lcd驱动的fb_read函数分析

staticssize_t fb_read(structfile*file,char__user *buf,size_t count,loff_t *ppos)

{

    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;

}