- 了解Linux Framebuffer的驱动框架
1.Framebuffer 驱动位置
对于驱动开发人员来说,其实只需要针对具体的硬件平台SOC和具体的LCD(焊接连接到该SOC的引脚上)来进行第一部分的寄存器编程(红色部分)。而第二、三、四部分内容(绿色部分)已经被抽象并实现在Linux driver发布源码中了,LCD驱动开发人员只需要理解framebuffer内部的框架和接口使用即可。
2.Framebuffer的驱动框架
帧缓冲设备为标准的字符型设备,在Linux中主设备号29,定义在/include/linux/major.h中的FB_MAJOR,次设备号定义帧缓冲的个数,最大允许有32个FrameBuffer,定义在/include/linux/fb.h中的FB_MAX,对应于文件系统下/dev/fb%d设备文件。
在嵌入式系统中一般没有专门的显存,而仅仅是从RAM(SDRAM)空间中分配一段显示缓冲区,典型结构如下图所示:
2.Framebuffer 相关的重要数据结构
从Framebuffer设备驱动程序结构看,该驱动主要跟fb_info结构体有关,该结构体记录了Framebuffer设备的全部信息,包括设备的设置参数、状态以及对底层硬件操作的函数指针。在Linux中,每一个Framebuffer设备都必须对应一个fb_info,fb_info在/linux/fb.h中的定义如下:
2.1.struct fb_info
1 struct fb_info {
2 atomic_t count;
3 int node; ///次设备号
4 int flags;
5 struct mutex lock; //用于open、release、ioctrl功能的锁
6 struct mutex mm_lock; /* Lock for fb_mmap and smem_* fields */
7 struct fb_var_screeninfo var; //LCD 可变参数
8 struct fb_fix_screeninfo fix; //LCD 固定参数
9 struct fb_monspecs monspecs; /* Current Monitor specs *///LCD 显示器标准
10 struct work_struct queue; //帧缓冲事件队列
11 struct fb_pixmap pixmap; ///图像硬件 mapper
12 struct fb_pixmap sprite; //光标硬件 mapper
13 struct fb_cmap cmap; //当前颜色表
14 struct list_head modelist; //模式列表
15 struct fb_videomode *mode; //当前的显示模式
16
17 #ifdef CONFIG_FB_BACKLIGHT
18 /* assigned backlight device */
19 /* set before framebuffer registration,
20 remove after unregister */
21 struct backlight_device *bl_dev;///对应的背光设备
22
23 /* Backlight level curve */
24 struct mutex bl_curve_mutex;
25 u8 bl_curve[FB_BACKLIGHT_LEVELS];///背光调整
26 #endif
27 #ifdef CONFIG_FB_DEFERRED_IO
28 struct delayed_work deferred_work;
29 struct fb_deferred_io *fbdefio;
30 #endif
31
32 struct fb_ops *fbops;///---->对底层硬件设备操作的函数指针
33 struct device *device; /* This is the parent *////父设备节点
34 struct device *dev; /* This is this fb device *////当前的帧缓冲设备
35 int class_flag; /* private sysfs flags */
36 #ifdef CONFIG_FB_TILEBLITTING
37 struct fb_tile_ops *tileops; /* Tile Blitting *///图块Blitting ?
