一、框架分析
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。
二、帧缓冲子系统（Framebuffer）
Linux下可支持多个帧缓冲设备，最多可达32个（通过内核宏定义设置），分别为/dev/fb0~/dev/fb31。帧缓冲设备为标准字符设备，主设备号为29，次设备号为0~31。
三、帧缓冲子系统数据结构
fb_info结构
包括了对帧缓冲设备属性和方法的完整描述。
每一个帧缓冲设备对应一个fb_info，fb_info在/linux/fb.h中定义。
struct fb_info {
int node; //用作次设备号索引
int flags;
struct mutex lock; //用于open/release/ioctl函数的锁
struct fb_var_screeninfo var; //可变参数，重点
struct fb_fix_screeninfo fix; //固定参数，重点
struct fb_monspecs monspecs; //显示器标准
struct work_struct queue; //帧缓冲区队列
struct fb_pixmap pixmap; //图像硬件映射
struct fb_pixmap sprite; //光标硬件映射
struct fb_cmap cmap; //当前颜色表
struct list_head modelist; //模式链表
struct fb_videomode *mode; //当前video模式
char __iomem *screen_base; //显存基地址
unsigned long screen_size; //显存大小
void *pseudo_palette; //伪16色颜色表
#define FBINFO_STATE_RUNNING 0
#define FBINFO_STATE_SUSPENDED 1
u32 state; //硬件状态，如挂起
void *fbcon_par; //用作私有数据区
void *par; //info->par指向了额外多申请内存空间的首地址
};
fb_var_screeninfo结构
记录可修改的控制器参数，如分辨率、像素比特数等。
fb_fix_screeninfo结构
记录不可修改的控制器参数，如屏幕缓冲区的物理地址、长度等。
fb_ops结构
对底层硬件操作的函数指针，实现对硬件的操作。
struct fb_ops {
struct module *owner;
int (*fb_open)(struct fb_info *info, int user);
int (*fb_release)(struct fb_info *info, int user);
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);
/* 检测可变参数，并调整到支持的值 */
int (*fb_check_var)(struct fb_var_screeninfo *var, struct fb_info *info);
/* 根据 info->var 设置 video 模式 */
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);
......
/* perform fb specific ioctl (optional) */
int (*fb_ioctl)(struct fb_info *info, unsigned int cmd,
unsigned long arg);
/* perform fb specific mmap */
int (*fb_mmap)(struct fb_info *info, struct vm_area_struct *vma);
......
};
fb_open
{
int fbidx = iminor(inode); //获取次设备号
struct fb_info *info;
info = get_fb_info(fbidx);
struct fb_info *fb_info;
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
return fb_info;
......
/*
* 从registered_fb[]数组项里找到fb_info结构体后，将其保存到
* 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...
* 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
*/
file->private_data = info;
//info->fbops->fb_open无定义，这是值得思考的问题！
if (info->fbops->fb_open) {
res = info->fbops->fb_open(info,1);
if (res)
module_put(info->fbops->owner);
}
......
}
fb_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
struct fb_info *info = file_fb_info(file);
struct inode *inode = file_inode(file);
int fbidx = iminor(inode);
//也是根据次设备号来获取fb_info结构
struct fb_info *info = registered_fb[fbidx];
if (info != file->private_data)
info = NULL;
return info;
//无定义
if (info->fbops->fb_read)
return info->fbops->fb_read(info, buf, count, ppos);
//获得显存的大小
total_size = info->screen_size;
//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
if (count >= total_size)
count = total_size;
//分配显存，最大只能是一页PAGE_SIZE=4KB
buffer = kmalloc((count > PAGE_SIZE) ? PAGE_SIZE : count,GFP_KERNEL);
//要读的源地址：显存虚拟基地址+偏移
src = (u8 __iomem *) (info->screen_base + p);
while (count) {
c = (count > PAGE_SIZE) ? PAGE_SIZE : count;
//读的目的地址
dst = buffer;
//读操作：拷贝数据
fb_memcpy_fromfb(dst, src, c);
dst += c;
src += c;
if (copy_to_user(buf, buffer, c)) {
err = -EFAULT;
break;
}
*ppos += c;
buf += c;
cnt += c;
count -= c;
}
kfree(buffer); //释放buffer，只起到临时中转站的作用
}
static ssize_t fb_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
{
