嵌入式Linux驱动开发（LCD屏幕专题）（四）-编程知识

嵌入式Linux驱动开发（LCD屏幕专题）（四）

单Buffer的缺点与改进方法

1. 单Buffer的缺点

如果APP速度很慢，可以看到它在LCD上缓慢绘制图案
即使APP速度很高，LCD控制器不断从Framebuffer中读取数据来显示，而APP不断把数据写入Framebuffer
- 假设APP想把LCD显示为整屏幕的蓝色、红色
- 很大几率出现这种情况：
  - LCD控制器读取Framebuffer数据，读到一半时，在LCD上显示了半屏幕的蓝色
  - 这是APP非常高效地把整个Framebuffer的数据都改为了红色
  - LCD控制器继续读取数据，于是LCD上就会显示半屏幕蓝色、半屏幕红色
  - 人眼就会感觉到屏幕闪烁、撕裂

2. 使用多Buffer来改进

上述两个缺点的根源是一致的：Framebuffer中的数据还没准备好整帧数据，就被LCD控制器使用了。
使用双buffer甚至多buffer可以解决这个问题：

假设有2个Framebuffer：FB0、FB1
LCD控制器正在读取FB0
APP写FB1
写好FB1后，让LCD控制器切换到FB1
APP写FB0
写好FB0后，让LCD控制器切换到FB0

3. 内核驱动程序、APP互相配合使用多buffer

流程如下：

在这里插入图片描述

驱动：分配多个buffer

fb_info->fix.smem_len = SZ_32M;
fbi->screen_base = dma_alloc_writecombine(fbi->device,fbi->fix.smem_len,(dma_addr_t *)&fbi->fix.smem_start,GFP_DMA | GFP_KERNEL);

驱动：保存buffer信息

fb_info->fix.smem_len  // 含有总buffer大小 
fb_info->var           // 含有单个buffer信息

APP：读取buffer信息

ioctl(fd_fb, FBIOGET_FSCREENINFO, &fix);
ioctl(fd_fb, FBIOGET_VSCREENINFO, &var);// 计算是否支持多buffer，有多少个buffer
screen_size = var.xres * var.yres * var.bits_per_pixel / 8;
nBuffers = fix.smem_len / screen_size;

APP：使能多buffer

var.yres_virtual = nBuffers * var.yres;
ioctl(fd_fb, FBIOPUT_VSCREENINFO, &var);

APP：写buffer

fb_base = (unsigned char *)mmap(NULL , fix.smem_len, PROT_READ | PROT_WRITE, MAP_SHARED, fd_fb, 0);/* get buffer */
pNextBuffer =  fb_base + nNextBuffer * screen_size;/* set buffer */
lcd_draw_screen(pNextBuffer, colors[i]);

APP：开始切换buffer

/* switch buffer */
var.yoffset = nNextBuffer * var.yres;
ioctl(fd_fb, FBIOPAN_DISPLAY, &var);

驱动：切换buffer

// fbmem.c
fb_ioctldo_fb_ioctlfb_pan_display(info, &var);err = info->fbops->fb_pan_display(var, info) // 调用硬件相关的函数

示例：
在这里插入图片描述

APP：等待切换完成(在驱动程序中已经等待切换完成了，所以这个调用并无必要)
```
ret = 0;
ioctl(fd_fb, FBIO_WAITFORVSYNC, &ret);
```

多buff代码

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <linux/fb.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <time.h>static int fd_fb;
static struct fb_fix_screeninfo fix;	/* Current fix */
static struct fb_var_screeninfo var;	/* Current var */
static int screen_size;
static unsigned char *fb_base;
static unsigned int line_width;
static unsigned int pixel_width;void lcd_put_pixel(void *fb_base, int x, int y, unsigned int color)
{unsigned char *pen_8 = fb_base+y*line_width+x*pixel_width;unsigned short *pen_16;	unsigned int *pen_32;	unsigned int red, green, blue;	pen_16 = (unsigned short *)pen_8;pen_32 = (unsigned int *)pen_8;switch (var.bits_per_pixel){case 8:{*pen_8 = color;break;}case 16:{/* 565 */red   = (color >> 16) & 0xff;green = (color >> 8) & 0xff;blue  = (color >> 0) & 0xff;color = ((red >> 3) << 11) | ((green >> 2) << 5) | (blue >> 3);*pen_16 = color;break;}case 32:{*pen_32 = color;break;}default:{printf("can't surport %dbpp\n", var.bits_per_pixel);break;}}
}void lcd_draw_screen(void *fb_base, unsigned int color)
{int x, y;for (x = 0; x < var.xres; x++)for (y = 0; y < var.yres; y++)lcd_put_pixel(fb_base, x, y, color);
}/* ./multi_framebuffer_test single* ./multi_framebuffer_test double*/
int main(int argc, char **argv)
{int i;int ret;int nBuffers;int nNextBuffer = 1;char *pNextBuffer;unsigned int colors[] = {0x00FF0000, 0x0000FF00, 0x000000FF, 0, 0x00FFFFFF};  /* 0x00RRGGBB */struct timespec time;time.tv_sec  = 0;time.tv_nsec = 100000000;if (argc != 2){printf("Usage : %s <single|double>\n", argv[0]);return -1;}fd_fb = open("/dev/fb0", O_RDWR);if (fd_fb < 0){printf("can't open /dev/fb0\n");return -1;}if (ioctl(fd_fb, FBIOGET_FSCREENINFO, &fix)){printf("can't get fix\n");return -1;}if (ioctl(fd_fb, FBIOGET_VSCREENINFO, &var)){printf("can't get var\n");return -1;}line_width  = var.xres * var.bits_per_pixel / 8;pixel_width = var.bits_per_pixel / 8;screen_size = var.xres * var.yres * var.bits_per_pixel / 8;nBuffers = fix.smem_len / screen_size;printf("nBuffers = %d\n", nBuffers);fb_base = (unsigned char *)mmap(NULL , fix.smem_len, PROT_READ | PROT_WRITE, MAP_SHARED, fd_fb, 0);if (fb_base == (unsigned char *)-1){printf("can't mmap\n");return -1;}if ((argv[1][0] == 's') || (nBuffers == 1)){while (1){/* use single buffer */for (i = 0; i < sizeof(colors)/sizeof(colors[0]); i++){lcd_draw_screen(fb_base, colors[i]);nanosleep(&time, NULL);}}}else{/* use double buffer *//* a. enable use multi buffers */var.yres_virtual = nBuffers * var.yres;ioctl(fd_fb, FBIOPUT_VSCREENINFO, &var);while (1){for (i = 0; i < sizeof(colors)/sizeof(colors[0]); i++){/* get buffer */pNextBuffer =  fb_base + nNextBuffer * screen_size;/* set buffer */lcd_draw_screen(pNextBuffer, colors[i]);/* switch buffer */var.yoffset = nNextBuffer * var.yres;ioctl(fd_fb, FBIOPAN_DISPLAY, &var);ret = 0;ioctl(fd_fb, FBIO_WAITFORVSYNC, &ret);nNextBuffer = !nNextBuffer;nanosleep(&time, NULL);}}}munmap(fb_base , screen_size);close(fd_fb);return 0;	
}