blk_qc_t类型是通用块层一个类型，其定义如下：
typedef unsigned int blk_qc_t;
请教了Lei ming两个问题，答复如下：
- qc, 很好奇qc是什么的缩写？queue code?
queue cookie, which is only used by poll code path.
- 下面3个宏定义代表什么含义呢？
#define BLK_QC_T_NONE -1U#define BLK_QC_T_SHIFT 16#define BLK_QC_T_INTERNAL (1U << 31)
NONE means the submitted bio needn't to be polled. It is used in request_to_qc_t(), and BLK_QC_T_INTERNAL means the tag is a internal tag, which is used for IO scheduler, and if it isn't set, it means the tag is a driver tag, which is for submitting IO to hardware/driver.