38 #endif
39 char __iomem *screen_base; /* Virtual address *///虚拟地址
40 unsigned long screen_size; /* Amount of ioremapped VRAM or 0 */ ///LCD IO映射的虚拟内存大小
41 void *pseudo_palette; /* Fake palette of 16 colors *///伪16色 颜色表
42 #define FBINFO_STATE_RUNNING 0
43 #define FBINFO_STATE_SUSPENDED 1
44 u32 state; /* Hardware state i.e suspend *////LCD 挂起或复位的状态
45 void *fbcon_par; /* fbcon use-only private area */
46 /* From here on everything is device dependent */
47 void *par;
48 /* we need the PCI or similar aperture base/size not
49 smem_start/size as smem_start may just be an object
50 allocated inside the aperture so may not actually overlap */
51 struct apertures_struct {
52 unsigned int count;
53 struct aperture {
54 resource_size_t base;
55 resource_size_t size;
56 } ranges[0];
57 } *apertures;
58
59 bool skip_vt_switch; /* no VT switch on suspend/resume required */
60 };
- fb_var_screeninfo 代表可修改的LCD显示参数,如分辨率和像素比特数;
- fb_fix_screeninfo 代表不可修改的LCD属性参数,如显示内存的物理地址和长度等。
- fb_ops,其是LCD底层硬件操作接口集。
fb_ops硬件操作接口集包含很多接口,如设置可变参数fb_set_par、设置颜色寄存器fb_setcolreg、清屏接口fb_blank、画位图接口fb_imageblit、内存映射fb_mmap等等。
These structures are fb_var_screeninfo, fb_fix_screeninfo, fb_cmap, and fb_info. The first three are made available to and from user space code.struct fb_info structure should always be allocated dynamically, using framebuffer_alloc(),which is a kernel (framebuffer core) helper functions to allocate memory for instance of framebuffer devices, along with their private data memory:
struct fb_info *framebuffer_alloc(size_t size, struct device *dev);
void framebuffer_release(struct fb_info *info)
fb_info结构体在调用register_framebuffer之前完成初始化。一般来说,LCD设备属于平台设备,其初始化是在平台设备驱动的probe接口完成。而LCD设备所涉及的硬件初始化则在平台设备初始化中完成。
2.2.fb_fix_screeninfo
主要记录用户不可以修改的控制器的参数,该结构体的定义如下:
1 struct fb_fix_screeninfo {
2 char id[16]; //字符串形式的标识符
3 unsigned long smem_start; /fb 缓存的开始位置
4 /* (physical address) */
5 __u32 smem_len; /* Length of frame buffer mem *///fb 缓存的长度
6 __u32 type; /* see FB_TYPE_* *////看FB_TYPE_* -->
7 __u32 type_aux; /* Interleave for interleaved Planes *///分界
8 __u32 visual; ///看FB_VISUAL_* -->
9 __u16 xpanstep; //如果没有硬件panning就赋值为0
10 __u16 ypanstep; //如果没有硬件panning就赋值为0
11 __u16 ywrapstep; //如果没有硬件ywrap就赋值为0
12 __u32 line_length; //一行的字节数
13 unsigned long mmio_start; //内存映射 IO的开始位置
14 /* (physical address) */
15 __u32 mmio_len; //内存映射 IO的长度
16 __u32 accel; /* Indicate to driver which */
17 /* specific chip/card we have */
18 __u16 capabilities; /* see FB_CAP_* *///功能 ---FB_CAP_FOURCC--- Device supports FOURCC-based formats
19 __u16 reserved[2]; //为以后的兼容性保留
20 };
2.3.struct fb_var_screeninfo
主要记录用户可以修改的控制器的参数,比如屏幕的分辨率和每个像素的比特数等,该结构体定义如下:
1 struct fb_var_screeninfo { ///显示屏信息
2 __u32 xres; /* visible resolution*//可视区域,一行有多少个像素点
3 __u32 yres; ///可视区域,一列有多少个像素点
4 __u32 xres_virtual; /* virtual resolution*//虚拟区域,一行有多少个像素点,简单的意思就是内存中定义的区间是比较大的
5 __u32 yres_virtual;////虚拟区域,一列有多少个像素点
6 __u32 xoffset; //虚拟到可见屏幕之间的行偏移
