Linux驱动：LCD驱动框架分析

一直想花时间来整理一下Linux内核LCD驱动，却一直都忙着做其他事情去了，这些天特意抽出时间来整理之前落下的笔记，故事就这样开始了。LCD驱动也是字符设备驱动的一种，框架上相对于字符设备驱动稍微复杂一点点，真的就是一点点，难点在对LCD硬件的配置上。

开发平台：TQ210，S5PV210处理器

内核版本：linux-3.10.46

LCD型号：AT070TN92，7英寸，TFT屏，分辨率800x480x3(RGB)，24位真彩色

一、框架分析

上图说明：①内核装载LCD驱动模块：设置并注册fb_info结构，初始化LCD硬件。②APP打开LCD设备，获取设备文件，根据设备文件进行读写显存。③在内核中，根据主设备号和次设备号定位一个fb_info结构，如果应用层的系统调用是读操作则调用fb_ops中对应的操作函数，写操作也是一样。

主要数据结构分析：

 1 struct fb_info { 2     int node;                        //用作次设备号索引 3     int flags; 4     struct mutex lock;                //用于open/release/ioctl函数的锁 5     struct fb_var_screeninfo var;    //可变参数，重点 6     struct fb_fix_screeninfo fix;    //固定参数，重点 7     struct fb_monspecs monspecs;    //显示器标准 8     struct work_struct queue;        //帧缓冲区队列 9     struct fb_pixmap pixmap;        //图像硬件映射10     struct fb_pixmap sprite;        //光标硬件映射11     struct fb_cmap cmap;            //当前颜色表12     struct list_head modelist;      //模式链表13     struct fb_videomode *mode;        //当前video模式14 15     char __iomem *screen_base;        //显存基地址16     unsigned long screen_size;        //显存大小17     void *pseudo_palette;            //伪16色颜色表18 #define FBINFO_STATE_RUNNING    019 #define FBINFO_STATE_SUSPENDED    120     u32 state;                        //硬件状态，如挂起21     void *fbcon_par;                //用作私有数据区22     void *par;                        //info->par指向了额外多申请内存空间的首地址23 };

 1 struct fb_ops { 2     struct module *owner; 3     int (*fb_open)(struct fb_info *info, int user); 4     int (*fb_release)(struct fb_info *info, int user); 5     ssize_t (*fb_read)(struct fb_info *info, char __user *buf,size_t count, loff_t *ppos); 6     ssize_t (*fb_write)(struct fb_info *info, const char __user *buf,size_t count, loff_t *ppos); 7  8     /* 检测可变参数，并调整到支持的值 */ 9     int (*fb_check_var)(struct fb_var_screeninfo *var, struct fb_info *info);10 11     /* 根据 info->var 设置 video 模式 */12     int (*fb_set_par)(struct fb_info *info);13 14     /* set color register */15     int (*fb_setcolreg)(unsigned regno, unsigned red, unsigned green,unsigned blue, unsigned transp, struct fb_info *info);16     /* set color registers in batch */17     int (*fb_setcmap)(struct fb_cmap *cmap, struct fb_info *info);18     /* blank display */19     int (*fb_blank)(int blank, struct fb_info *info);20     /* pan display */21     int (*fb_pan_display)(struct fb_var_screeninfo *var, struct fb_info *info);22 23     /* Draws a rectangle */24     void (*fb_fillrect) (struct fb_info *info, const struct fb_fillrect *rect);25     /* Copy data from area to another */26     void (*fb_copyarea) (struct fb_info *info, const struct fb_copyarea *region);27     /* Draws a image to the display */28     void (*fb_imageblit) (struct fb_info *info, const struct fb_image *image);29 30     ......31 32     /* perform fb specific ioctl (optional) */33     int (*fb_ioctl)(struct fb_info *info, unsigned int cmd,34             unsigned long arg);35     /* perform fb specific mmap */36     int (*fb_mmap)(struct fb_info *info, struct vm_area_struct *vma);37 38     ......39 };

主要操作代码分析：

 1 fb_open 2 { 3     int fbidx = iminor(inode);         //获取次设备号 4     struct fb_info *info; 5     info = get_fb_info(fbidx); 6         struct fb_info *fb_info; 7         fb_info = registered_fb[fbidx];//根据次设备号从已注册的fb_info数组中获取响应的结构 8         return fb_info; 9     ......10     /* 11      * 从registered_fb[]数组项里找到fb_info结构体后，将其保存到 12      * struct file结构中的私有信息成员，难道这是为了以后在某些情况方便找到并调用？？先放着...13      * 回过来发现：这样做是为了验证在read、write、ioctl等系统调用中获得的fb_info结构和open获得的是否一样14      */  15     file->private_data = info;16     //info->fbops->fb_open无定义，这是值得思考的问题！17     if (info->fbops->fb_open) {18         res = info->fbops->fb_open(info,1);19         if (res)20             module_put(info->fbops->owner);21     }22     ......23 }

