Host:Ubuntu14.04(64bit)
Target:smdk2410
Kernel:linux-2.6.39.4
2 前言
我們在剛開始寫Linux設備驅動程序的時候, 很多時候都是利用mknod命令手動創建設備節點,實際上Linux內核爲我們提供了一組函數,可以用來在模塊加載的時候自動在/dev目錄下創建相應設備節點,並在卸載模塊時刪除該節點,當然前提條件是用戶空間移植了udev[2]。從linux 內核2.6的某個版本之後,devfs不復存在,udev成爲devfs的替代。與devfs不同的是,udev是用戶空間的[2]。
內核中定義struct class結構體,該結構體的一個變量對應一個類。內核同時提供了class_create(),可以用它來創建一個類,這個類存放於sysfs下面。一旦創建好了這個類,再調用device_create()在/dev目錄下創建相應的設備節點。這樣,加載模塊的時候,用戶空間中的udev會自動響應device_create()函數,去/sysfs下尋找對應的類從而創建設備節點[1]。
在驅動用加入對udev的支持主要做的就是:在驅動初始化的代碼裏調用class_create()爲該設備創建一個struct class,再爲每個設備調用device_create()創建對應的設備[1],大致用法如下[2]:
struct class *myclass = class_create(THIS_MODULE, “my_device_driver”);
device_create(myclass, NULL, MKDEV(major_num, 0), NULL, “my_device”);
這樣的module被加載時,udev daemon就會自動在/dev下創建my_device設備文件[2]。
注: 在2.6較早的內核中device_create()的曾用名爲class_device_create()[1]。
3 FrameBuffer設備節點
創建FrameBuffer設備節點的主要流程是,先定義一個struct class指針變量fb_class,然後在fbmem_init()中調用class_create()創建類並且賦值給fb_class,然後在do_register_framebuffer()中調用device_create()根據上述fb_class創建設備節點。
(1)fb_class
struct class *fb_class;
EXPORT_SYMBOL(fb_class);
/* 源文件:drivers/video/fbmem.c */
(2)class_create()
static int __init fbmem_init(void)
{
proc_create("fb", 0, NULL, &fb_proc_fops);
if (register_chrdev(FB_MAJOR,"fb",&fb_fops))
printk("unable to get major %d for fb devs\n", FB_MAJOR);
fb_class = class_create(THIS_MODULE, "graphics"); /* 創建一個graphics類! */
if (IS_ERR(fb_class)) {
printk(KERN_WARNING "Unable to create fb class; errno = %ld\n", PTR_ERR(fb_class));
fb_class = NULL;
}
return 0;
}
/* 源文件:drivers/video/fbmem.c */
(3)device_create()
在do_register_framebuffer()函數中,就調用device_create()創建fbn設備節點(n=0~31),如下面函數第25行所示:
static int do_register_framebuffer(struct fb_info *fb_info)
{
int i;
struct fb_event event;
struct fb_videomode mode;
if (fb_check_foreignness(fb_info))
return -ENOSYS;
do_remove_conflicting_framebuffers(fb_info->apertures, fb_info->fix.id,
fb_is_primary_device(fb_info));
if (num_registered_fb == FB_MAX)
return -ENXIO;
num_registered_fb++;
for (i = 0 ; i < FB_MAX; i++)
if (!registered_fb[i])
break;
fb_info->node = i;
atomic_set(&fb_info->count, 1);
mutex_init(&fb_info->lock);
mutex_init(&fb_info->mm_lock);
fb_info->dev = device_create(fb_class, fb_info->device, /* 創建設備節點! */
MKDEV(FB_MAJOR, i), NULL, "fb%d", i);
if (IS_ERR(fb_info->dev)) {
/* Not fatal */
printk(KERN_WARNING "Unable to create device for framebuffer %d; errno = %ld\n", i, PTR_ERR(fb_info->dev));
fb_info->dev = NULL;
} else
fb_init_device(fb_info);
if (fb_info->pixmap.addr == NULL) {
fb_info->pixmap.addr = kmalloc(FBPIXMAPSIZE, GFP_KERNEL);
if (fb_info->pixmap.addr) {
fb_info->pixmap.size = FBPIXMAPSIZE;
fb_info->pixmap.buf_align = 1;
fb_info->pixmap.scan_align = 1;
fb_info->pixmap.access_align = 32;
fb_info->pixmap.flags = FB_PIXMAP_DEFAULT;
}
}
fb_info->pixmap.offset = 0;
if (!fb_info->pixmap.blit_x)
fb_info->pixmap.blit_x = ~(u32)0;
if (!fb_info->pixmap.blit_y)
fb_info->pixmap.blit_y = ~(u32)0;
if (!fb_info->modelist.prev || !fb_info->modelist.next)
INIT_LIST_HEAD(&fb_info->modelist);
fb_var_to_videomode(&mode, &fb_info->var);
fb_add_videomode(&mode, &fb_info->modelist);
registered_fb[i] = fb_info;
event.info = fb_info;
if (!lock_fb_info(fb_info))
return -ENODEV;
fb_notifier_call_chain(FB_EVENT_FB_REGISTERED, &event);
unlock_fb_info(fb_info);
return 0;
}
/* 源文件:drivers/video/fbmem.c */
參考資料