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：

• 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);
  1
  2
  3
  4
  5

• 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
  1
  2

• 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;
  1
  2
  3
  4
  5
  6

• APP：使能多buffer

  var.yres_virtual = nBuffers * var.yres;
  ioctl(fd_fb, FBIOPUT_VSCREENINFO, &var);
  1
  2

• 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]);
  1
  2
  3
  4
  5
  6
  7

• APP：开始切换buffer

  /* switch buffer */
  var.yoffset = nNextBuffer * var.yres;
  ioctl(fd_fb, FBIOPAN_DISPLAY, &var);
  1
  2
  3

• 驱动：切换buffer

  // fbmem.c
  fb_ioctl
      do_fb_ioctl
      	fb_pan_display(info, &var);
  			err = info->fbops->fb_pan_display(var, info) // 调用硬件相关的函数            
  1
  2
  3
  4
  5

  示例：
  

• 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);
  1
  2

多buff代码

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

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 \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;	
}



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176