 1 fb_read(struct file *file, char __user *buf, size_t count, loff_t *ppos) 2 { 3     struct fb_info *info = file_fb_info(file); 4         struct inode *inode = file_inode(file); 5         int fbidx = iminor(inode); 6         //也是根据次设备号来获取fb_info结构 7         struct fb_info *info = registered_fb[fbidx]; 8      9         if (info != file->private_data)10             info = NULL;11         return info;12     //无定义13     if (info->fbops->fb_read)14         return info->fbops->fb_read(info, buf, count, ppos);15     //获得显存的大小16     total_size = info->screen_size;17     //如果应用层要读的数据count比实际最大的显存还要大，修改count值为最大显存值18     if (count >= total_size)19         count = total_size;20     //分配显存，最大只能是一页PAGE_SIZE=4KB21     buffer = kmalloc((count > PAGE_SIZE) ? PAGE_SIZE : count,GFP_KERNEL);22     //要读的源地址：显存虚拟基地址+偏移23     src = (u8 __iomem *) (info->screen_base + p);24     while (count) {25         c  = (count > PAGE_SIZE) ? PAGE_SIZE : count;26         //读的目的地址27         dst = buffer;28         //读操作：拷贝数据29         fb_memcpy_fromfb(dst, src, c);30         dst += c;31         src += c;32 33         if (copy_to_user(buf, buffer, c)) {34             err = -EFAULT;35             break;36         }37         *ppos += c;38         buf += c;39         cnt += c;40         count -= c;41     }42     kfree(buffer); //释放buffer，只起到临时中转站的作用43 }

 fb_write

 1 /*  2  * 函数功能：将内核空间分配的物理显存空间映射到用户空间中  3  * 用户空间就能访问这段内存空间了 4  */   5 static int fb_mmap(struct file *file, struct vm_area_struct * vma) 6 { 7     struct fb_info *info = file_fb_info(file); 8     struct fb_ops *fb; 9     unsigned long mmio_pgoff;10     unsigned long start;11     u32 len;12 13     if (!info)14         return -ENODEV;15     fb = info->fbops;16     if (!fb)17         return -ENODEV;18     mutex_lock(&info->mm_lock);19     //如果fb_info->fbops->fb_mmap存在就调用该函数，实际中没有！20     if (fb->fb_mmap) {21         int res;22         res = fb->fb_mmap(info, vma);23         mutex_unlock(&info->mm_lock);24         return res;25     }26     /*27     * fb缓冲内存的开始位置(物理地址)28     * info->fix.smem_start这个地址是在哪里被设置的？29     * 在驱动程序xxx_lcd_init()函数中：30     * clb_fbinfo->screen_base = dma_alloc_writecombine(NULL,clb_fbinfo->fix.smem_len, (u32*)&(clb_fbinfo->fix.smem_start), GFP_KERNEL);31     * dma_alloc_writecombine函数返回的是内核虚拟起始地址，同时第3个参数fix.smem_start会被设置成对应的物理起始地址。32     * 内核中操作这个分配的空间只能操作虚拟的地址空间！！！33     * dma_alloc_writecombine函数的调用只是把物理显存映射到内核空间，并没有映射到用户空间，因此用户在操作物理显存之前要先把34     * 物理显存空间映射到用户可见的用户空间中来，这就是该函数的意义所在。35     */36     start = info->fix.smem_start; 37     //帧缓冲长度38     len = info->fix.smem_len;39     mmio_pgoff = PAGE_ALIGN((start & ~PAGE_MASK) + len) >> PAGE_SHIFT;40     if (vma->vm_pgoff >= mmio_pgoff) {41         if (info->var.accel_flags) {42             mutex_unlock(&info->mm_lock);43             return -EINVAL;44         }45 46         vma->vm_pgoff -= mmio_pgoff;47         start = info->fix.mmio_start;48         len = info->fix.mmio_len;49     }50     mutex_unlock(&info->mm_lock);51 52     vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);53     fb_pgprotect(file, vma, start);54     //映射物理内存到用户空间虚拟地址55     return vm_iomap_memory(vma, start, len);56 }