7 __u32 yoffset; /* resolution *//虚拟到可见屏幕之间的列偏移
8
9 __u32 bits_per_pixel; /* guess what*/ 每个像素的 bit 数,这个参数不需要自己配置,而是通过上层在调用 checkvar 函数传递 bpp 的时候赋值的
10 __u32 grayscale; /* 0 = color, 1 = grayscale,*////等于零就成黑白 (灰度)
11 /* >1 = FOURCC */
12 // 通过 pixel per bpp 来设定 red green 和 blue 的位置; pixel per bpp 可以通过 ioctl 设定
13 struct fb_bitfield red; //fb缓存的R位域
14 struct fb_bitfield green; /* else only length is significant *//fb缓存的G位域
15 struct fb_bitfield blue; //fb缓存的B位域
16 struct fb_bitfield transp; /* transparency *//透明度
17
18 __u32 nonstd; /* != 0 Non standard pixel format *///如果nonstd 不等于0,非标准的像素格式
19
20 __u32 activate; /* see FB_ACTIVATE_* */
21
22 __u32 height; //内存中的图像高度
23 __u32 width; //内存中的图像宽度
24
25 __u32 accel_flags; /* (OBSOLETE) see fb_info.flags *////加速标志
26
27 /* Timing: All values in pixclocks, except pixclock (of course) */
28 ///时序,这些部分就是显示器的显示方法了,和具体的液晶显示屏有关,在驱动中一般放在 具体液晶屏的配置文件
29 __u32 pixclock; /* pixel clock in ps (pico seconds) *///像素时钟
30 __u32 left_margin; /* time from sync to picture *////行切换,从同步到绘图之间的延迟
31 __u32 right_margin; /* time from picture to sync *///行切换,从绘图到同步之间的延迟
32 __u32 upper_margin; /* time from sync to picture *///帧切换,从同步到绘图之间的延迟
33 __u32 lower_margin; ////帧切换,从绘图到同步之间的延迟
34 __u32 hsync_len; /* length of horizontal sync */ //水平同步的长度
35 __u32 vsync_len; /* length of vertical sync */ //垂直同步的长度
36 __u32 sync; /* see FB_SYNC_* *////---->看 FB_SYNC_*
37 __u32 vmode; /* see FB_VMODE_* *////---->看 FB_VMODE_*
38 __u32 rotate; /* angle we rotate counter clockwise */
39 __u32 colorspace; /* colorspace for FOURCC-based modes */
40 __u32 reserved[4]; /* Reserved for future compatibility */
41 };
通过下图可以对fb_var_screeninfo中涉及的时序理解更清楚一些:
2.4.fb_ops结构体
对底层硬件操作的函数指针,该结构体中定义了对硬件的操作有:
1 struct fb_ops {
2 /* open/release and usage marking */
3 struct module *owner;
4 int (*fb_open)(struct fb_info *info, int user);
5 int (*fb_release)(struct fb_info *info, int user);
/*对于非线性布局的/常规内存映射无法工作的帧缓冲设备需要*/
10 ssize_t (*fb_read)(struct fb_info *info, char __user *buf,
11 size_t count, loff_t *ppos);
12 ssize_t (*fb_write)(struct fb_info *info, const char __user *buf,
13 size_t count, loff_t *ppos);
15 /* checks var and eventually tweaks it to something supported,
16 * DO NOT MODIFY PAR */
17 int (*fb_check_var)(struct fb_var_screeninfo *var, struct fb_info *info);///检查可变参数并进行设置
19 /* set the video mode according to info->var */
20 int (*fb_set_par)(struct fb_info *info);///根据设置的值进行更新,使之有效
22 /* set color register */
23 int (*fb_setcolreg)(unsigned regno, unsigned red, unsigned green, ////设置颜色寄存器
24 unsigned blue, unsigned transp, struct fb_info *info);
26 /* set color registers in batch */
27 int (*fb_setcmap)(struct fb_cmap *cmap, struct fb_info *info);
29
30 int (*fb_blank)(int blank, struct fb_info *info);///显示空白
32 /*pan显示*/
33 int (*fb_pan_display)(struct fb_var_screeninfo *var, struct fb_info *info);
36 void (*fb_fillrect) (struct fb_info *info, const struct fb_fillrect *rect);///矩形填充
