在Linux的应用平台上，很多时候我们需要用到从SD卡读数据来使用，比如升级触摸屏固件，比如载入调试文本等，都需要用到Linux的文件系统来操作。通过举一个实例来说明VFS FILE操作细节，即在ADB中通过echo 到DEVICE_ATTR的store来升级TP固件，相关升级函数可以参见博文。

（1）store属性函数

static ssize_t melfas_fwupdate_store(struct device *dev,struct device_attribute *attr,const char *buf, size_t count){	int UpResult;	mt65xx_eint_mask(CUST_EINT_TOUCH_PANEL_NUM);printk("run here \r\n");	UpResult = ms6000_firmware_upgrade_from_TCard();	//T卡升级主函数	if(UpResult== MS6000_RET_SUCCESS){		printk("MFS6000 DOWNLOAD SUCCESS \r\n");		msleep(100);	}	else		mfs6000_print_fail_result(UpResult);	i2c_client->addr = MS6000_8BIT_I2CADDR;	mt_set_gpio_mode(GPIO_I2C0_SCA_PIN, GPIO_I2C0_SCA_PIN_M_SCL);	mt_set_gpio_mode(GPIO_I2C0_SDA_PIN, GPIO_I2C0_SDA_PIN_M_SDA);	mt65xx_eint_unmask(CUST_EINT_TOUCH_PANEL_NUM);	//恢复中断和I2C脚的使用	return count;}

（2）从T卡读文件并处理

static int ms6000_firmware_upgrade_from_TCard(){	unsigned char NewFwVersion,OldFwVersion;	UINT16 rdlen;			//实际读入内存的文件长度	loff_t file_size;			//文件大小	UINT8 *buf = NULL;			//分配的内存指针	struct file *fp = NULL;	struct inode *inode = NULL;	int ret = MS6000_RET_FILE_ACCESS_FAILED;	mm_segment_t old_fs;	const struct cred *old_cred;	struct cred *override_cred;	override_cred = prepare_creds();	if (!override_cred)		return -ENOMEM;	override_cred->fsuid = 0;	override_cred->fsgid = 0;	old_cred = (struct cred *)override_creds(override_cred);	//权限属性		old_fs = get_fs();	set_fs(KERNEL_DS);					//权限属性	if(mfs_i2c_read_single_reg(0x21,&OldFwVersion) == true){		NewFwVersion = ms6000_GetLibVer();	//从T卡中读文件，并取出存于特定字节的版本号值		printk("mfs6000 NewFwVersionis 0x%x,OldFwVersion is 0x%x \r\n",NewFwVersion,OldFwVersion);	}	i2c_client->addr = MS6000_8BIT_DOWNLOAD_ADDR;		if(1NewFwVersion != OldFwVersion){		fp = filp_open("/mnt/sdcard/MCH_U5(S)_R01_VF6.bin", O_RDONLY, 0);  //载入BIN格式的固件		if (IS_ERR(fp)) {			printk("ERROR opening file 	\r\n");			return MS6000_RET_OPENFILE_ERROR;		} else {					if (!(fp->f_op)) {				printk("File has no file operations registered!\r\n");							filp_close(fp, NULL);				return MS6000_RET_OPENFILE_ERROR;					}					if (!fp->f_op->read) {				printk(" File has no READ operations registered!\r\n");							filp_close(fp, NULL);				return MS6000_RET_OPENFILE_ERROR;					}		}		inode = fp->f_dentry->d_inode;		file_size = inode->i_size;		buf = (UINT8 *)kmalloc(file_size, GFP_KERNEL);		if(buf < 0)			return -ENOMEM;		rdlen = vfs_read(fp, buf, file_size, &(fp->f_pos));		printk("start download, file_size is %d,rdlen is %d\r\n",file_size,rdlen);		mtk_wdt_disable();		ret = ms6000_download(buf,rdlen);	//升级主函数参见文章头部博文		mtk_wdt_get_en_setting();		kfree(buf);		filp_close(fp, NULL);		revert_creds(old_cred);		put_cred(override_cred);		set_fs(old_fs);		//恢复权限	}	else{		printk("because of the same lib, update abort!\r\n");	}	return ret;}

按以上方式升级完后不用重启手机即可正常操作。验证成功与否的话，进入工厂菜单中即可看到正确的版本号F6，则说明升级成功。需要注意的是：该种方式只能用BIN固件升级，不能用C格式固件，因为读入的是流文件，也解释了之前升级C固件却导致TP不能用的原因。