FrameBuffer驱动学习

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最新推荐文章于 2022-07-31 12:03:39 发布

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本文深入介绍了Linux系统的帧缓冲（Framebuffer）机制，详细解析了fb_info、fb_ops等关键数据结构的功能与作用，以及如何注册与注销帧缓冲设备。

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FrameBuffer驱动学习

Daniel Wood 20110301

转载时请注明出处和作者

文章出处：http://danielwood.cublog.cn

作者：Daniel Wood

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写在前面的话：

这篇文章是自己学习Framebuffer所用，其实里面的东西狠水狠基本的，很多都是从他人的文章中Ctrl+C过来的，如果你没有太多的时间，还是直接跳过了，免得耽误了你的青春，污了你的双眸。

Framebuffer翻译过来是帧缓冲，是Linux系统为显示设备提供的一个接口，它将显示缓冲区抽象，屏蔽图像硬件的底层差异，允许上层应用程序在图形模式下直接对显示缓冲区进行读写操作。用户不必关心物理显示缓冲区的具体位置及存放方式，这些都由帧缓冲设备驱动本身来完成。对于帧缓冲设备而言，只要在显示缓冲区中与显示点对应的区域写入颜色值，对应的颜色会自动在屏幕上显示。

帧缓冲设备是标准的字符设备，主设备号为29，对应于/dev/fb（/dev/fb1，/dev/fb2…）设备文件。

注：代码版本2.6.35.6

帧缓冲数据结构

fb_info结构体

fb.h [include\linux]

struct fb_info { int node; int flags; struct mutex lock; /* Lock for open/release/ioctl funcs */ struct mutex mm_lock; /* Lock for fb_mmap and smem_* fields */ struct fb_var_screeninfo var; /* Current var */ struct fb_fix_screeninfo fix; /* Current fix */ struct fb_monspecs monspecs; /* Current Monitor specs */ struct work_struct queue; /* Framebuffer event queue */ struct fb_pixmap pixmap; /* Image hardware mapper */ struct fb_pixmap sprite; /* Cursor hardware mapper */ struct fb_cmap cmap; /* Current cmap */ struct list_head modelist; /* mode list */ struct fb_videomode *mode; /* current mode */ #ifdef CONFIG_FB_BACKLIGHT /* assigned backlight device */ /* set before framebuffer registration, remove after unregister */ struct backlight_device *bl_dev; /* Backlight level curve */ struct mutex bl_curve_mutex; u8 bl_curve[FB_BACKLIGHT_LEVELS]; #endif #ifdef CONFIG_FB_DEFERRED_IO struct delayed_work deferred_work; struct fb_deferred_io *fbdefio; #endif struct fb_ops *fbops; struct device *device; /* This is the parent */ struct device *dev; /* This is this fb device */ int class_flag; /* private sysfs flags */ #ifdef CONFIG_FB_TILEBLITTING struct fb_tile_ops *tileops; /* Tile Blitting */ #endif char __iomem *screen_base; /* Virtual address */ unsigned long screen_size; /* Amount of ioremapped VRAM or 0 */ void *pseudo_palette; /* Fake palette of 16 colors */ #define FBINFO_STATE_RUNNING 0 #define FBINFO_STATE_SUSPENDED 1 u32 state; /* Hardware state i.e suspend */ void *fbcon_par; /* fbcon use-only private area */ /* From here on everything is device dependent */ void *par; /* we need the PCI or similiar aperture base/size not smem_start/size as smem_start may just be an object allocated inside the aperture so may not actually overlap */ struct apertures_struct { unsigned int count; struct aperture { resource_size_t base; resource_size_t size; } ranges[0]; } *apertures; };

fb_info是Linux为帧缓冲设备定义的驱动层接口。它不仅包含了设备的设置参数，状态以及操作函数。每个帧缓冲设备都与一个fb_info结构相对应。

其中node成员域标示了特定的FrameBuffer，实际上也就是一个FrameBuffer设备的次设备号。fb_var_screeninfo结构体成员记录用户可修改的显示控制器参数，包括屏幕分辨率和每个像素点的比特数。fb_fix_screeninfo中记录用户不能修改的显示控制器的参数，如屏幕缓冲区的物理地址，长度。

fb_ops结构体

fb.h [include\linux]

