lcd驱动的fb_read函数分析

{

unsigned long p = *ppos;

struct inode *inode = file->f_dentry->d_inode;

int fbidx = iminor(inode);//取次设备号

struct fb_info *info = registered_fb[fbidx];

u32 *buffer, *dst;

u32 __iomem *src;

int c, i, cnt = 0, err = 0;

unsigned long total_size;

if (!info || ! info->screen_base)

return -ENODEV;

if (info->state != FBINFO_STATE_RUNNING)

return -EPERM;

if (info->fbops->fb_read)

return info->fbops->fb_read(file, buf, count, ppos);

total_size = info->screen_size;

if (total_size == 0)

total_size = info->fix.smem_len;

if (p >= total_size)

return 0;

if (count >= total_size)

count = total_size;

if (count + p > total_size)

count = total_size - p;

cnt = 0;

buffer = kmalloc((count > PAGE_SIZE) ? PAGE_SIZE : count,

GFP_KERNEL);

if (!buffer)

return -ENOMEM;

src = (u32 __iomem *) (info->screen_base + p);

if (info->fbops->fb_sync)

info->fbops->fb_sync(info);

while (count) {

c  = (count > PAGE_SIZE) ? PAGE_SIZE : count;//每次操作不能大于PAGE_SIZE

dst = buffer;

for (i = c >> 2; i--; )   //c>>2,表示除以4,也就是下面的数据,一次性读4个字节,这样总共读取c/4次

*dst++ = fb_readl(src++);   // 注意这里的src++实质:src是个u32*所以,每次++就会自增四个字节

if (c & 3) {  //如果读取字节数不是4的倍数，则最后3个字节另外处理

u8 *dst8 = (u8 *) dst;// u8* , 使其能够按照一个一个char读取

u8 __iomem *src8 = (u8 __iomem *) src;

for (i = c & 3; i--;) //i取最后2位,即最后未处理的数据

*dst8++ = fb_readb(src8++);

src = (u32 __iomem *) src8;// 让src指向最后读取的地址,为下次while(count)做准备

}

if (copy_to_user(buf, buffer, c)) {

err = -EFAULT;

break;

}

*ppos += c;

buf += c;

cnt += c;

count -= c;

}

kfree(buffer);

return (err) ? err : cnt;

}