plan的显示区域

（6）FBIO_WAITFORVSYNC

等待vsync，内核态对应接口drm_fb_helper_ioctl：

drm_fb_helper_ioctlswitch(cmd){case FBIO_WAITFORVSYNC://仅支持第一个crtc的vsync等待crtc = fb_helper->client.modeset[0].crtc;drm_crtc_wait_one_vblank(crtc)}

（7）测试程序

static int fd_fb;
static struct fb_fix_screeninfo fix;    /* Current fix */
static struct fb_var_screeninfo var;    /* Current var */
static int screen_size;
static unsigned char *fb_base;
static unsigned int line_width;
static unsigned int pixel_width;int main(int argc, char **argv)
{int i;int ret;int buffer_num;int buf_idx = 1;char *buf_next;unsigned int colors[] = {0x00FF0000, 0x0000FF00, 0x000000FF, 0, 0x00FFFFFF};  /* 0x00RRGGBB */struct timespec time;...fd_fb = open("/dev/fb0", O_RDWR);ioctl(fd_fb, FBIOGET_FSCREENINFO, &fix);ioctl(fd_fb, FBIOGET_VSCREENINFO, &var);line_width  = var.xres * var.bits_per_pixel / 8;pixel_width = var.bits_per_pixel / 8;screen_size = var.xres * var.yres * var.bits_per_pixel / 8;// 1. 获得 buffer 个数buffer_num = fix.smem_len / screen_size;printf("buffer_num = %d\n", buffer_num);fb_base = (unsigned char *)mmap(NULL , fix.smem_len, PROT_READ | PROT_WRITE, MAP_SHARED, fd_fb, 0);if (fb_base == (unsigned char *)-1) {printf("can't mmap\n");return -1;}if ((argv[1][0] == 's') || (buffer_num == 1)) {printf("single buffer:\n");while (1) {for (i = 0; i < sizeof(colors)/sizeof(colors[0]); i++) {lcd_draw_screen(fb_base, colors[i]);nanosleep(&time, NULL);}}} else {printf("double buffer:\n");// 2. 使能多 buffervar.yres_virtual = buffer_num * var.yres;ioctl(fd_fb, FBIOPUT_VSCREENINFO, &var);while (1) {for (i = 0; i < sizeof(colors)/sizeof(colors[0]); i++) {// 3. 更新 buffer 里的数据buf_next =  fb_base + buf_idx * screen_size;lcd_draw_screen(buf_next, colors[i]);// 4. 通知驱动切换 buffervar.yoffset = buf_idx * var.yres;ret = ioctl(fd_fb, FBIOPAN_DISPLAY, &var);if (ret < 0) {perror("ioctl() / FBIOPAN_DISPLAY");}// 5. 等待帧同步完成ret = 0;ioctl(fd_fb, FBIO_WAITFORVSYNC, &ret);if (ret < 0) {perror("ioctl() / FBIO_WAITFORVSYNC");}buf_idx = !buf_idx;nanosleep(&time, NULL);}}}munmap(fb_base , screen_size);close(fd_fb);return 0;
}

关于fbdev双缓冲可以参考：

Linux 驱动开发 / fbdev 双缓存 / 快速入门_嵌入式Hacker-CSDN博客