38 void (*fb_copyarea) (struct fb_info *info, const struct fb_copyarea *region);///复制数据
40 void (*fb_imageblit) (struct fb_info *info, const struct fb_image *image);///图形填充
42 /* 绘制光标 */
43 int (*fb_cursor) (struct fb_info *info, struct fb_cursor *cursor);
45 /* 旋转显示 */
46 void (*fb_rotate)(struct fb_info *info, int angle);
48 /* 等待blit空闲 */
49 int (*fb_sync)(struct fb_info *info);
51 /* fb 特定的 ioctl */
52 int (*fb_ioctl)(struct fb_info *info, unsigned int cmd,
53 unsigned long arg);
55 /* 处理32bit compat ioctl (optional) */
56 int (*fb_compat_ioctl)(struct fb_info *info, unsigned cmd,
57 unsigned long arg);
59 /* fb 特定的 mmap */
60 int (*fb_mmap)(struct fb_info *info, struct vm_area_struct *vma);
62 /* get capability given var */
63 void (*fb_get_caps)(struct fb_info *info, struct fb_blit_caps *caps,
64 struct fb_var_screeninfo *var);
66 /* teardown any resources to do with this framebuffer */
67 void (*fb_destroy)(struct fb_info *info);
69 /* called at KDB enter and leave time to prepare the console */
70 int (*fb_debug_enter)(struct fb_info *info);
71 int (*fb_debug_leave)(struct fb_info *info);
72 };
fb_ops结构与file_operations 结构不同,fb_ops是底层操作的抽象,而file_operations是提供给上层系统调用的接口,可以直接调用。
2.5.fb_cmap
设备独立的 colormap 信息,可以通过 ioctl 的 FBIOGETCMAP 和 FBIOPUTCMAP 命令设置 colormap;
struct fb_cmap { //颜色映射表
__u32 start; /* First entry *////第一个入口
__u32 len; /* Number of entries *///入口的数字
__u16 *red; /* Red values *///红色
__u16 *green; ///绿色
__u16 *blue; ///蓝色
__u16 *transp; //透明度,允许为空
};
这些结构相互之间的关系如下所示:
3.Framebuffer模块初始化
3.1.初始化Framebuffer
FrameBuffer驱动是以模块的形式注册到系统中,在模块初始化时,创建FrameBuffer对应的设备文件及proc文件,并注册FrameBuffer设备操作接口函数fb_fops。
static int __init fbmem_init(void)
{
proc_create("fb", 0, NULL, &fb_proc_fops);///向 proc 文件系统报告驱动状态和参数
if (register_chrdev(FB_MAJOR,"fb",&fb_fops))///注册字符设备驱动,主设备号是29
printk("unable to get major %d for fb devs\n", FB_MAJOR);
fb_class = class_create(THIS_MODULE, "graphics");///创建 /sys/class/graphics 设备类,配合 mdev生成设备文件
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;
}
Framebuffer作为一个子系统,在fbmem_init中通过register_chrdev接口向系统注册一个主设备号位29的字符设备驱动。通过class_create创建graphics设备类,配合mdev机制生成供用户访问的设备文件(位于/dev目录)。
3.2.Framebuffer设备驱动的接口集fb_fops的定义为:
static const struct file_operations fb_fops = {
.owner = THIS_MODULE,
.read = fb_read,
.write = fb_write,//二次拷贝
.unlocked_ioctl = fb_ioctl,
#ifdef CONFIG_COMPAT
.compat_ioctl = fb_compat_ioctl,
#endif
.mmap = fb_mmap,///映射,一次拷贝
.open = fb_open,
.release = fb_release,
#ifdef HAVE_ARCH_FB_UNMAPPED_AREA
.get_unmapped_area = get_fb_unmapped_area,
#endif
#ifdef CONFIG_FB_DEFERRED_IO
.fsync = fb_deferred_io_fsync,
#endif
.llseek = default_llseek,
};
3.3. 注册Framebuffer
linux 提供了register_framebuffer()和unregister_framebuffer()函数分别作为注册和注销帧缓冲设备,这两个函数都接受fb_info指针为参数,原型为:
int register_framebuffer(struct fb_info *fb_info);
int Unregister_framebuffer(struct fb_info *fb_info);
对于register_framebuffer()而言,如果注册的帧缓冲设备超过了FB_MAX(目前定义为32),则返回为-ENXIO,注册成功则返回为0。