unsigned long p = *ppos;
struct fb_info *info = file_fb_info(file); //获取fb_info结构
/************************************************************
函数跟进分析：
static struct fb_info *file_fb_info(struct file *file)
{
struct inode *inode = file_inode(file);
int fbidx = iminor(inode); //获取次设备号
struct fb_info *info = registered_fb[fbidx]; //根据次设备号获取相应的fb_info结构
if (info != file->private_data)
info = NULL;
return info; //返回fb_info结构
}
************************************************************/
u8 *buffer, *src;
u8 __iomem *dst;
int c, cnt = 0, err = 0;
unsigned long total_size;
//获取fb_info失败或者fb_info结构中没有设置显存基址，返回
if (!info || !info->screen_base)
return -ENODEV;
if (info->state != FBINFO_STATE_RUNNING)
return -EPERM;
//如果帧缓冲操作函数结构中有重定义fb_write函数，优先使用！实际上没有。
if (info->fbops->fb_write)
return info->fbops->fb_write(info, buf, count, ppos);
//获取显存大小
total_size = info->screen_size;
if (total_size == 0)
total_size = info->fix.smem_len;
//如果写偏移位置p比整个显存还要大，出错返回。
if (p > total_size)
return -EFBIG;
if (count > total_size) {
err = -EFBIG;
count = total_size;
}
if (count + p > total_size) {
if (!err)
err = -ENOSPC;
count = total_size - p;
}
//内核空间分配临时帧缓冲区
buffer = kmalloc((count > PAGE_SIZE) ? PAGE_SIZE : count,GFP_KERNEL);
if (!buffer)
return -ENOMEM;
//计算写目的地址（虚拟地址：内核空间中能够操作的也就是虚拟地址）
dst = (u8 __iomem *) (info->screen_base + p);
if (info->fbops->fb_sync)
info->fbops->fb_sync(info);
while (count) {
c = (count > PAGE_SIZE) ? PAGE_SIZE : count;
//源地址
src = buffer;
if (copy_from_user(src, buf, c)) {
err = -EFAULT;
break;
}
// 从内存buffer拷贝数据到帧缓冲区
fb_memcpy_tofb(dst, src, c);
dst += c;
src += c;
*ppos += c;
buf += c;
cnt += c;
count -= c;
}
kfree(buffer);
return (cnt) ? cnt : err;
}
/*
* 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
* 用户空间就能访问这段内存空间了
*/
static int fb_mmap(struct file *file, struct vm_area_struct * vma)
{
struct fb_info *info = file_fb_info(file);
struct fb_ops *fb;
unsigned long mmio_pgoff;
unsigned long start;
u32 len;
if (!info)
return -ENODEV;
fb = info->fbops;
if (!fb)
return -ENODEV;
mutex_lock(&info->mm_lock);
//如果fb_info->fbops->fb_mmap存在就调用该函数，实际中没有！
if (fb->fb_mmap) {
int res;
res = fb->fb_mmap(info, vma);
mutex_unlock(&info->mm_lock);
return res;
}
/*
* fb缓冲内存的开始位置(物理地址)
* info->fix.smem_start这个地址是在哪里被设置的？
* 在驱动程序xxx_lcd_init()函数中：
* clb_fbinfo->screen_base = dma_alloc_writecombine(NULL,clb_fbinfo->fix.smem_len,
* (u32*)&(clb_fbinfo->fix.smem_start), GFP_KERNEL);
* 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。
* 内核中操作这个分配的空间只能操作虚拟的地址空间！！！
* 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
* 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。
*/
start = info->fix.smem_start;
//帧缓冲长度
len = info->fix.smem_len;
mmio_pgoff = PAGE_ALIGN((start & ~PAGE_MASK) + len) >> PAGE_SHIFT;
if (vma->vm_pgoff >= mmio_pgoff) {
if (info->var.accel_flags) {
mutex_unlock(&info->mm_lock);
return -EINVAL;
}
vma->vm_pgoff -= mmio_pgoff;
start = info->fix.mmio_start;
len = info->fix.mmio_len;
}
mutex_unlock(&info->mm_lock);
vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
fb_pgprotect(file, vma, start);
//映射物理内存到用户空间虚拟地址
return vm_iomap_memory(vma, start, len);
}
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。
四、驱动代码
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/delay.h>
#include <linux/fb.h>