问题思考：

问1.什么叫帧缓冲区，他有哪些特性指标？

答1.对于应用层来说，显示图像到LCD设备就相当于往“一块内存”中写入数据，获取LCD设备上的图像就相当于拷贝“这块内存”中的数据。因此，LCD就和“一块内存”一样，专业一点术语叫帧缓冲区，它和普通的内存不太一样，除了可以“读写”操作之外还可以进行其他操作和功能设置，特性指标就是LCD的特性指标。在内核中，一个LCD显示器就相当于一个帧缓冲设备，对应一个fb_info结构。

问2.为什么要通过 registered_fb[] 数组来找到对应的 fb_info 结构体？

答2.通过对上边这几个函数的剖析发现，不管是fb_read、fb_write、fb_ioctl、fb_mmap系统调用，都是通过次设备号在已注册的fb_info结构数组中找到匹配的那一个结构之后，判断其中的fbops结构中的操作函数是否有定义，有的话就优先调用该函数，没有就使用往下的方案策略。这样的好处就是多个相同的LCD设备可以使用同一套代码，减少代码的重复性，同时对于需要特殊定义的函数又可以方便实现重定义。

问3.这个数组在哪里被注册？

答3.在register_framebuffer()函数中被注册 register_framebuffer(struct fb_info *fb_info) ret = do_register_framebuffer(fb_info); ...... registered_fb[i] = fb_info; ......

问4.fb_mmap()函数在什么场合使用？

答4.在用户空间中通过mmap()函数来进行系统调用，该函数执行成功返回的是指向被映射的帧缓冲区的指针，这样用户直接可以通过该指针来读写缓冲区。

问5.在用户程序中调用write函数和直接使用mmap函数返回的fbp指针有什么不一样？

答5.用户空间使用fbp指针操作的地址是用户空间和物理显存空间直接映射的关系，而使用write是将用户中的数据拷贝到内核空间，然后再将这些数据写到内核中已映射的虚拟地址空间中；write是操作整个fb，而fbp只操作一个像素点。