struct fb_ops { /* open/release and usage marking */ struct module *owner; int (*fb_open)(struct fb_info *info, int user); int (*fb_release)(struct fb_info *info, int user); /* For framebuffers with strange non linear layouts or that do not * work with normal memory mapped access */ ssize_t (*fb_read)(struct fb_info *info, char __user *buf, size_t count, loff_t *ppos); ssize_t (*fb_write)(struct fb_info *info, const char __user *buf, size_t count, loff_t *ppos); /* checks var and eventually tweaks it to something supported, * DO NOT MODIFY PAR */ int (*fb_check_var)(struct fb_var_screeninfo *var, struct fb_info *info); /* set the video mode according to info->var */ int (*fb_set_par)(struct fb_info *info); /* set color register */ int (*fb_setcolreg)(unsigned regno, unsigned red, unsigned green, unsigned blue, unsigned transp, struct fb_info *info); /* set color registers in batch */ int (*fb_setcmap)(struct fb_cmap *cmap, struct fb_info *info); /* blank display */ int (*fb_blank)(int blank, struct fb_info *info); /* pan display */ int (*fb_pan_display)(struct fb_var_screeninfo *var, struct fb_info*info); /* Draws a rectangle */ void (*fb_fillrect) (struct fb_info *info, const struct fb_fillrect*rect); /* Copy data from area to another */ void (*fb_copyarea) (struct fb_info *info, const struct fb_copyarea*region); /* Draws a image to the display */ void (*fb_imageblit) (struct fb_info *info, const struct fb_image *image); /* Draws cursor */ int (*fb_cursor) (struct fb_info *info, struct fb_cursor *cursor); /* Rotates the display */ void (*fb_rotate)(struct fb_info *info, int angle); /* wait for blit idle, optional */ int (*fb_sync)(struct fb_info *info); /* perform fb specific ioctl (optional) */ int (*fb_ioctl)(struct fb_info *info, unsigned int cmd, unsigned long arg); /* Handle 32bit compat ioctl (optional) */ int (*fb_compat_ioctl)(struct fb_info *info, unsigned cmd, unsigned long arg); /* perform fb specific mmap */ int (*fb_mmap)(struct fb_info *info, struct vm_area_struct *vma); /* get capability given var */ void (*fb_get_caps)(struct fb_info *info, struct fb_blit_caps *caps, struct fb_var_screeninfo *var); /* teardown any resources to do with this framebuffer */ void (*fb_destroy)(struct fb_info *info); };

它是提供给底层设备驱动的一个接口。通常我们编写字符驱动的时候，要填写一个file_operations结构体，并使用register_chrdev()注册之，以告诉Linux如何操控驱动。当我们编写一个FrameBuffer的时候，就要依照Linux FrameBuffer编程的套路，填写fb_ops结构体。这个fb_ops也就相当于通常的file_operations结构体。

fb_var_screeninfo结构体

struct fb_var_screeninfo { __u32 xres; /* visible resolution */ __u32 yres; __u32 xres_virtual; /* virtual resolution */ __u32 yres_virtual; __u32 xoffset; /* offset from virtual to visible */ __u32 yoffset; /* resolution */ __u32 bits_per_pixel; /* guess what */ __u32 grayscale; /* != 0 Graylevels instead of colors */ struct fb_bitfield red; /* bitfield in fb mem if true color, */ struct fb_bitfield green; /* else only length is significant */ struct fb_bitfield blue; struct fb_bitfield transp; /* transparency */ __u32 nonstd; /* != 0 Non standard pixel format */ __u32 activate; /* see FB_ACTIVATE_* */ __u32 height; /* height of picture in mm */ __u32 width; /* width of picture in mm */ __u32 accel_flags; /* (OBSOLETE) see fb_info.flags */ /* Timing: All values in pixclocks, except pixclock (of course) */ __u32 pixclock; /* pixel clock in ps (pico seconds) */ __u32 left_margin; /* time from sync to picture */ __u32 right_margin; /* time from picture to sync */ __u32 upper_margin; /* time from sync to picture */ __u32 lower_margin; __u32 hsync_len; /* length of horizontal sync */ __u32 vsync_len; /* length of vertical sync */ __u32 sync; /* see FB_SYNC_* */ __u32 vmode; /* see FB_VMODE_* */ __u32 rotate; /* angle we rotate counter clockwise */ __u32 reserved[5]; /* Reserved for future compatibility */ };

fb_var_screeninfo结构体成员记录用户可修改的显示控制器参数，包括屏幕分辨率和每个像素点的比特数。fb_var_screeninfo中的xres定义屏幕一行有多少个点, yres定义屏幕一列有多少个点, bits_per_pixel定义每个点用多少个字节表示。

fb_fix_screeninfo结构体

struct fb_fix_screeninfo { char id[16]; /* identification string eg "TT Builtin" */ unsigned long smem_start; /* Start of frame buffer mem */ /* (physical address) */ __u32 smem_len; /* Length of frame buffer mem */ __u32 type; /* see FB_TYPE_* */ __u32 type_aux; /* Interleave for interleaved Planes */ __u32 visual; /* see FB_VISUAL_* */ __u16 xpanstep; /* zero if no hardware panning */ __u16 ypanstep; /* zero if no hardware panning */ __u16 ywrapstep; /* zero if no hardware ywrap */ __u32 line_length; /* length of a line in bytes */ unsigned long mmio_start; /* Start of Memory Mapped I/O */ /* (physical address) */ __u32 mmio_len; /* Length of Memory Mapped I/O */ __u32 accel; /* Indicate to driver which */ /* specific chip/card we have */ __u16 reserved[3]; /* Reserved for future compatibility */ };