static ssize_t mousedev_read(struct file *file, char __user *buffer,
     size_t count, loff_t *ppos)
{
 struct mousedev_client *client = file->private_data;
 struct mousedev *mousedev = client->mousedev;
 // mousedev_client结构体里查找到ps2的大小是6个字节。
 signed char data[sizeof(client->ps2)];
 int retval = 0;

 spin_lock_irq(&client->packet_lock);

 if (!client->buffer && client->ready) {
  // 这里就是核心了，继续跟过去
  mousedev_packet(client, client->ps2);
  client->buffer = client->bufsiz;
 }
 ...

static void mousedev_packet(struct mousedev_client *client,
    signed char *ps2_data)
{
 struct mousedev_motion *p = &client->packets[client->tail];

 ps2_data[0] = 0x08 |
  ((p->dx < 0) << 4) | ((p->dy < 0) << 5) | (p->buttons & 0x07);
 ps2_data[1] = mousedev_limit_delta(p->dx, 127);
 ps2_data[2] = mousedev_limit_delta(p->dy, 127);
 p->dx -= ps2_data[1];
 p->dy -= ps2_data[2];
...

#include <stdio.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <linux/fb.h>
#include <stdlib.h>

// 正方形边长为100个像素点
#define LENGTH 100

// 显示器显存的抽象
unsigned int *mem = NULL;
// 保存上一次的屏幕
unsigned int *old_mem = NULL;
// 屏幕信息
static struct fb_var_screeninfo info;
int mouse_fd, fb_fd;

// 正方形涂成红色
int start = 0xffff0000;

int main(int argc, char **argv)
{
 signed char mouse_event[6];
 char rel_x, rel_y;
 int old_x = 0, old_y = 0;
 int abs_x = 0, abs_y = 0;

 mouse_fd = open("/dev/input/mouse0", O_RDONLY);
 fb_fd = open("/dev/fb0", O_RDWR);

 ioctl(fb_fd, FBIOGET_VSCREENINFO, &info);

 mem = (unsigned int *)mmap(NULL, info.xres*info.yres*info.bits_per_pixel/8, PROT_READ|PROT_WRITE, MAP_SHARED, fb_fd, 0);

 while(read(mouse_fd, &mouse_event[0], 6)) {
 int i, w, h;
 static int idx = 0;

 // 按照内核mousedev_packet的定义，解析出相对位移。
 rel_x = (char) mouse_event[1];
 rel_y = (char) mouse_event[2];
 // 计算绝对位移
 abs_x += rel_x;
 abs_y -= rel_y;
 if (abs_x <= 0 || abs_x >= info.xres - LENGTH || abs_y <= 0 || abs_y >= info.yres - LENGTH) {
 continue;
 }

 if (old_mem == NULL) {
 old_mem = (unsigned int *)mmap(NULL, info.xres*info.yres*info.bits_per_pixel/8, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
 if (old_mem == NULL) {
 exit(1);
 }
 } else {
 // 恢复上一次正方形区域里的像素
 for (w = old_x; w < old_x + LENGTH; w++) {
 for (h = old_y; h < old_y + LENGTH; h++) {
  idx = h*info.xres + w;
  mem[idx] = old_mem[idx];
 }
 }
 old_x = abs_x;
 old_y = abs_y;
 }

 // 保存当前的像素，以便下一次恢复
 for (w = abs_x; w < abs_x + LENGTH; w++) {
 for (h = abs_y; h < abs_y + LENGTH; h++) {
 idx = h*info.xres + w;
 old_mem[idx] = mem[idx];
 }
 }

 // 根据鼠标的位置涂抹红色矩形
 for (w = abs_x; w < abs_x + LENGTH; w++) {
 for (h = abs_y; h < abs_y + LENGTH; h++) {
 idx = h*info.xres + w;
 mem[idx] = start;
 }
 }
 }

 return 0;
}