二、驱动代码编写

  1 #include <linux/module.h>  2 #include <linux/kernel.h>  3 #include <linux/errno.h>  4 #include <linux/string.h>  5 #include <linux/mm.h>  6 #include <linux/slab.h>  7 #include <linux/delay.h>  8 #include <linux/fb.h>  9 #include <linux/init.h> 10 #include <linux/dma-mapping.h> 11 #include <linux/interrupt.h> 12 #include <linux/platform_device.h> 13 #include <linux/clk.h> 14 #include <linux/workqueue.h> 15  16 #include <asm/io.h> 17 #include <asm/div64.h> 18 #include <asm/uaccess.h> 19  20 #include <asm/mach/map.h> 21 #include <mach/regs-gpio.h> 22 #include <linux/fb.h> 23  24 #define VSPW        9   //4 25 #define VBPD        13  //17 26 #define LINEVAL     479   27 #define VFPD        21  //26 28  29 #define HSPW        19    //4 30 #define HBPD        25   //40 31 #define HOZVAL      799    32 #define HFPD        209   //214 33  34 #define LeftTopX    0 35 #define LeftTopY    0 36 #define RightBotX   799 37 #define RightBotY   479 38  39 static struct fb_info *clb_fbinfo; 40  41 /* LCD GPIO Pins */ 42 static long unsigned long *gpf0con; 43 static long unsigned long *gpf1con; 44 static long unsigned long *gpf2con; 45 static long unsigned long *gpf3con; 46 static long unsigned long *gpd0con; 47 static long unsigned long *gpd0dat; 48 static long unsigned long *display_control; 49  50 /* LCD Controler Pins */ 51 struct s5pv210_lcd_regs{ 52     volatile unsigned long vidcon0; 53     volatile unsigned long vidcon1; 54     volatile unsigned long vidcon2; 55     volatile unsigned long vidcon3; 56      57     volatile unsigned long vidtcon0; 58     volatile unsigned long vidtcon1; 59     volatile unsigned long vidtcon2; 60     volatile unsigned long vidtcon3; 61      62     volatile unsigned long wincon0; 63     volatile unsigned long wincon1; 64     volatile unsigned long wincon2; 65     volatile unsigned long wincon3; 66     volatile unsigned long wincon4; 67      68     volatile unsigned long shadowcon; 69     volatile unsigned long reserve1[2]; 70      71     volatile unsigned long vidosd0a; 72     volatile unsigned long vidosd0b; 73     volatile unsigned long vidosd0c; 74 }; 75  76 struct clk      *lcd_clk; 77 static struct s5pv210_lcd_regs *lcd_regs; 78  79 static long unsigned long *vidw00add0b0; 80 static long unsigned long *vidw00add1b0; 81  82 static u32  pseudo_palette[16]; 83  84 /* from pxafb.c */ 85 static  unsigned int chan_to_field(unsigned int chan, struct fb_bitfield *bf) 86 { 87     chan &= 0xffff; 88     chan >>= 16 - bf->length; 89     return chan << bf->offset; 90 } 91  92 static int  clb210_lcdfb_setcolreg(unsigned regno, 93                    unsigned red, unsigned green, unsigned blue, 94                    unsigned transp, struct fb_info *info) 95 { 96     unsigned int val; 97      98     if (regno > 16)  99         return 1;100     101     /* 用red,green,blue三原色构造出val */102     val  = chan_to_field(red,   &info->var.red);103     val |= chan_to_field(green, &info->var.green);104     val |= chan_to_field(blue,  &info->var.blue);105 106     pseudo_palette[regno] = val;107         108     return 0;    109 }110 111 //帧缓冲操作函数112 static struct fb_ops clb210_lcdfb_ops = 113 {114     .owner            = THIS_MODULE,115     .fb_setcolreg    = clb210_lcdfb_setcolreg, //设置color寄存器和调色板116     //下面这3个函数是通用的117     .fb_fillrect    = cfb_fillrect,  //画一个矩形118     .fb_copyarea    = cfb_copyarea,  //数据拷贝119     .fb_imageblit    = cfb_imageblit, //图像填充120 };121 122 static int  __init clb210_lcd_init(void)123 {124     /* 1.分配一个fb_info */125     clb_fbinfo = framebuffer_alloc(0 , NULL);126 127     /* 2. 设置 */128     /* 2.1 设置固定的参数 */129     strcpy(clb_fbinfo->fix.id, "clb210_lcd");130     clb_fbinfo->fix.smem_len = 800 * 480 * 32/8;131     clb_fbinfo->fix.type = FB_TYPE_PACKED_PIXELS;132     clb_fbinfo->fix.visual = FB_VISUAL_TRUECOLOR;133     clb_fbinfo->fix.line_length = 800 * 32/8;134 135     /* 2.2 设置可变的参数 */136     clb_fbinfo->var.xres = 800;137     clb_fbinfo->var.yres = 480;138     clb_fbinfo->var.xres_virtual   = 800;139     clb_fbinfo->var.yres_virtual   = 480;140     clb_fbinfo->var.bits_per_pixel = 32;141     142     /*RGB:888*/143     clb_fbinfo->var.red.offset = 16;144     clb_fbinfo->var.red.length = 8;145     146     clb_fbinfo->var.green.offset = 8;147     clb_fbinfo->var.green.length = 8;148     149     clb_fbinfo->var.blue.offset = 0;150     clb_fbinfo->var.blue.length = 8;151     152     clb_fbinfo->var.activate = FB_ACTIVATE_NOW    ;153 154     /* 2.3 设置操作函数 */155     clb_fbinfo->fbops = &clb210_lcdfb_ops;156     157     /* 2.4 其他的设置 */158     /* 2.4.1 设置显存的大小 */159     clb_fbinfo->screen_size =  800 * 480 * 32/8;    160 161     /* 2.4.2 设置调色板 */162     clb_fbinfo->pseudo_palette = pseudo_palette;163 164     /* 2.4.3 设置显存的虚拟起始地址 */165     clb_fbinfo->screen_base = dma_alloc_writecombine(NULL,166             clb_fbinfo->fix.smem_len, (u32*)&(clb_fbinfo->fix.smem_start), GFP_KERNEL);167     168 169     /* 3. 