视频驱动程序概念整理

1. 分析驱动之前，先说明下LCD有关概念：
   1.1. LCD，即液晶显示器，有多种类型，比如STN，TFT，LTPS TFT，OLED等，各有优缺点。
   1.2. Cpu或显卡发出的数据都是TTL信号，通常是LCD控制器发出，LCD本身接收的也是TTL信号，不管采用何种的信号传输，本质都是TTL信号。
   1.3. LCD控制器输出的数据会将18 bit的数据分组，红绿蓝6 bit。
   1.4. 一幅图像称为一帧，每帧多行组成，每行由多个像素组成，每个像素由若干位来表示，比如由8 bit表示一个像素，称为8Bpp。
   1.5. 传输数据之前，需要设置一系列信号，比如VSYNC等，然后将帧缓存地址告诉LCD控制器，LCD控制器会自动发起DMA传输，并在上述设置好的信号下将数据传到VD[23:0]上。
2. Linux 视频子系统：
   2.1 帧缓冲（frame buffer）是Linux视频子系统的一个重要概念，帧缓冲进行了一般化的抽象并规定编程接口，帧缓冲允许App与图形硬件的变化无关。
   2.2 子系统一些参数
   为了方便理解，在ubuntu执行sudo fbset，会打印一下信息：

destop@ubuntu:~$ sudo fbsetmode "800x600"    geometry 800 600 2048 1920 32    timings 0 0 0 0 0 0 0    rgba 8/16,8/8,8/0,0/0endmode

geometry（几何形状）后面表示分辨率，一个像素使用32 bit，
timings 第一个参数是一个像素的绘制速率，在带有LCD设备上执行会有相应数字出现。
fbset可以选择帧缓冲设备的设备性能。
注： 打印信息有会D： xxMHz，D为dotclock，表示视频硬件绘制像素的速率。
3. 帧缓冲API
3.1 帧缓冲核心层，向App层生成设备节点，供App操作设备。
3.2 帧缓冲核心层的数据结构主要存在include/linux/fb.h中，用户层的数据存在usr/include/linux/fb.h。

   3.3 视频卡的基本属性，比如分辨率，每个像素占的bit等；数据结构如下：

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 */};

3.4 视频卡的一些固定信息保存在fb_fix_screeninfo中，用户无权更改。

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 */};

3.5 fb_cmap 规定了颜色映射，将用户定义的颜色分配信息传给底层硬件，可以用这个结构体定义RGB的配比来获得不同颜色的分配。

struct fb_cmap {    __u32 start;            /* First entry  */    __u32 len;          /* Number of entries */    __u16 *red;         /* Red values   */    __u16 *green;    __u16 *blue;    __u16 *transp;          /* transparency, can be NULL */};

3.6 fb_info结构体

struct fb_info {    int node;    int flags;    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;  };

fb_info是帧缓冲设备的核心数据结构，成员包括上文所述的数据结构，通过framebuffer_alloc()分配。
3.6 fb_ops结构体
fb_ops 包括了底层帧缓冲驱动程序提供的所有函数指针.

/* * Frame buffer operations * * LOCKING NOTE: those functions must _ALL_ be called with the console * semaphore held, this is the only suitable locking mechanism we have * in 2.6. Some may be called at interrupt time at this point though. */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);    /* save current hardware state */    void (*fb_save_state)(struct fb_info *info);    /* restore saved state */    void (*fb_restore_state)(struct fb_info *info);    /* get capability given var */    void (*fb_get_caps)(struct fb_info *info, struct fb_blit_caps *caps,                struct fb_var_screeninfo *var);};

后续内容以后再补充。