#include <linux/init.h>
#include <linux/dma-mapping.h>
#include <linux/interrupt.h>
#include <linux/platform_device.h>
#include <linux/clk.h>
#include <linux/workqueue.h>
#include <asm/io.h>
#include <asm/div64.h>
#include <asm/uaccess.h>
#include <asm/mach/map.h>
#include <mach/regs-gpio.h>
#include <linux/fb.h>
#define VSPW 9 //4
#define VBPD 13 //17
#define LINEVAL 479
#define VFPD 21 //26
#define HSPW 19 //4
#define HBPD 25 //40
#define HOZVAL 799
#define HFPD 209 //214
#define LeftTopX 0
#define LeftTopY 0
#define RightBotX 799
#define RightBotY 479
static struct fb_info *clb_fbinfo;
/* LCD GPIO Pins */
static long unsigned long *gpf0con;
static long unsigned long *gpf1con;
static long unsigned long *gpf2con;
static long unsigned long *gpf3con;
static long unsigned long *gpd0con;
static long unsigned long *gpd0dat;
static long unsigned long *display_control;
/* LCD Controler Pins */
struct s5pv210_lcd_regs{
volatile unsigned long vidcon0;
volatile unsigned long vidcon1;
volatile unsigned long vidcon2;
volatile unsigned long vidcon3;
volatile unsigned long vidtcon0;
volatile unsigned long vidtcon1;
volatile unsigned long vidtcon2;
volatile unsigned long vidtcon3;
volatile unsigned long wincon0;
volatile unsigned long wincon1;
volatile unsigned long wincon2;
volatile unsigned long wincon3;
volatile unsigned long wincon4;
volatile unsigned long shadowcon;
volatile unsigned long reserve1[2];
volatile unsigned long vidosd0a;
volatile unsigned long vidosd0b;
volatile unsigned long vidosd0c;
};
struct clk *lcd_clk;
static struct s5pv210_lcd_regs *lcd_regs;
static long unsigned long *vidw00add0b0;
static long unsigned long *vidw00add1b0;
static u32 pseudo_palette[16];
/* from pxafb.c */
static unsigned int chan_to_field(unsigned int chan, struct fb_bitfield *bf)
{
chan &= 0xffff;
chan >>= 16 - bf->length;
return chan << bf->offset;
}
static int clb210_lcdfb_setcolreg(unsigned regno,
unsigned red, unsigned green, unsigned blue,
unsigned transp, struct fb_info *info)
{
unsigned int val;
if (regno > 16)
return 1;
/* 用red,green,blue三原色构造出val */
val = chan_to_field(red, &info->var.red);
val |= chan_to_field(green, &info->var.green);
val |= chan_to_field(blue, &info->var.blue);
pseudo_palette[regno] = val;
return 0;
}
//帧缓冲操作函数
static struct fb_ops clb210_lcdfb_ops =
{
.owner = THIS_MODULE,
.fb_setcolreg = clb210_lcdfb_setcolreg, //设置color寄存器和调色板
//下面这3个函数是通用的
.fb_fillrect = cfb_fillrect, //画一个矩形
.fb_copyarea = cfb_copyarea, //数据拷贝
.fb_imageblit = cfb_imageblit, //图像填充
};
static int __init clb210_lcd_init(void)
{
/* 1.分配一个fb_info */
clb_fbinfo = framebuffer_alloc(0 , NULL);
/* 2. 设置 */
/* 2.1 设置固定的参数 */
strcpy(clb_fbinfo->fix.id, "clb210_lcd");
clb_fbinfo->fix.smem_len = 800 * 480 * 32/8;
clb_fbinfo->fix.type = FB_TYPE_PACKED_PIXELS;
clb_fbinfo->fix.visual = FB_VISUAL_TRUECOLOR;
clb_fbinfo->fix.line_length = 800 * 32/8;
/* 2.2 设置可变的参数 */
clb_fbinfo->var.xres = 800;
clb_fbinfo->var.yres = 480;
clb_fbinfo->var.xres_virtual = 800;
clb_fbinfo->var.yres_virtual = 480;
clb_fbinfo->var.bits_per_pixel = 32;
/*RGB:888*/
clb_fbinfo->var.red.offset = 16;
clb_fbinfo->var.red.length = 8;
clb_fbinfo->var.green.offset = 8;
clb_fbinfo->var.green.length = 8;
clb_fbinfo->var.blue.offset = 0;
clb_fbinfo->var.blue.length = 8;