硬件相关的操作 */170     /* 3.1 获取lcd时钟，使能时钟 */171     lcd_clk = clk_get(NULL, "lcd");172     if (!lcd_clk || IS_ERR(lcd_clk)) {173         printk(KERN_INFO "failed to get lcd clock source\n");174     }175     clk_enable(lcd_clk);176 177     /* 3.2 配置GPIO用于LCD */178     gpf0con = ioremap(0xE0200120, 4);179     gpf1con = ioremap(0xE0200140, 4);180     gpf2con = ioremap(0xE0200160, 4);181     gpf3con = ioremap(0xE0200180, 4);182     gpd0con = ioremap(0xE02000A0, 4);183     gpd0dat = ioremap(0xE02000A4, 4);184     185     gpd0con      = ioremap(0xE02000A0, 4);  186     gpd0dat      = ioremap(0xE02000A4, 4);  187       188     vidcon0      = ioremap(0xF8000000, 4);  189     vidcon1      = ioremap(0xF8000004, 4);  190     vidtcon0     = ioremap(0xF8000010, 4);  191     vidtcon1     = ioremap(0xF8000014, 4);  192     vidtcon2     = ioremap(0xF8000018, 4);  193     wincon0      = ioremap(0xF8000020, 4);  194     vidosd0a     = ioremap(0xF8000040, 4);  195     vidosd0b     = ioremap(0xF8000044, 4);  196     vidosd0c     = ioremap(0xF8000048, 4);  197     vidw00add0b0 = ioremap(0xF80000A0, 4);  198     vidw00add1b0 = ioremap(0xF80000D0, 4);  199     shodowcon    = ioremap(0xF8000034, 4); 200     201     display_control = ioremap(0xe0107008, 4);202 203     /* 设置相关GPIO引脚用于LCD */204     *gpf0con = 0x22222222;205     *gpf1con = 0x22222222;206     *gpf2con = 0x22222222;207     *gpf3con = 0x22222222;208 209     /* 使能LCD本身 */210     *gpd0con |= 1<<4;211     *gpd0dat |= 1<<1;212 213     /* 显示路径的选择, 0b10: RGB=FIMD I80=FIMD ITU=FIMD */214     *display_control  = 2<<0;215 216     /* 3.3 映射LCD控制器对应寄存器 */    217     lcd_regs = ioremap(0xF8000000, sizeof(struct s5pv210_lcd_regs));    218     vidw00add0b0 = ioremap(0xF80000A0, 4);219     vidw00add1b0 = ioremap(0xF80000D0, 4);220     221     lcd_regs->vidcon0 &= ~((3<<26) | (1<<18) | (0xff<<6)  | (1<<2));   222     lcd_regs->vidcon0 |= ((5<<6) | (1<<4) );223 224     lcd_regs->vidcon1 &= ~(1<<7);            /* 在vclk的下降沿获取数据 */225     lcd_regs->vidcon1 |= ((1<<6) | (1<<5));  /* HSYNC极性反转, VSYNC极性反转 */226 227     lcd_regs->vidtcon0 = (VBPD << 16) | (VFPD << 8) | (VSPW << 0);228     lcd_regs->vidtcon1 = (HBPD << 16) | (HFPD << 8) | (HSPW << 0);229     lcd_regs->vidtcon2 = (LINEVAL << 11) | (HOZVAL << 0);230     lcd_regs->wincon0 &= ~(0xf << 2);231     lcd_regs->wincon0 |= (0xB<<2)|(1<<15);232     lcd_regs->vidosd0a = (LeftTopX<<11) | (LeftTopY << 0);233     lcd_regs->vidosd0b = (RightBotX<<11) | (RightBotY << 0);234     lcd_regs->vidosd0c = (LINEVAL + 1) * (HOZVAL + 1);235 236     *vidw00add0b0 = clb_fbinfo->fix.smem_start;  237     *vidw00add1b0 = clb_fbinfo->fix.smem_start + clb_fbinfo->fix.smem_len;  238 239     lcd_regs->shadowcon = 0x1;  /* 使能通道0 */    240     lcd_regs->vidcon0  |= 0x3;  /* 开启总控制器 */    241     lcd_regs->wincon0 |= 1;     /* 开启窗口0 */242 243     244     /*4.注册*/245     register_framebuffer(clb_fbinfo);246 247     return 0;248 }249 static void __exit clb210_lcd_exit(void)250 {251     unregister_framebuffer(clb_fbinfo);252     dma_free_writecombine(NULL,  clb_fbinfo->fix.smem_len, clb_fbinfo->screen_base, clb_fbinfo->fix.smem_start);253     iounmap(gpf0con);254     iounmap(gpf1con);255     iounmap(gpf2con);256     iounmap(gpf3con);257     iounmap(gpd0con);258     iounmap(gpd0dat);259     iounmap(display_control);260     iounmap(lcd_regs);261     iounmap(vidw00add0b0);262     iounmap(vidw00add1b0);263     framebuffer_release(clb_fbinfo);264 }265 266 module_init(clb210_lcd_init);267 module_exit(clb210_lcd_exit);268 269 MODULE_LICENSE("GPL");270 MODULE_AUTHOR("clb");271 MODULE_DESCRIPTION("Lcd driver for clb210 board");

这份代码没有基于platform设备驱动来编写，在内核源码中的demo就是基于platform驱动模型来搭建的，主要内容其实一样。