clb_fbinfo->var.activate = FB_ACTIVATE_NOW ;
/* 2.3 设置操作函数 */
clb_fbinfo->fbops = &clb210_lcdfb_ops;
/* 2.4 其他的设置 */
/* 2.4.1 设置显存的大小 */
clb_fbinfo->screen_size = 800 * 480 * 32/8;
/* 2.4.2 设置调色板 */
clb_fbinfo->pseudo_palette = pseudo_palette;
/* 2.4.3 设置显存的虚拟起始地址 */
clb_fbinfo->screen_base = dma_alloc_writecombine(NULL,
clb_fbinfo->fix.smem_len, (u32*)&(clb_fbinfo->fix.smem_start), GFP_KERNEL);
/* 3. 硬件相关的操作 */
/* 3.1 获取lcd时钟，使能时钟 */
lcd_clk = clk_get(NULL, "lcd");
if (!lcd_clk || IS_ERR(lcd_clk)) {
printk(KERN_INFO "failed to get lcd clock source\n");
}
clk_enable(lcd_clk);
/* 3.2 配置GPIO用于LCD */
gpf0con = ioremap(0xE0200120, 4);
gpf1con = ioremap(0xE0200140, 4);
gpf2con = ioremap(0xE0200160, 4);
gpf3con = ioremap(0xE0200180, 4);
gpd0con = ioremap(0xE02000A0, 4);
gpd0dat = ioremap(0xE02000A4, 4);
gpd0con = ioremap(0xE02000A0, 4);
gpd0dat = ioremap(0xE02000A4, 4);
vidcon0 = ioremap(0xF8000000, 4);
vidcon1 = ioremap(0xF8000004, 4);
vidtcon0 = ioremap(0xF8000010, 4);
vidtcon1 = ioremap(0xF8000014, 4);
vidtcon2 = ioremap(0xF8000018, 4);
wincon0 = ioremap(0xF8000020, 4);
vidosd0a = ioremap(0xF8000040, 4);
vidosd0b = ioremap(0xF8000044, 4);
vidosd0c = ioremap(0xF8000048, 4);
vidw00add0b0 = ioremap(0xF80000A0, 4);
vidw00add1b0 = ioremap(0xF80000D0, 4);
shodowcon = ioremap(0xF8000034, 4);
display_control = ioremap(0xe0107008, 4);
/* 设置相关GPIO引脚用于LCD */
*gpf0con = 0x22222222;
*gpf1con = 0x22222222;
*gpf2con = 0x22222222;
*gpf3con = 0x22222222;
/* 使能LCD本身 */
*gpd0con |= 1<<4;
*gpd0dat |= 1<<1;
/* 显示路径的选择, 0b10: RGB=FIMD I80=FIMD ITU=FIMD */
*display_control = 2<<0;
/* 3.3 映射LCD控制器对应寄存器 */
lcd_regs = ioremap(0xF8000000, sizeof(struct s5pv210_lcd_regs));
vidw00add0b0 = ioremap(0xF80000A0, 4);
vidw00add1b0 = ioremap(0xF80000D0, 4);
lcd_regs->vidcon0 &= ~((3<<26) | (1<<18) | (0xff<<6) | (1<<2));
lcd_regs->vidcon0 |= ((5<<6) | (1<<4) );
lcd_regs->vidcon1 &= ~(1<<7); /* 在vclk的下降沿获取数据 */
lcd_regs->vidcon1 |= ((1<<6) | (1<<5)); /* HSYNC极性反转, VSYNC极性反转 */
lcd_regs->vidtcon0 = (VBPD << 16) | (VFPD << 8) | (VSPW << 0);
lcd_regs->vidtcon1 = (HBPD << 16) | (HFPD << 8) | (HSPW << 0);
lcd_regs->vidtcon2 = (LINEVAL << 11) | (HOZVAL << 0);
lcd_regs->wincon0 &= ~(0xf << 2);
lcd_regs->wincon0 |= (0xB<<2)|(1<<15);
lcd_regs->vidosd0a = (LeftTopX<<11) | (LeftTopY << 0);
lcd_regs->vidosd0b = (RightBotX<<11) | (RightBotY << 0);
lcd_regs->vidosd0c = (LINEVAL + 1) * (HOZVAL + 1);
*vidw00add0b0 = clb_fbinfo->fix.smem_start;
*vidw00add1b0 = clb_fbinfo->fix.smem_start + clb_fbinfo->fix.smem_len;
lcd_regs->shadowcon = 0x1; /* 使能通道0 */
lcd_regs->vidcon0 |= 0x3; /* 开启总控制器 */
lcd_regs->wincon0 |= 1; /* 开启窗口0 */
/*4.注册*/
register_framebuffer(clb_fbinfo);
return 0;
}
static void __exit clb210_lcd_exit(void)
{
unregister_framebuffer(clb_fbinfo);
dma_free_writecombine(NULL, clb_fbinfo->fix.smem_len, clb_fbinfo->screen_base, clb_fbinfo->fix.smem_start);
iounmap(gpf0con);
iounmap(gpf1con);
iounmap(gpf2con);
iounmap(gpf3con);
iounmap(gpd0con);
iounmap(gpd0dat);
iounmap(display_control);
iounmap(lcd_regs);
iounmap(vidw00add0b0);
iounmap(vidw00add1b0);
framebuffer_release(clb_fbinfo);
}
module_init(clb210_lcd_init);
module_exit(clb210_lcd_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("clb");
MODULE_DESCRIPTION("Lcd driver for clb210 board");